Neuropathology

What are the two major disciplines in the field of pathology?

The two disciplines of pathology are anatomic pathology and clinical pathology. Pathologists commonly use the shorthand “AP” and “CP” to refer to these fields. To become certified by the American Board of Pathology in AP/CP in the United States requires a 4-year residency, whereas standalone AP or CP certifications typically require a 3-year residency.

What does anatomic pathology entail? How does this relate to neurologic and neurosurgical practice?

Anatomic pathology deals with the examination of tissues and fluids from patients (living, sometimes deceased) in order to make a diagnosis. Examination may involve microscopic examination of a biopsy specimen from an enhancing brain tumor, examination of a resection specimen from a dural-based mass, or examination of a cytology specimen prepared from the cerebrospinal fluid of a patient with primary central nervous system (CNS) lymphoma or medulloblastoma (looking for leptomeningeal dissemination). Least commonly, but every bit as important, anatomic pathology involves the performance of an autopsy.

What does clinical pathology entail? How does this relate to neurology and neurosurgical practice?

Neurologists, surgeons, and other physicians work more often with the clinical pathology laboratory than with the anatomic pathology lab even though they may not be aware of this. Clinical pathology deals with the examinations of patient tissues and fluids (whole blood, serum) to assist clinicians in making a diagnosis. In a hospital, the clinical pathology lab reports routine laboratories such as chemistry panels. Clinical pathologists must oversee the function of blood banks, clinical chemistry and hematopathology laboratories, and clinical microbiology and molecular pathology laboratories, among other divisions.

Where do samples go to when they go “to the lab”?

When neurologists send a specimen “to the lab,” it is important to know the basic divisions between AP and CP as described above. By knowing this, you will make sure the appropriate individuals are notified if there is an issue in how the specimen should be submitted (for example, a cerebrospinal fluid sample) or if there is an unexpected result.

What is a neuropathologist?

Neuropathologists are physicians who have certification in AP, occasionally in CP, and who have undergone two additional years of fellowship training in neuropathology. Typically these individuals have subspecialty certification in neuropathology, as well as certification in AP and even CP. In a hospital-based practice, it is common for neuropathologists to handle AP testing on nervous system tissues, including autopsy, as well as neuromuscular pathology and even ophthalmic pathology.

What is a frozen section?

This term refers to the intraoperative consultation performed on unfixed, fresh tissue. Relative to the neurologic patient, this occurs when a neurosurgeon is asked to obtain tissue to facilitate diagnosis when other methods are not specific enough to render an appropriate treatment. At the time of surgery, the neurosurgeon contacts the AP frozen section laboratory and places an order for a frozen section. The neuropathologist is notified and prepares to receive the fresh tissue. Typically, the patient remains under anesthesia so that the frozen section diagnosis can guide surgical treatment as needed (infiltrating versus circumscribed glioma) or rebiopsy can be performed if the tissue is not diagnostic.

What decisions are made during intraoperative consultation/frozen section? What are the goals?

First and foremost, the goal is not to render a final diagnosis. Instead, the neuropathologist must decide at frozen section how the tissue is to be triaged in consultation with the neurosurgeon. These decisions include whether tissue should be sent sterilely to the microbiology lab, sent fresh to the flow cytometry lab in order to rule out lymphoma or other hematopoietic malignancy, frozen for future molecular and/or research studies, or even placed in glutaraldehyde fixative for electron microscopy. It is crucial to understand that once a piece of tissue is fixed in formalin, it is forever lost to a number of possible studies that require unfixed tissue. This may mean a definitive diagnosis is not possible because the triaging of tissues was inappropriate (this is fortunately rare).

How is an intraoperative consultation actually performed?

With these triaging issues settled (or at least under way), the pathologist often makes a cytologic preparation, typically by smearing a small piece of tissue on a slide and staining the slide to examine the cellular characteristics of the tissue. A portion of the tissue is then rapidly frozen in a cryostat at about –25° C and thinly sectioned at 5 to 7 μm. This rapidly frozen and sectioned tissue is typically stained with hematoxylin and eosin (H&E) and examined microscopically in the frozen section room by the pathologist. The frozen section process is a caricature of the routine process of fixing, embedding, and staining tissues and as such the quality is never that of permanent sections (see below).

Is the frozen section diagnosis the final diagnosis for a patient?

Not usually. In conjunction with the cytologic preparation, and with knowledge of the clinical examination and radiologic findings, the pathologist renders a preliminary diagnosis or “frozen section diagnosis.” Rarely, this may be a specific and definitive diagnosis (myxopapillary ependymoma or pituitary adenoma). More commonly, this is a diagnostic category (high-grade glioma) or a descriptive diagnosis (reactive gliosis with perivascular lymphocytic and plasmacytic inflammation; no tumor identified).

What happens to the tissues after the frozen section is complete?

The unfrozen tissue is submitted in formalin, which fixes the tissue. Frozen tissue is thawed to room temperature and also submitted in formalin for routine histologic processing (the artifacts introduced by freezing this tissue will remain in the permanent sections, however). Overnight and the following morning, routine histologic processing is performed and a definitive and final diagnosis is ideally made from this formalin-fixed tissue.

How do pathologists refer to the formalin-fixed, unfrozen tissue they rely on to make a final diagnosis?

These are called permanent sections , or in pathology jargon, permanents . Rarely, an intraoperative consultation yields a diagnosis of “lesional tissue present; defer to permanents,” which means that any further classification of the lesion requires processing of the formalin-fixed tissue.

Why do results in anatomic pathology take so long to obtain (>24 hours or even days in some cases)?

