Brain Stem Disease

What is the functional importance of the brain stem?

The brain stem is a small, narrow region connecting the spinal cord with the diencephalon and cerebrum. It lies ventral to the cerebellum, which it links via the cerebellar peduncles. Its functions are critical to survival. The brain stem is densely packed with many vital structures such as long ascending and descending pathways that carry sensory and motor information to and from higher brain regions. It contains the nuclei of cranial nerves III through XII and their intramedullary fibers. It also possesses groups of neurons that are the major source of noradrenergic, dopaminergic, and serotonergic inputs to most parts of the brain. In addition, other specific nuclear groups, such as the reticular formation, olivary bodies, and red nucleus, lie within the brain stem. In short, it is a complicated but highly organized structure that controls motor and sensory activities, respiration, cardiovascular functions, and mechanisms related to sleep and consciousness. Consequently, a small lesion in the brain stem can affect contiguous structures and cause disastrous neurologic deficits.

Describe the function of the medulla.

The medulla (bulb) is the direct rostral extension of the spinal cord. It contains the nuclei of the lower cranial nerves (mainly IX, X, XI, and XII) and the inferior olivary nucleus. The dorsal column pathways decussate in its central region to form the medial lemniscus, whereas the corticospinal tracts cross on the ventral side as they descend caudally. Together with the pons, the medulla also participates in vital autonomic functions such as digestion, respiration, and regulation of heart rate and blood pressure ( Fig. 9-1 ).

Describe the function of the pons.

The pons (bridge) lies rostral to the medulla and appears as a bulge mounting from the ventral surface of the brain stem. The pons contains nuclei for cranial nerves V, VI, VII, and VIII as well as a large number of neurons that relay information about movement from the frontal cerebral hemispheres to the cerebellum (frontopontocerebellar pathway). Other clinically pertinent pathways in the pons are those for the control of saccadic eye movements (medial longitudinal fasciculus) and the auditory and vestibular connections ( Fig. 9-2 ).

Describe the function of the midbrain.

The midbrain, the smallest and most rostral component of the brain stem, plays an important role in the control of eye movements and coordination of visual and auditory reflexes. It contains the nuclei for cranial nerves III and IV. Other important structures are the red nuclei and substantia nigra. The periaqueduct area has an important but poorly understood influence on consciousness and pain perception ( Fig. 9-3 ).

Which cranial nerves are not found in the brain stem?

The 12 pairs of cranial nerves are numbered in rostral caudal sequence. The brain stem contains the nuclei of all cranial nerves except two: the optic (II) nerve, which terminates in the thalamus, and the olfactory (I) nerve, which synapses in the olfactory bulb.

What are the locations and functions of the individual cranial nerves?

See Table 9-1 .

How can understanding the anatomy and function of individual cranial nerves assist in localizing brain stem lesions?

The relatively compact positioning of the cranial nerve nuclei and their intramedullary nerve fibers at specific levels, as well as their proximity to certain vertically directed fiber tracts, create a series of anatomic patterns that provide a basis for the localization of brain stem lesions. Generally speaking, the motor nuclei of the cranial nerves are situated medially, the spinothalamic fibers run along the dorsal lateral portion, and the corticospinal fibers run along the ventral portion of the brain stem.

What is the approach to localizing a brain stem lesion?

As a consequence of the unique anatomic arrangements in the brain stem, a unilateral lesion within this structure often causes “crossed syndromes,” in which ipsilateral dysfunction of one or more cranial nerves is accompanied by hemiplegia and/or hemisensory loss on the contralateral body. Exquisite localization of a brain stem lesion depends on signs of long-tract (corticospinal and spinothalamic pathways) dysfunction to identify the lesion in the longitudinal (or sagittal) plane and on signs of cranial nerve dysfunction to establish its position in the cross-sectional (or axial) plane. Localization of disorders of the brain stem can be simplified by summarizing the patient’s neurologic deficits to answer the following questions: Is the lesion affecting unilateral or bilateral structures of the brain stem? What is the level of the lesion? If the lesion is unilateral, is it medial or lateral in the brain stem?

