Management of Chiari Malformations and Syringomyelia

Definition and History

In 1891, Hans Chiari described three types of cerebellar malformations that were associated with hydrocephalus. ^, He added a fourth type 5 years later. ^, The type I Chiari malformation was described as herniation restricted to the tonsils and adjacent cerebellum, which was associated with elongation of these structures into a conical shape. The cerebellar tonsils were atrophic and attached to the dorsal medulla by fibrous adhesions. Chiari malformation type II included protrusion of the medulla, fourth ventricle, cerebellar vermis, and cerebellar tonsils into the spinal canal. The insertion of the tentorium was low, the tentorial hiatus widened, and posterior fossa small in these patients. This malformation was present at birth and arose in association with myelomeningocele and hydrocephalus. Chiari III malformation consisted of a rare deformity in which the cerebellum herniated through a defect in the suboccipital bone or upper cervical lamina and created an occipitocervical encephalocele. Beaking of the tectum, elongation and kinking of the brainstem, hydrocephalus, and lumbar spina bifida were also present. This condition resulted in severe neurologic deficits and early mortality. Chiari type IV malformation was extremely rare and was characterized by severe cerebellar hypoplasia, but with the cerebellum and brainstem remaining within the posterior fossa. Mortality in infancy is the rule with this malformation.

In recent years other authors have expanded the Chiari classification system to include other types of lesions involving the hindbrain and cerebrospinal fluid (CSF) pathways at the foramen magnum. The “Chiari 0 malformation” is a condition in which the obex, but not the cerebellar tonsils, is inferiorly located and a syrinx is found within the cervical segments of the spinal cord in an identical anatomic location as syringomyelia associated with Chiari I malformation. In this condition a membrane covers the foramen of Magendie or bridges the subarachnoid space, and the volume of the posterior fossa is smaller than normal. ^, The “Chiari 1.5 malformation” is a condition in which cerebellar ectopia is restricted to the cerebellar tonsils, as in Chiari I malformation, but contrary to Chiari’s original description of the Chiari I malformation, the brainstem is also caudally displaced. The term Chiari V malformation was also proposed to describe extremely rare cases wherein the occipital lobe herniates through the foramen magnum.

In syringomyelia, a cyst forms within the spinal cord and produces myelopathy. The word syrinx is derived from the Greek word for “reed or pipe,” which in classical mythology is the form that the nymph Syrinx assumed to escape pursuit from the Greek god Pan. Surgical treatment for syringomyelia was initiated by Abbe and Coley, who performed a syringostomy in 1892. In 1938, based on intraoperative and postmortem observations, the Chiari I malformation was first described in adults and in a patient without hydrocephalus. ^,

In 1950, James Gardner and associates at the Cleveland Clinic recognized the association of the Chiari I malformation with syringomyelia. They postulated that the outlets of the fourth ventricle were occluded by the Chiari I malformation and that a water-hammer pulsation was directed from the fourth ventricle, through the obex, and into the central canal of the spinal cord, leading to pulsatile expansion of the central canal to form a syrinx. To reverse this process, Gardner performed a surgical procedure that removed the bone from the posterior aspect of the foramen magnum, opened the fourth ventricle to the subarachnoid space, and plugged the obex.

In the 1970s, Logue introduced a less-invasive alternative to Gardner’s procedure. His procedure consisted of simple bony decompression, expansion of the dura with a tissue graft, and avoidance of opening of the arachnoid membrane and entrance into the subarachnoid space or fourth ventricle. His group performed a clinical study comparing Gardner’s procedure with their procedure of simple decompression and duraplasty and demonstrated that there was no difference in syrinx resolution between the procedures, although Gardner’s operation resulted in a higher complication rate. ^, Since then, some investigators have advocated a decompressive procedure that opens the arachnoid membrane, removes or shrinks the inferior portion of the cerebellar tonsils, and attempts to enlarge the CSF pathways beyond what is achievable with bony decompression and duraplasty alone. Syringomyelia resolves following this latter procedure in about 80% of cases, which is similar to the results reported for simple decompression and duraplasty with preservation of the arachnoid membrane and tonsils ( Table 167.1 ).

Development

In patients with the Chiari I malformation, the bones of the skull base are often underdeveloped, which results in reduced volume of the posterior fossa. It was thought that Chiari I malformations result not from a primary cerebellar anomaly but rather from a smaller-than-normal posterior fossa. Because the volume is insufficient to contain the entire cerebellum, the cerebellar tonsils are displaced into the cervical spinal canal. Tonsillar impaction into the foramen magnum has been demonstrated to occur in a pulsatile fashion during the systolic portion of the cardiac cycle, leading to the abnormal appearance of the tonsils and brainstem. Recent studies have demonstrated that a disproportionately small posterior fossa is not present in many patients with a Chiari I malformation and have dichotomized patients into those with a “crowded” or “spacious” posterior fossa based on volumetric analysis. Thus crowding of the posterior fossa alone cannot explain the pathogenesis of Chiari I malformation in many patients, suggesting another mechanism leading to the tonsillar impaction in the foramen magnum in these patients. Genetic disorders and diseases that affect skull development and reduce intracranial volume can result in the development of secondary Chiari I malformation. However, most patients with Chiari I malformation do not have an associated disease to explain the development of this condition. In them, environmental and genetic factors presumably contribute to underdevelopment of the posterior fossa and development of the Chiari I malformation.