It is important to keep in mind that tissues for pathologic analysis are fixed in formalin, sampled and submitted by a pathologist and undergo processing steps on machines that dehydrate the tissues to allow them to be penetrated by paraffin (a wax). The same tissues are embedded in a block, sectioned on a microtome, placed on a slide, and stained with H&E, special stains (e.g., Luxol fast blue for myelin), or immunostains (glial fibrillary acidic protein, or GFAP, to demonstrate glial differentiation in a tumor). The slides of an individual case must then be cover-slipped and ordered together with the relevant requisition forms for delivery to the pathologist, sometimes over multiple iterations if special studies are performed. This process is akin to an assembly line and requires many skilled technical personnel prior to the pathologist even receiving slides that are sufficient to render a final diagnosis.

What are some of the clinical indications for a frozen section?

Frozen sections are ideally performed if this information would guide the surgical approach or medical treatment. An example would be the differential diagnosis of ependymoma versus diffuse glioma in the spinal cord, with the former being amenable to gross total resection and the latter not. Likewise, if a preliminary diagnosis would immediately impact the treatment strategy (e.g., confirming a suspected diagnosis of pineal region germinoma) or triaging of tissues (for cultures, flow cytometry), intraoperative consultations may be appropriate. However, microscopic examination of formalin-fixed tissues is always preferable to frozen section as fixed tissues have preserved anatomic detail and can be used for special studies. If a frozen section neither guides surgical approach nor immediately impacts either treatment or triaging of the tissues, it is not indicated and may cause critical diagnostic tissue to be expended in the form of a suboptimal histologic preparation.

What are special stains? What are immunostains?

Special stains, or “specials,” are histochemical stains neuropathologists utilize to identify some specific feature of a tissue. Examples include silver-based stains to identify neurofibrillary tangles, periodic acid–Schiff stains to identify glycogen in various tissue types such as tumor cells, muscle fibers, and basement membrane material in capillaries. Special stains do not rely on an antigen–antibody reaction. In contrast, immunohistochemistry relies on dilutions of polyclonal and monoclonal antibodies (immunostains or “immunos”) and their reactions to specific tissue and tumor antigens. The binding of antibodies in tissue is visualized using a secondary antibody reaction. Pathologists use the presence/absence, distribution, and intensity of immunostaining to assist in making a diagnosis (for example, to demonstrate neuronal differentiation in a tumor).

What is lipofuscin? Where in the brain is this commonly identified?

Lipofuscin is a pigment that accumulates with aging in the normal nervous system. Examples of neurons with prominent lipofuscin pigment include those of the dorsal thalamus, inferior olive, subiculum of the hippocampal formation, lateral geniculate nucleus, motor neurons of both the primary motor cortex and ventral horn of the spinal cord, and dentate nucleus of the cerebellum.

What is central chromatolysis?

This process takes place when there is axonal damage distal to the neuronal cell body. In central chromatolysis, the nucleus becomes eccentrically placed, the normal, granular, basophilic Nissl substance dissipates, and the cytoplasm becomes distended like a balloon.

What is a cellular inclusion? Name some examples.

This is a nonspecific term referring to the accumulation of abnormal material in either the nucleus or cytoplasm of a cell, either glial or neuronal, recognized on either H&E or special studies. Important examples include (1) the globular cytoplasmic deposits of alpha-synuclein in the substantia nigra with a surrounding clear halo ( Lewy bodies ), (2) the violaceous intranuclear inclusion of a patient with herpes encephalitis and bilateral hemorrhagic necrosis of the temporal lobes ( Cowdry type A inclusions ), (3) the intranuclear inclusions of a patient with Huntington’s disease, a trinucleotide repeat disorder, (4) the eosinophilic, cytoplasmic bodies of a patient with rabies ( Negri bodies ), (5) the eosinophilic, refractile cytoplasmic rods in the pyramidal neurons of the hippocampus in an older patient with Alzheimer’s disease (AD) neuropathology ( Hirano bodies ), and (6) the small, round, lightly eosinophilic inclusions in the ventral horn motor neurons of a patient with amyotrophic lateral sclerosis (ALS) ( Bunina bodies ).

What is the significance of a “red” neuron?

Red neurons (also called ischemic or acidophilic neurons , eosinophilic neuronal necrosis ) indicate acute hypoxic/ischemic damage with neuronal death. The “red” in the name refers to the dense staining of eosin in the cytoplasm of these neurons. Simultaneously, the nuclei of these cells shrink in size and their chromatin condenses (these nuclei are said to be “pyknotic”). The nuclei eventually fragment in a process called karyorrhexis .

What is dark cell change? Are all dark neurons in a brain specimen indicative of hypoxic/ischemic injury?

No. “Dark” cell change may be an artifact in brain tissues that are manipulated either during biopsy or shortly after death (at autopsy). Distinguishing dark cell change from red neurons is not always easy. In general, the nuclei of neurons truly undergoing hypoxic injury are shrunken and their chromatin condensed (they appear pyknotic).

What is reactive gliosis?

This term is nonspecific and indicative of some injury to the nervous system, which may imply a reactive proliferation of astrocytes, oligodendroglia, or even microglia. Typically, however, pathologists use reactive gliosis to imply one of two situations: either reactive astrocytosis (increased number of nonneoplastic astrocytes) or astrogliosis (increased ramification of astrocytes). Reactive astrocytes are conspicuous on H&E stain as they appear larger than resting astrocytes and have dense eosinophilic cytoplasm and star-shaped astrocytic processes.

How does granular ependymitis look microscopically and what does it indicate?