What is the approach to localizing an isolated cranial nerve deficit?

An isolated cranial nerve defect, especially of VI and VII, is most often due to a peripheral and not a brain stem lesion.

How do the presentations of intra-axial and extra-axial lesions of the brain stem differ?

A lesion that directly affects the tissues of the brain stem is called intra-axial or intramedullary . It usually presents with simultaneous cranial nerve and long-tract symptoms and signs. A lesion outside the brain stem is called extra-axial . It affects the brain stem by initially compressing and interfering with the functions of individual cranial nerves. Later, as it enlarges, neighboring structures within the brain stem may be affected, causing additional long-tract signs.

What is the radiographic examination of choice for brain stem lesions?

Magnetic resonance imaging (MRI) is the examination of choice for suspected brain stem lesions. It provides a highly sensitive and noninvasive method of evaluating the posterior fossa, unhampered by skull base artifact. Enhancement with gadolinium may be useful to characterize breakdown of the blood–brain barrier. Magnetic resonance angiography also may be helpful to investigate further the major branches of the vertebrobasilar system in brain stem ischemia or infarction.

Describe the vascular supply of the medulla.

The medulla is supplied by the vertebral arteries and their branches. Its blood supply may be further subdivided into two groups, the paramedian bulbar and the lateral bulbar arteries. The paramedian bulbar arteries are penetrating branches, mainly from the vertebral artery, that supply the midline structures of the medulla. At the lower medulla, branches from the anterior spinal artery also contribute to this paramedian zone. The lateral portion of the medulla is supplied by the lateral bulbar branches of the vertebral artery or the posterior inferior cerebellar artery.

Describe the vascular supply of the pons.

The basilar artery is the principal supplier of the pons. It gives off three types of branches. The paramedian arteries supply the medial basal pons, including the pontine nuclei, corticospinal fibers, and medial lemniscus. The short circumferential arteries supply the lateral aspect of the pons and the middle and superior cerebellar peduncles. The long circumferential arteries together with branches from the anterior inferior cerebellar and superior cerebellar arteries supply the pontine tegmentum and the dorsolateral quadrant of the pons.

Describe the vascular supply of the midbrain.

Arteries supplying the midbrain include branches of the superior cerebellar artery, posterior cerebral artery, posterior communicating artery, and anterior choroidal artery. Branches of these arteries, like those of the pons, can be grouped into paramedian arteries, which supply the midline structures, and the long and short circumferential arteries, which supply the dorsal and lateral midbrain.

Because the blood supply to the brain stem at each level is divided into several territories (usually medial and lateral), occlusion of specific arteries manifests clinical features that reflect their vascular distribution.

What is the medial medullary syndrome?

The medial medullary (Dejerine’s) syndrome is caused by occlusion of the anterior spinal artery or its parent vertebral artery, resulting in the following signs:

• Ipsilateral paresis of the tongue (damage to cranial nerve XII), which deviates toward the lesion

• Contralateral hemiplegia (damage to corticospinal tract) with sparing of the face

• Contralateral loss of position and vibratory sensation (damage to medial lemniscus)

What is the consequence of occlusion of a dominant anterior spinal artery?

The central medullary area may be supplied by a single dominant anterior spinal artery. Occlusion of this vessel then leads to bilateral infarction of the medial medulla, resulting in quadriplegia (with face sparing), complete paralysis of the tongue, and complete loss of position and vibratory sensation. The patient will be mute although fully conscious.

What is the lateral medullary syndrome?

The lateral medullary (Wallenberg’s) syndrome is often due to vertebral artery or posterior inferior cerebellar artery occlusion. Vertebral artery dissection can also be a cause. Damage to the dorsolateral medulla and the inferior cerebellar peduncle results in the following signs:

• Ipsilateral loss of pain and temperature sensation of the face (damage to descending spinal tract and nucleus of cranial nerve V)

• Ipsilateral paralysis of palate, pharynx, and vocal cord (damage to nuclei or fibers of IX and X) with dysphagia and dysarthria

• Ipsilateral Horner’s syndrome (damage to descending sympathetic fibers)

• Ipsilateral ataxia and dysmetria (damage to inferior cerebellar peduncle and cerebellum)

• Contralateral loss of pain and temperature on the body (damage to spinothalamic tract)

• Vertigo, nausea, vomiting, and nystagmus (damage to vestibular nuclei)

• Other signs and symptoms may include hiccups, diplopia, or unilateral posterior headache. See Figure 9-4 .