Chiari type II malformation is associated with myelomeningocele and primary and secondary brain anomalies. Cerebellar gliosis and atrophy occur, along with distortion and hypoplasia of cranial nerve, olivary, and pontine nuclei. Cerebral findings include focal cortical dysplasia, gray heterotopias in the hemispheric white matter and subependymal zone of the lateral ventricles, and thickening of the massa intermedia. The posterior fossa in Chiari II malformation is even smaller than that in Chiari I malformation. Over the years, multiple theories have aimed to explain the pathoembryologic etiology and anatomic findings of Chiari II malformations. Briefly summarized, these theories include the following: (1) a traction theory positing that, as a result of tethering, the caudal spinal cord may pull the cerebellum and medulla into the lower cervical canal; (2) a theory suggesting inadequate posterior fossa size resulting from mesodermal insufficiency and overgrowth of the neuroepithelium, causing a neural tube defect; (3) developmental arrest with associated dysgenesis of the hindbrain; and (4) a theory speculating that herniation of the of the posterior fossa contents is the result of supratentorial hydrocephalus with leakage of CSF into the amnion and herniation of the hindbrain. Although no theory fully accounts for all features of Chiari II malformations, the unified theory is perhaps the most widely accepted. This theory suggests that a neural tube that is open at the caudal end allows leakage or venting of CSF from the central nervous system (CNS) of the developing embryo or fetus. This, in turn, leads to a lack of distention of the caudally located rhombencephalic and mesencephalic embryonic vesicles, resulting in mesenchymal abnormalities and a small posterior fossa. Subsequently, as the cerebellum develops, the small posterior fossa is insufficient to contain the enlarging hindbrain and herniation ensues both cranially and caudally. It is also theorized that the loss of CSF may lead to a lack of distention of the ventricular system. Lateral ventricular collapse disrupts neuronal migration from the ventricular germinal zone during corticogenesis, resulting in cortical defects such as heterotopias and polymicrogyria. Other more complex genetic and environmental factors are also likely to play a role in the development of Chiari II malformations and require further elucidation to account for additional features of the condition. The unified theory is further supported by the results of a randomized controlled trial comparing the results of prenatal versus postnatal repair of myelomeningocele, which demonstrated a significantly improved degree of hindbrain herniation associated with the Chiari II malformation with prenatal surgical intervention. Drainage of CSF through the myelomeningocele in utero encourages herniation of the posterior fossa structures through the foramen magnum. Following repair of the myelomeningocele, obstruction of the CSF pathways in the basilar cisterns, and often at the cerebral aqueduct, prevents normal CSF flow, which results in hydrocephalus. Ventricular shunting effectively treats hydrocephalus in these patients. Symptomatic syringomyelia can subsequently arise years later in the context of shunt malfunction, spinal cord tethering, or compression of CSF pathways by the malformation at the foramen magnum. Treatment for syringomyelia is directed toward its etiology and includes restoring CSF drainage by shunt revision or craniocervical decompression, thus relieving tension on the spinal cord by spinal cord untethering, or draining the syrinx directly using a syringopleural or syringoperitoneal shunt. Medullary compression can develop and requires prompt craniocervical decompression.

Syringomyelia occurs as a consequence of another abnormal process. There is general agreement that in syringomyelia the CSF pathways are encroached upon by an underlying condition. Because syrinx fluid is identical in chemical composition to CSF, syrinx formation likely results from a process that increases the movement of CSF into the spinal cord. Autopsy and radiographic studies rarely demonstrate a patent central canal in adult patients with syringomyelia ; therefore it appears that syringomyelia does not develop with expansion of the central canal of the spinal cord by CSF transmitted from the fourth ventricle (Gardner’s water hammer theory).

Another mechanism of syrinx formation and progression has been proposed that does not require a patent central canal. This mechanism is initiated by the Chiari malformation partially obstructing CSF pathways at the foramen magnum, which prevents the normally rapid efflux and influx of CSF between the head and the spine that compensates for brain expansion and contraction during the cardiac cycle. In lieu of CSF, the cerebellar tonsils are displaced during the cardiac cycle, creating a piston effect on the partially enclosed spinal subarachnoid space and produces enlarged cervical subarachnoid pressure waves; these compress the spinal cord from without, direct CSF into the spinal cord, and cause pulsatile syrinx flow, which leads to syrinx progression ( Figs. 167.1 and 167.2 ). A prospective study of patients with Chiari I malformation and syringomyelia who were evaluated before, during, and after surgery provided radiographic and physiologic findings consistent with this mechanism.