Ventricular spaces in the nervous system are lined by columnar cells with intercellular tight junctions and apical microvilli and cilia. These are the ependymal cells. Whenever there is injury to the ventricular lining due to hydrocephalus, intraventricular hemorrhage, or infections with a predilection for the ventricular lining (e.g., cytomegalovirus), there is loss of ependymal cells. A small nodule of reactive gliosis without overlying ependyma is formed and this is termed granular ependymitis . Often seen by the neuropathologist long after the initial insult, the presence of granular ependymitis indicates that the aforementioned mechanisms of ependymal injury must be considered.

Where in the nervous system does Bergmann gliosis occur? What does it represent?

Bergmann gliosis is a proliferation of the normal resting Bergmann glia (astrocytes) of the cerebellar cortex. This typically occurs in response to hypoxic injury with loss of ischemia-susceptible Purkinje cells but may also be seen in degenerative diseases that involve the cerebellar cortex with Purkinje cell loss (e.g., multiple system atrophy). This pathologic process is typically apparent on H&E.

What normal structures in the aging brain may be mistaken for fungi or even Lafora bodies?

These structures are corpora amylacea, which accumulate in astrocytic processes adjacent to periventricular regions, blood vessels, around the base of the orbitofrontal cortex, and within the olfactory bulb. They represent normal aging in most brains, similar to lipofuscin pigment. Corpora amylacea are basophilic, round structures on H&E stain that may appear to have multiple concentric rings. These structures appear similar to encapsulated fungi or to the polyglucosan bodies that may be see in Lafora disease and type IV glycogen storage disease.

What is an infarct?

Infarction occurs when there is frank tissue necrosis and cell death. This may be due to either hypoperfusion (e.g., cardiogenic or septic shock) or vessel occlusion due to an embolic event or thrombus formation (e.g., hypercoagulable state).

How long does it take for red neurons to appear following acute hypoxic/ischemic injury?

The best answer is that this estimate varies widely in both the literature and among knowledgeable observers. A safe answer is approximately 6 hours following an ischemic event. In reality, this is not a hard and fast rule and shorter intervals have been reported in autopsy studies.

What is the other form of neuronal cell death besides the formation of red neurons? What takes place in this process?

Apoptosis with the formation of apoptotic bodies. This may be seen in ischemic injury (particularly in infant brains where red neurons are rarely formed), infection, and neurodegenerative disease. Similar to red neurons, the nuclear chromatin condenses in apoptotic cells and then fragments (termed karyorrhexis). These nuclear fragments are seen as multiple, small, densely basophilic fragments called apoptotic bodies . This process is not unique to neurons that have been injured, since apoptosis with formation of apoptotic bodies is characteristic of some high-grade tumors, including primary CNS lymphoma and small blue cell tumors of childhood such as medulloblastoma and retinoblastoma.

How is a subacute infarct differentiated from an acute infarct?

Subacute infarcts demonstrate reactive vascular proliferation, gliosis, and may begin to demonstrate accumulation of macrophages to clear necrotic debris. Axonal retraction balls may also begin to form. The boundaries are not absolute, however, and red neurons may still be present in infarcts that otherwise appear to be subacute.

How do infarcts appear macroscopically and how are these separated from hemorrhagic contusions?

The appearance of an infarct may vary greatly by location, age, and whether it has been reperfused. The characteristic appearance, however, is a dusky discoloration of the cerebral cortical ribbon in the depths of cerebral sulci with variable amounts of hemorrhage. Unlike contusions, the pial surface and molecular layer of the cortex are spared (contusions disrupt these layers).

What is a lacunar infarct?

These are small infarcts that typically involve the basal ganglia, cerebral white matter, and brainstem in patients with a history of hypertension.

What is a “watershed” zone?

These are brain regions positioned at the periphery of the zones of perfusion for two or more vessels (hence the use of the term arterial border zones ). Blood flow to these areas is less than other brain regions, which explains their selective vulnerability for hypoxic/ischemic injury. For example, the cerebral convexity in the lateral parietal area is a watershed zone for the anterior and middle cerebral arteries.

What neuronal populations are particularly susceptible to hypoxic/ischemic injury?

The most notable sites to examine for hypoxic/ischemic injury include sector CA1 of the hippocampus, the Purkinje cell layer of the cerebellar cortex, and the middle laminae of the cerebral cortex. Sector CA2 of the hippocampus, unlike CA1, is particularly resistant to hypoxic injury and is thus referred to as the dorsal resistant zone .

What is the end stage of any necrotizing lesion in the brain (infarct, contusion)?

If the patient survives such an insult, these lesions become cystic cavities with a rim of gliotic brain tissue. Macrophages infiltrate the lesion to remove the necrotic debris and may persist for a long time following the insult. In the case of cerebral infarcts, the necrotic tissue is resorbed, the red neurons are no longer present, and a cavity with a glial scar is all that remains (residual macrophages may be present).

What is the characteristic macroscopic appearance of a venous infarct?

These infarcts result from dural venous sinus thrombosis and produce hemorrhagic necrosis that is parasagittal, bilateral, and may appear symmetrical. These involve both white matter and cortex of the affected regions. Associated clinical conditions include hypercoagulability, dehydration, and meningitis.

What are the major causes of intraparenchymal hemorrhage?

These include (1) trauma with massive contusion and laceration of the brain surface, (2) chronic hypertension with small vessel disease (subcortical hemorrhage following rupture of the lenticulostriate arteries), (3) cerebral amyloid angiopathy (typically as cerebral lobar hemorrhage), (4) tumor-associated hemorrhage (most often with melanoma, glioblastoma, and metastases of highly vascular tumors), (5) infectious etiologies (mycotic aneurysms, angioinvasive fungal infections), (6) reperfusion of an infarct, and (7) rupture of a vascular malformation.