  

What is the ventral pontine syndrome?

The ventral pontine (Millard–Gubler) syndrome is caused by paramedian infarction of the pons and results in the following signs:

• Ipsilateral paresis of the lateral rectus (damage to cranial nerve VI) with diplopia

• Ipsilateral paresis of the upper and lower face (damage to cranial nerve VII)

• Contralateral hemiplegia (damage to corticospinal tract) with sparing of the face

What is the lower dorsal pontine syndrome?

The lower dorsal pontine (Foville’s) syndrome is caused by lesions in the dorsal tegmentum of the lower pons, resulting in the following signs:

• Ipsilateral paresis of the whole face (damage to nucleus and fibers of VII)

• Ipsilateral horizontal gaze palsy (damage to paramedian pontine reticular formation and/or VI nucleus)

• Contralateral hemiplegia (damage to corticospinal tract) with sparing of the face

What is the upper dorsal pontine syndrome?

The upper dorsal pontine (Raymond–Cestan) syndrome is caused by obstruction of the long circumferential branches of the basilar artery and results in:

• Ipsilateral ataxia and coarse intention tremor (damage to the superior and middle cerebellar peduncles)

• Ipsilateral paralysis of muscles of mastication and sensory loss in face (damage to sensory and motor nuclei and tracts of V)

• Contralateral loss of all sensory modalities in the body (damage to medial lemniscus and spinothalamic tract)

• Contralateral hemiparesis of the face and body (damage to corticospinal tract) may occur with ventral extension of the lesion

• Horizontal gaze palsy may occur, as in the lower dorsal pontine syndrome

What is the ventral midbrain syndrome?

The ventral midbrain (Weber’s) syndrome is caused by occlusion of median and paramedian perforating branches and may result in:

• Ipsilateral oculomotor paresis, ptosis, and dilated pupil (damage to fascicle of cranial nerve III, including parasympathetic fibers)

• Contralateral hemiplegia, including the lower face (damage to corticospinal and corticobulbar tracts)

What is the dorsal midbrain syndrome?

The dorsal midbrain (Benedikt’s) syndrome results from a lesion in the midbrain tegmentum caused by occlusion of paramedian branches of the basilar or posterior cerebral arteries or both. Its signs are:

• Ipsilateral oculomotor paresis, ptosis, and dilated pupil (damage to fascicle of cranial nerve III, including parasympathetic fibers as in Weber’s syndrome).

• Contralateral involuntary movements, such as intention tremor, ataxia, and chorea (damage to red nucleus).

• Contralateral hemiparesis may be present if the lesion extends ventrally.

• Contralateral hemianesthesia may be present if the lesion extends laterally, affecting the spinothalamic tract and medial lemniscus.

What is the dorsolateral midbrain syndrome?

The dorsolateral midbrain syndrome is caused by infarction of the circumferential arteries and results in:

• Ipsilateral Horner’s syndrome (damage to sympathetic tract).

• Ipsilateral severe tremor that may be present at rest and grossly worsened by attempted movement (damage to superior cerebellar peduncle prior to crossing to the opposite red nucleus). Tremor and ataxia can be present bilaterally, if both the superior cerebellar peduncle and red nucleus are affected.

• Contralateral loss of all sensory modalities (damage to spinothalamic tract and medial lemniscus that now ascend together).

What are the symptoms of brain stem transient ischemic attacks?

Transient circulatory insufficiency in the vertebrobasilar distribution causes brief episodes of brain stem dysfunction characterized by a more patchy and variable presentation. The symptoms of the recurrent attacks may be identical or varying in detail. In basilar artery disease, each side of the body may be affected alternately. All of the structures in the same ischemic distribution may be affected simultaneously, or symptoms of brain stem dysfunction may spread from one region to another. The symptoms may then end abruptly or fade gradually. They are often premonitory symptoms of impending brain stem strokes that may result in devastating consequences.