Improved understanding of the pathophysiology of syringomyelia has encouraged the design and implementation of procedures directed toward eliminating the obstruction of CSF pathways—that is, strategies designed to reverse the pathophysiologic process underlying syringomyelia. In the case of Chiari I malformation and basilar invagination, procedures that effectively open the CSF pathways at the foramen magnum or in the spinal canal (in the setting of posttraumatic and postinflammatory syringomyelia) provide effective and lasting treatment of syringomyelia with low morbidity ( Figs. 167.3 and 167.4 ). Since the 1990s, the use of syrinx shunts for the treatment of syringomyelia has declined to the extent that shunts are now used only as a last resort.

Clinical Characteristics

In many patients, Chiari I malformation is diagnosed on the basis of nonneurologic symptoms such as suboccipital and cough headache; headache and neck pain are typical symptoms in young children. Neurologic signs and symptoms arising from the cerebellum, medulla, or central spinal cord are often found in patients with Chiari I malformation, either from direct compression of the cerebellum or medulla at the foramen magnum or from syringomyelia or syringobulbia. Lateral or downbeat nystagmus, ataxia, and reduced gag reflex are neurologic findings consistent with Chiari I malformation. In cases where neither neurologic findings nor typical symptoms occur, the clinical significance of a mild degree of tonsillar ectopia cannot be determined. Generalized headache, chronic fatigue, and similar symptoms usually have etiologies other than Chiari I malformation. Thus it is important not to overinterpret the magnetic resonance imaging (MRI) finding of a mild degree of tonsillar ectopia in the absence of syringomyelia.

The prevalence of syringomyelia has not been clearly established; one estimate is that symptomatic syringomyelia occurs in about 8.4 cases per 100,000 population. The Chiari I malformation is present in 70% of patients with syringomyelia. Basilar invagination causes 10% of cases of syringomyelia. Syringomyelia associated with conditions of the spine below the craniocervical junction is known as primary spinal syringomyelia and accounts for about 16% of all cases. This type of syringomyelia may be posttraumatic, postinflammatory, or compressive. ^, Patients with posttraumatic syringomyelia present several months or years after the initial trauma. ^, The incidence of syringomyelia following trauma that produces paraplegia was estimated to be 1% to 4% in the pre-MRI era, when it was difficult to detect and could not be detected with noninvasive diagnostic techniques. Since the availability of MRI, however, the incidence has been found to be much higher.

Postinflammatory syringomyelia results from a delayed reaction to chronic meningitis, either infectious or chemical. Syringomyelia also occurs in association with compression of the CSF pathways by extramedullary tumors or cysts, osteophytes, or herniated intervertebral discs. Intramedullary spinal cord tumors cause about 4% of the cases of syringomyelia and can do so without narrowing the CSF pathways. Syrinx fluid in primary spinal syringomyelia is identical to CSF, whereas syrinx fluid in syringomyelia associated with an intramedullary tumor is highly proteinaceous. ^{, ,}

Patients with syringomyelia present with symptoms of paralysis, sensory loss, and chronic pain, which most commonly develop during the second through the fifth decades of life. The natural history of syringomyelia is typically one of gradual, stepwise neurologic deterioration over many years. Although syringomyelia is uncommon, typical signs and symptoms often suggest its diagnosis. The cervical segments of the spinal cord are affected first in syringomyelia associated with the Chiari I malformation, resulting in upper extremity symptoms of pain, weakness, atrophy, and loss of pain and temperature sensation. Early in the course of the disease, the signs and symptoms may be mild and confined to a restricted area of the body. As the syrinx displaces and destroys the central gray matter of the spinal cord, the upper extremities become weak, atrophic, hyporeflexic, and devoid of normal pain and temperature sensation (see Fig. 167.2 ). Sensory loss is considered to be disassociated (because the sensation of pain and temperature is lost, whereas light touch is preserved) and suspended (because the sensory loss hangs between regions of normal sensation). Both sides of the body are usually affected, but asymmetric extension of the syrinx to one side of the spinal cord results in more severe involvement of the upper extremity on that side.

If this condition is left untreated, the upper extremity dysfunction progresses over months to years and is eventually accompanied by spasticity in the lower extremities as the syrinx expands outside the gray matter and into the corticospinal tracts. In less advanced cases, the diagnosis of Chiari I malformation and syringomyelia is often made by MRI in the course of evaluation for another, more common suspected condition, such as a herniated cervical disc, cervical spondylosis, or scoliosis.

In untreated patients with Chiari I malformation and syringomyelia, the pace of neurologic deterioration is more rapid initially and slows after the signs have become well established. This time course is consistent with an incidence of neurologic deterioration of 10% to 24% per year in studies in the MRI era, in which deterioration is measured early in the disease, compared with 2% to 3% per year in the pre-MRI era, ^, when deterioration was measured later in the disease and after definite central myelopathy had developed.

Primary spinal syringomyelia is associated with lesions within the spinal subarachnoid space or with deformity of the spinal canal and is often recognized months or years after spinal trauma or meningitis. It manifests as ascending loss of motor and sensory function, which usually signals involvement of the spinal cord above the original level of injury.