Describe the histologic appearance of an arteriovenous malformation (AVM).

On H&E examination, AVMs comprise thick-walled, aberrant vascular channels with intervening gliotic brain tissue. Some of the large vessels will be arterial, which is demonstrated by the presence of an elastic lamina on H&E stain and special stains that highlight the elastic lamina (e.g., Movat pentachrome or Verhoeff–Van Gieson stains).

What process may be seen in the cortex adjacent to an AVM or any vascular malformation?

Cortical dysplasia may be present. Dysplasia consists of cortex containing dysmorphic neurons that are abnormally shaped, maloriented, enlarged, and possibly multinucleated, with an atypical columnar/vertical and laminar/horizontal organization of cortex. In any setting, cortical dysplasia may range from very mild to severe and a variety of grading schemes for this pathology have been proposed.

Describe the histologic appearance of cavernous hemangioma (cavernoma)?

Cavernous hemangioma has no intervening brain tissue (at least for the purposes of a board exam!) and consists of venous channels, including thick-walled, muscularized veins without an elastic lamina. These lesions often have a rim of tissue with gliosis and hemosiderin-laden macrophages, the latter giving rise to the famous “ferruginous penumbra” on T2-weighted MRI sequences.

What are the most common locations of hypertensive intraparenchymal hemorrhage?

Basal ganglia (specifically the putamen), basis pontis/brainstem, and cerebellum.

An elderly patient with lobar hemorrhage and cognitive impairment may have what combination of findings? What genetic features predispose to this (in nonfamilial AD)?

In cerebral amyloid angiopathy (CAA), amyloid deposits in the vessel walls of leptomeningeal and cortical capillaries and small arterioles, giving them a rigid appearance often likened to “lead pipes.” CAA often goes hand in hand with other AD neuropathology and is a major risk factor for cerebral lobar hemorrhage. When CAA is severe, there may be fibrinoid necrosis of the vessel wall, luminal occlusion, and a foreign body-type giant cell reaction. In contrast to arterioles and capillaries, which are affected by CAA, cerebral veins, venules, and white matter vessels typically are not affected. Both CAA and AD neuropathology are significantly associated with the presence of an apolipoprotein E epsilon 4 (ApoE ε4) allele.

Which ApoE allele is thought to be protective against cerebral amyloid angiopathy and AD neuropathology?

Apolipoprotein E epsilon 2 (ApoE ε2).

What is Wallerian degeneration and how does it look microscopically?

This is a secondary axonal degeneration that follows neuronal death proximally (e.g., infarct, ALS) or following tract/axonal destruction (e.g., in diffuse axonal injury). This appears microscopically as atrophy of the involved white matter structure with accumulation of macrophages. In the case of axonal damage with transection this may induce bulbous swellings of the axon termed axonal “ retraction balls .” It may take 1 week or more for these changes to become apparent on routine microscopy.

How does cerebral edema look macroscopically? What herniation syndromes may result from severe edema?

Cerebral edema is characterized by a “heavy” brain (subjectively assessed first by handling the brain at autopsy and then objectively by weighing the brain). There is typically effacement of the normal sulcal spaces. Gyral contours may be flattened as the cerebral hemispheres press up against the skull and on serial sections the ventricular spaces often appear slit-like. If the brain has been formalin fixed, there may be pink discoloration of brain tissue due to the fact that formalin could not easily penetrate into the effaced sulcal spaces. If the edema is severe enough, there may be downward displacement of the cerebellar tonsils into the foramen magnum (tonsillar herniation) and/or of the mesial temporal lobes over the edge of the tentorium cerebelli (uncal herniation).

If a patient has a significant ischemic injury in the cerebrum with subsequent cerebral edema and unilateral uncal herniation, they may develop Duret hemorrhages. Who was Duret and what are Duret hemorrhages?

Henri Duret was a French surgeon who trained in part with the famed neurologist Jean-Martin Charcot and was active in the late nineteenth century. The hemorrhages named after Duret (also called secondary brainstem hemorrhage ) occur in the brainstem following cerebral edema with herniation. These are thought to be due to the rupture of small penetrating vessels, particularly into the central portions of the midbrain, pons, and medulla.

If the same patient demonstrated a Kernohan’s notch, what does that refer to?

This is observed as disruption and hemorrhage of the cerebral peduncle opposite the side of uncal herniation and thus opposite the side of a space-occupying hemispheric lesion (hematoma, intracerebral hemorrhage). It arises when the cerebral peduncle is forced up against the “sharp” edge of the tentorium as the uncus of the opposite hemisphere herniates.

How does a Kernohan’s notch create a false localizing sign?

In a patient with a right-sided epidural hematoma, the right uncus would herniate and the left cerebral peduncle would develop the destructive lesion known as Kernohan’s notch . Since the motor fibers within the affected peduncle have yet to decussate, the motor deficits would appear largely on the right side of the body. Thus, motor deficits would be paradoxically ipsilateral to the hemispheric lesion rather than contralateral.

Who was Kernohan?

James Kernohan was an Irish-born neuropathologist who worked for many years at the Mayo Clinic in Rochester, Minnesota.

How does an epidural hematoma arise?