Transient brain stem ischemic attacks affecting the medulla occur particularly often. Vertigo, dysarthria, dysphagia, and tingling around the mouth suggest dysfunction in this region. At pontine levels, frequent symptoms are vertigo; imbalance; hearing abnormalities; tingling, numbness, or weakness of the limbs; and diplopia. Midbrain ischemia may cause diplopia, ataxia, sudden loss of consciousness, and weakness of limbs. Symptoms of brain stem ischemia are usually multiple, and isolated findings (such as vertigo or diplopia) are more often caused by peripheral lesions affecting individual cranial nerves.

What is the “top of the basilar” syndrome?

Occlusion of the rostral basilar artery, usually embolic, often results in the “top of the basilar” syndrome caused by infarction of the midbrain, thalamus, and portions of the temporal and occipital lobes. This syndrome should be suspected in a patient with sudden onset of unresponsiveness, confusion, amnesia, abnormal eye movement, and visual defect. The neurologic signs may be variable, but the most common include:

• Impairments of ocular movements —unilateral or bilateral vertical (upgaze, downgaze, or complete) gaze palsy, skew deviation, hyperconvergence or convergence spasms causing pseudo-VI nerve palsy, convergence-retraction nystagmus, and retraction of the upper eyelids.

• Abnormalities in pupils —small with incomplete light reactivity (diencephalic dysfunction), large or midposition and fixed (midbrain dysfunction), ectopic pupils (corectopia), oval pupils.

• Alterations of consciousness and behavior —stupor, somnolence, apathy, lack of attention, memory deficits, agitated delirium.

• Defects in vision —homonymous hemianopsia, cortical blindness, Balint’s syndrome (impaired visual form discrimination and color dysnomia), and abnormal color vision.

• Motor weakness, sensory deficits, and reflex abnormalities are usually variable and subtle and due to the involvement of long tracts at the infarcted region.

This syndrome may be reversible in patients who are younger and do not have significant risks for cerebrovascular disease.

What is the locked-in syndrome?

The locked-in syndrome occurs in patients with bilateral ventral pontine lesions. Its most common cause is pontine infarction. Other common causes include pontine hemorrhage, trauma, central pontine myelinolysis, tumor, and encephalitis. The patient is quadriplegic because of bilateral damage to the corticospinal tracts in the ventral pons. He or she is unable to speak and incapable of facial movement because of involvement of the corticobulbar tracts. Horizontal eye movements are also limited by the bilateral involvement of the nuclei and fibers of cranial nerve VI. Consciousness is preserved because the reticular formation is not damaged. The patient has intact vertical eye movements and blinking because the supranuclear ocular motor pathways that run dorsally are spared. The patient is able to communicate by movement of the eyelids, but otherwise is completely immobile. Sometimes an incomplete state of this syndrome may occur when the patient retains some horizontal gaze and facial movement. The locked-in syndrome must be distinguished from the persistent neurovegetative state (such as coma vigil or akinetic mutism), in which the patient appears awake but does not react to environmental stimuli and is unable to communicate in any form (thought to be due to a lesion in the rostral midbrain, basal–medial frontal region, or limbic lobes).

What are the common causes of brain stem hemorrhage?

Pontine hemorrhage is usually caused by uncontrolled systemic hypertension, resulting in a sudden loss of consciousness, quadriparesis, and pinpoint pupils. Progressive central herniation from supratentorial mass lesions can compress the brain stem and cause hemorrhage in the midline of the midbrain (Duret hemorrhage), producing coma and bilateral large and fixed pupils. Diencephalic bleeding, such as thalamic hemorrhage, can dissect into the cerebral peduncles and midbrain, producing acute severe headache, hemiparesis, and III nerve palsy. Small petechial hemorrhages occur in the brain stem of patients with head injuries, blood dyscrasias, or hemorrhagic disorders. Ruptured aneurysms or arteriovenous malformations of the vertebrobasilar system may result in subarachnoid hemorrhage that injures the brain stem.