Epidural hematoma arises following laceration of the middle meningeal typically, which courses in the superficial-most aspect of the dura. This typically follows skull fracture. After the artery is torn, blood accumulates between periosteal dura and skull. Formation of the hematoma is limited by the sutures, resulting in an elliptical hematoma that compresses brain parenchyma as it expands.

How and where does subdural hematoma arise? Why is this region susceptible?

Subdural hematoma occurs when hemorrhage begins in the border cell layer of the dura (innermost part of dura adjacent to arachnoid) due to tears in cortical bridging veins. This is a good location for hematomas to form due to the paucity of tight junctions between cells of this layer.

How does a subdural hematoma progress?

As the hematoma expands, the dura responds by forming granulation tissue, which comprises fibroblastic and capillary proliferation. This may result in further hemorrhage into the hematoma within the potential “subdural” space. As a result, many subdural hematomas have some combination of recent and remote hemorrhage, the latter being characterized by blood breakdown products (e.g., hemosiderin) and degenerated erythrocytes.

What are the “membranes” of a subdural hematoma, where do these begin forming, and why are they important?

Subdural membranes begin on the dural side (where fibroblasts reside). In the first week after the hematoma develops, a membrane of fibroblasts, collagen, and capillaries is formed on the dural side to “wall off” the hematoma while clot lysis is under way. In the second week, fibroblasts, collagen, and capillaries of granulation tissue grow along the arachnoid side of the hematoma from the dural side. If the hematoma is stable, over many months the clot will resorb and only the inner and outer hematoma membranes remain. This is why examination of the dura at autopsy will always include an evaluation for membrane formation on the inner aspect of the dura, even if the blood clot is not visible grossly.

How do chronic subdural hematomas further expand?

Particularly with slowly developing, chronic hematomas, the granulation tissue formed contains capillaries susceptible to further hemorrhage. These are termed microbleeds and may further expand the hematoma.

In subdural hematoma over a convexity, how does the contralateral hemisphere appear?

Often the contralateral cerebral hemisphere will be smooth surfaced with loss of normal gyral contours. This is due to the mass effect caused by the hematoma with shifting of the uninvolved cerebral hemisphere up against the skull.

Define the term extramedullary hematopoiesis . Why does this happen (though rarely) in subdural hematoma?

Extramedullary hematopoiesis is when blood precursor cells typically found in bone marrow (erythroblasts, megakaryocytes, myeloid precursors) accumulate outside of the bone marrow. This is an uncommon but curious finding in chronic subdural hematoma. The mechanisms underlying this are not understood.

List several mechanisms by which subarachnoid hemorrhages may form.

Ruptured berry aneurysm within the circle of Willis (the vast majority in the anterior circulation), traumatic brain injury, iatrogenic causes (e.g., a very rare complication of stereotactic brain biopsy), and bleeding from vascular malformations (e.g., arteriovenous malformations).

Describe a contusion of the brain and how this is formed.

Contusions are hemorrhagic lesions of the brain surface, which are essentially brain “bruises.” When these take place in the cortex (mesial temporal lobes) they result in disruption of the molecular layer and pial surface. These typically form either when the brain is displaced against a bony surface (temporal tips against the petrous ridges or orbitofrontal surface of the brain against the sphenoid wings), or alternately, when a depressed fracture displaces bone inward to disrupt the brain surface.

What is a coup contusion?

Coup contusions result from a traumatic closed head injury to a head that is not in motion. True coup contusions are not common. Much more common are countercoup contusions, as seen when the temporal tips slide against the petrous ridges after falling backward and hitting the back of the head, or fracture contusions (see previous answer).

Infarcts of the brain and contusions both have what shape? How might these be distinguished?

Both of these may be wedge-shaped, hemorrhagic lesions involved the gray matter. The base of the wedge is at the pial surface, and its apex points toward the white matter. Contusions hemorrhage due to laceration of vessels. Infarcts may hemorrhage as dead tissue is reperfused. However, infarcts, unlike contusions, have an intact pial surface and molecular layer.

Contusive brain injuries are very rare in infants and very young children. Therefore, what should be considered when countercoup contusion hemorrhages are identified in the brain of a small child?

Nonaccidental head injury.

What triad of findings is characteristic of nonaccidental head injury in infants?

Retinal hemorrhage, subarachnoid hemorrhage, and subdural hematoma. These may be present even in the absence of skin and scalp bruising or skull fracture.

What is diffuse axonal injury (DAI)?

DAI is a pathologic processes thought to mediate the cognitive disturbance following severe head trauma. DAI follows rapid acceleration–deceleration head injuries that generate rotational forces on the brain. This pathologic process may be subtle on autopsy examination yet result in very significant cognitive impairment or even death.

How might DAI be recognized macroscopically?

DAI is observed as punctate or linear hemorrhage in the white matter and brainstem of a patient with head trauma.

What is the microscopic correlate of DAI?

In DAI, axonal swellings form in various brain locations, including the dorsolateral brainstem (tectal plate), corpus callosum, and other sites. These axonal swellings may be followed by axonal disruption with formation of bulb-like structures termed retraction balls (these are visible on H&E stain). Neuropathologists may perform an immunohistochemical evaluation with amyloid precursor protein (APP), which highlights DAI.

Along with DAI, what other two components form a triad that is common in closed head injury?

Subarachnoid hemorrhage and subdural hemorrhage may also be seen with DAI in patients with closed head injury.

What is a “gliding contusion” and what does it indicate?

Gliding contusions are indicative of DAI and significant head trauma following closed head injury. Their macroscopic correlate is punctate and linear hemorrhages in the bilateral parasagittal white matter of the frontal and parietal lobes.

What are the respective patterns of injury in methanol and carbon monoxide toxicities?

Methanol toxicity results in bilateral putaminal necrosis. Carbon monoxide poisoning causes bilateral, internal segment of globus pallidus necrosis.

What is the macroscopic appearance of a brain affected by bilirubin deposition (kernicterus)?

The subcortical nuclei (basal ganglia), cranial nerve nuclei, olives, and periventricular regions of the brainstem have bright yellow pigmentation in hyperbilirubinemia. The microscopic correlate of these findings is neuronal necrosis due to the toxic nature of the unconjugated bilirubin.

What is the role of tau in normal cells?

Tau is a protein involved in the stabilization of microtubules. Microtubules have a critical role in stability of the cytoskeleton.

Describe the differences between pathologic and normal tau in brain cells.

Normal tau is a microtubule-associated protein (MAP). This protein is soluble, nonphosphorylated, and naturally has a range of isoforms (up to six) that result from variations in mRNA splicing. In contrast, pathologic tau is hyperphosphorylated, insoluble, and cannot have its normal interaction with microtubules. The accumulation of this insoluble form of pathologic tau is detected on H&E examination as basophilic, flame-shaped or globose tangles, or by special techniques (e.g., silver-based stains or antibodies to phosphorylated tau).

Describe the role of MAPT in the production of tau.

MAPT resides on the long arm of chromosome 17 (17q21-22) and is a large gene comprising 16 exons (or coding regions). Alternative splicing in three exons (E2, E3, and E10) results in six different forms of normal tau protein. These isoforms are further subdivided into 3-repeat (3R) and 4-repeat (4R) tau (also see the next question). MAPT mutations result in an autosomal dominant form of frontotemporal dementia characterized by both dementia and parkinsonism.

How do 3R and 4R tauopathies differ?

These differ in the number of microtubule-binding repeats contained in the final tau protein. 3R tau isoforms contain three such repeats, whereas 4R tau isoforms contain four.

Which neurodegenerative diseases are associated with the accumulation of pathologic tau in brain cells?

These include AD and also progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), argyrophilic grain disease (AGD), frontotemporal lobar degeneration (FTLD) with tau inclusions (FTLD-tau), and chronic traumatic encephalopathy (CTE), among others.

Which of these tauopathies are characterized by accumulation of 3R tau? 4R tau? Both 3R and 4R?

AD tangle pathology consists of both 3R and 4R tau isoforms. PSP, AGD, and CBD are all 4R tauopathies, whereas FTLD-tau (Pick’s disease) is a 3R tauopathy. FTDP-17 resulting from MAPT mutations may contain either 3R or 4R tau.

What is the characteristic macroscopic (gross) appearance of the brain affected by severe AD neuropathology?

The brain may be profoundly atrophic with hydrocephalus ex vacuo (expansion of the ventricular system secondary to tissue volume loss), marked thinning of the cortical ribbon, bilateral hippocampal atrophy, and widening of the cerebral sulci.

What two cardinal pathologic features characterize AD?

AD is characterized by the accumulation of pathologic forms of intracellular hyperphosphorylated tau in the form of neurofibrillary tangles and extracellular deposits of parenchymal amyloid in the form of plaques.

Describe the amyloid plaque pathology of AD.

Amyloid-β (Aβ) deposition in brain parenchyma may occur as multiple isoforms generated from proteolysis of amyloid precursor protein (APP). The deposition of Aβ occurs as either extracellular diffuse or dense cored plaques visualized with immunostains to Aβ/APP and special stains like Congo red and thioflavin-S. Dense cored plaques are also visible on H&E stain.

What is a neuritic plaque?

A dense-cored plaque surrounded by a rim of silver or tau-positive dystrophic neurites.

What is the Thal staging system for amyloid plaque pathology?

The Thal staging system describes the extent and progression of Aβ parenchymal deposition in five progressive phases. This evaluation may be based on Aβ immunohistochemistry as well as a silver stain. The first phase is deposition of extracellular amyloid in the six-layered isocortex with progressive extension to the mesial temporal lobe (phase 2), basal ganglia and diencephalon (phase 3), brainstem (phase 4), and, less commonly, cerebellar cortex (phase 5).

Is neurofibrillary tangle pathology ever encountered outside of AD?

Neurofibrillary tangles are frequent pathologic findings in cognitively normal older patients. Further, neurofibrillary tangles are among the diagnostic features of other tauopathies including PSP, FTLD-tau, CBD, and CTE. Tau inclusions are also seen in a wide range of other degenerative diseases (e.g., ALS/motor neuron disease) and in nondegenerative brain diseases as well (e.g., subacute sclerosing panencephalitis following measles infection).

What is the CERAD system for estimating the likelihood of AD?

The CERAD system was established in 1991 and uses a neuritic plaque score based on semiquantitative estimation of plaque density as sparse, moderate, and frequent in cortex. CERAD stands for the Consortium to Establish a Registry for Alzheimer’s disease. CERAD criteria propose that a neuritic plaque score is assigned for a given brain to make a determination of “normal,” “definite AD,” “probable AD,” and “possible AD” in conjunction with the patient’s age, presence/absence of clinically defined dementia, and other lesions likely to contribute to dementia.

In what forms does pathologic tau accumulate in AD?

Filamentous, hyperphosphorylated tau accumulates as intracytoplasmic neurofibrillary tangles, neuropil threads, and dystrophic neurites within neuritic plaques. These may be identified by either silver-based stains (e.g., Bielschowsky, Gallyas) or immunohistochemistry. On H&E, tangles can be identified as basophilic, flame-, or crescent-shaped inclusions in neurons of the cortex and hippocampus, whereas in subcortical and brainstem neurons these form globose tangles. Unlike more “pure” tauopathies, tau in AD does not typically accumulate within glia.

What are “pretangles”? What are “ghost tangles”?

“Pretangles” are tau-positive, perinuclear, and nonfibrillary structures in the neuronal cytoplasm thought to represent an incipient version of a mature neurofibrillary tangle. “Ghost” tangles are disintegrating extracellular neurofibrillary tangles identified after the involved neurons are lost.

What is the Braak and Braak staging system for AD neuropathology?

This is a six-stage system for recording the hierarchical distribution of neurofibrillary pathology. Stage I shows neurofibrillary tangles within the transentorhinal cortex straddling the rhinal sulcus in the anteromedial temporal lobe. Stage II is characterized by involvement of the transentorhinal cortex and also the superficial entorhinal cortex. Stage III is characterized by involvement of stage I and II areas but with more extensive and deeper involvement of the entorhinal cortex and additional neurofibrillary pathology in the hippocampal formation and temporal isocortex (sparse). Stage IV has involvement of the preceding regions as well as tangle pathology in cingulate and insular cortices; rare isocortical tangles may be present in stage IV. Stages V and VI are characterized by diffuse isocortical involvement with stage VI distinguished by significant involvement of primary sensory cortices.

How do the 2012 National Institute on Aging/National Institutes of Health (NIA/NIH) criteria bring together CERAD score, Thal phase, and the Braak staging scheme?

These criteria suggest that an “ABC” score be provided, which describes AD neuropathologic changes associated with Aβ deposition and neurofibrillary tangle pathology. The score incorporates the Thal phase of the extent of amyloid deposition (“A”), the Braak and Braak neurofibrillary stage (“B”), and the neuritic plaque density score of the CERAD system (“C”). For example, a brain with Thal phase 1 or 2 amyloid pathology, Braak stage 1 or 2 neurofibrillary pathology, and a CERAD neuritic plaque score of “sparse” would be A1 B1 C1 in the 2012 NIA/NIH criteria. The final ABC scores are assigned one of four levels of AD neuropathologic change as “not,” “low,” “intermediate,” and “high.” These values are correlated with the clinical level of cognitive impairment to determine whether the AD neuropathologic change provides an adequate explanation for the clinical findings. The recommendations also encourage the reporting of Lewy body pathology, vascular pathology, TAR DNA binding protein 43-kDa (TDP-43) protein deposition, and hippocampal sclerosis.

What differentiates the tau pathology of AD from age-related neurofibrillary tangle pathology?

Age-related neurofibrillary pathology may be seen in cognitively normal patients or in patients with mild cognitive impairment. Likewise, mild neocortical amyloid plaque pathology is a very frequent finding in normal aging. What differentiates AD from non-AD pathology is the extent and density of tau pathology, which very frequently involves a six-layered neocortex (isocortex). AD patients also typically have more abundant amyloid plaque pathology in the form of diffuse and neuritic plaques.

Can a neuropathologist confidently diagnose AD from autopsy material alone?

No. The final diagnosis of AD requires a correlation of premortem clinical findings and postmortem pathologic findings. Because of this requirement, current guidelines from the NIA/NIH provide a “likelihood” estimate of whether the pathologic findings observed would have contributed to a clinically recognized dementia.

What differentiates the tau pathology of AD from that seen in other tauopathies?

These diseases differ in the (1) the spatial distribution of tau pathology, (2) the relative extent of amyloid pathology (typically far less severe than seen in AD), (3) the number of repeats in the tau isoform accumulating in cells (3R vs 4R), and (4) in some diseases, the presence of glial cytoplasmic inclusions.

Which of the non-AD tauopathies have accumulation of pathologic tau in glial cells?

These include AGD, CBD, FTLD-17, and PSP. In the diseases both neurons and glial cells accumulate the pathologic tau. Glial inclusions are also seen in diseases with accumulation of TDP-43 (FTLD TDP-43 and ALS) and alpha-synuclein (multiple system atrophy, discussed below).

What is the neuropathologic basis of “Pick’s disease”?

Pick’s disease is a type of FTLD with tau-positive inclusion (FTLD-tau). These patients have a severe dementing illness with prominent frontal executive dysfunction and disinhibition. Macroscopically there is pronounced frontotemporal atrophy with “knife-edge,” sharp-appearing gyral crests. The parieto-occipital regions are spared in contrast (Note: AD may have a similar macroscopic appearance so frontotemporal atrophy is not pathognomonic for FTLD or Pick’s disease.) These patients have eosinophilic, globular inclusions that are tau and silver-stain positive termed Pick bodies . Unlike AD, amyloid pathology is not typically prominent.

What is AGD?

AGD is a non-AD tauopathy occurring in elderly patients and is thought to account for 5% of all dementias. The original pathologic descriptions were in clinically demented patients without significant AD pathology (not as incidental autopsy findings).

How is AGD diagnosed pathologically?

First, most patients lack the profound atrophy seen in AD brains on gross examination. Second, on microscopic examination, AGD brains demonstrate short, stubby, and spindle-shaped threads in the neuropil that are positive on tau immunostain and silver special stain (“argyrophilic”). These “grains” have been likened to grains of rice and appear dot-like on cross-section. These patients also have tau accumulation in glial cells in the form of oligodendroglial coiled bodies. Mild AD pathology (Braak stage ≤III) is very common in these patients and does not make their diagnosis AD.

Where do “grains” in AGD frequently occur?

These are most abundant within the mesial temporal lobe (superficial entorhinal cortex, amygdala, and hippocampus). Much like AD, these lesions progress over time to involve other limbic regions (anterior cingulate) and neocortex.

Since AGD also features neurofibrillary tangle pathology and occurs in elderly patients, why isn’t this just considered AD neuropathology?

First and foremost, AGD is a 4R tauopathy, unlike the mixture of 3R and 4R tau isoforms seen in AD. Second, unlike AD, AGD has no significant association with the frequency of the apolipoprotein E epsilon 4 (ApoE ε4) allele. However, AD and AGD pathology can coexist on neuropathologic examination. In fact “low stage” AD-type neuropathology is present in many non-AD disorders, particularly those of older patients (e.g., ALS, multiple system atrophy [MSA], dementia with Lewy bodies). Third, AGD brains demonstrate both the hallmark “grains” of the disease as well as glial cytoplasmic inclusions (see above).

What is MSA? How old are typical MSA patients?

MSA is a sporadic, progressive alpha-synucleinopathy characterized by (1) dysautonomia, (2) cerebellar dysfunction, and/or (3) parkinsonism. These features correspond to the earlier clinical designations of Shy–Drager syndrome, olivopontocerebellar atrophy, and striatonigral degeneration. Patients may also experience stridor (due to vocal cord palsy), dysphagia, rapid eye movement sleep disturbance, cognitive impairment, and upper motor neuron signs. Onset in the sixth or seventh decade of life is most typical, and patients rarely live beyond 10 years after diagnosis.

What neuropathologic finding is the core diagnostic feature of MSA?

In life, “possible” and “probable” MSA diagnoses may be made using clinical and laboratory criteria. However, the clinical findings of MSA may overlap those of atypical Parkinson’s disease, corticobasal degeneration, and progressive supranuclear palsy. Therefore, a definitive diagnosis requires demonstration of alpha-synuclein immunoreactive glial cytoplasmic inclusions (GCIs) that are the hallmark of MSA. These GCIs also react on silver stains (they are “argyrophilic”), as well as ubiquitin, and the ubiquitin pathway-related protein, p62.

In the literature, what is the difference between the clinicopathologic findings of MSA in US and Western European populations versus those in Japan?

US and European studies report a clear predominance of MSA with predominant parkinsonian features, or MSA-P (∼60%). In the Japanese literature, the vast majority of patients have MSA with predominant cerebellar symptoms, or MSA-C (>80%). The reasons for these differences are not clear.

What four alpha-synucleinopathies may have significant clinical symptoms of dysautonomia with pathologic involvement of structures involved in autonomic function (e.g., intermediolateral cell column, peripheral ganglia, hypothalamus)?

MSA, Parkinson’s disease, dementia with Lewy bodies, and primary autonomic failure.

What do glial cytoplasmic inclusions in MSA consist of? Are they seen in other alpha-synucleinopathies?

Granulofilamentous material with filaments having a diameter of 20 to 40 nm. Rare reports of glial inclusions in non-MSA alpha-synucleinopathies exist, but these are uncommon. MSA patients may also have pleomorphic neuronal inclusions and alpha-synuclein-positive processes or neurites.

What is the neuropathologic process underlying primary autonomic failure?

Loss of postganglionic sympathetic nerve fibers with Lewy body formation and neuronal loss in autonomic ganglia. In contrast, MSA patients have pathologic involvement of preganglionic neurons in the intermediolateral cell column, within Onuf’s nucleus of S2–S4 that controls urinary continence, within the hypothalamus, and within catecholaminergic and serotonergic neuron groups of the brainstem.

What is the characteristic macroscopic appearance of MSA? How might this overlap with that of Parkinson’s disease?

MSA-P patients show findings of striatonigral degeneration, which results from massive neuronal loss, volume loss, and gliosis within the dorsolateral putamen, caudate nucleus, and substantia nigra pars compacta with relative sparing of globus pallidus. The putamen in these patients is often slit-like and discolored. Pallor within the substantia nigra and locus ceruleus, reflecting loss of pigmented catecholaminergic neurons, is also common. The loss of pigmentation in brainstem nuclei overlaps with a key gross feature of Parkinson’s disease. However, Parkinson’s disease patients are not expected to have profound striatal degeneration.

What is the key macroscopic finding in patients with a predominant olivopontocerebellar atrophy form of MSA, or MSA-C?

These patients experience severe neuronal loss with volume loss and gliosis in the basis pontis and inferior olives, as well as loss of pontocerebellar and olivocerebellar fibers, and severe atrophy of the middle cerebellar peduncle and cerebellar white matter. The cerebellar cortex is usually atrophic with Purkinje cell loss. Unlike patients with dementia with either Lewy bodies or AD, cortical atrophy in MSA patients is not common.

What is the characteristic neuropathology of ALS?

ALS neuropathology is characterized by motor neuron loss and gliosis in the frontal cortex, spinal cord, and brainstem somatic motor nuclei (though sparing the nuclei of cranial nerves III, IV, and VI). Ubiquitinated inclusions of transactivating responsive sequence (TAR) DNA-binding protein 43 kDa (TDP-43) are also present in both neurons and glia.

What is the most common genetic risk factor identified to date in both sporadic and familial ALS?

Hexanucleotide expansion of C9orf72 . This results from an expansion of a noncoding GGGGCC hexanucleotide repeat in the C9orf72 locus at chromosome 9p21, leading to the accumulation of small nuclear RNA fragments and accumulation of 25-kDa C-terminal TDP-43 fragments that become phosphorylated and cytotoxic to the cell.