Posterior Fossa Anomalies

Pathogenesis and Pathophysiology. The development of the brain stem and cerebellum has been extensively investigated at the anatomical, cellular, and molecular levels.y The cerebellum is entirely derived from the rhombic lip, a dorsal ridge of the developing neural tube located at the junction between the midbrain and hindbrain (rhombencephalon). Proliferation and migration at the rhombic lip results in caudal growth of the cerebellar anlagen, with the cerebellum forming the roof of the fourth ventricle. Aplasia of the cerebellum is an extremely rare condition, and most such cases are believed to represent secondary disruptions of normal development, primarily on a vascular basis.

In the 1890s, Chiari described a group of malformations of the posterior fossa, each characterized by a displacement of the cerebellum. y Chiari II malformations, also known as the Arnold-Chiari malformation, have been described previously in this chapter (see the discussion of neural tube defects). Chiari I malformations are distinguished from Chiari II malformations by the displacement of the cerebellar tonsils (as opposed to the vermis in cases of the Chiari II malformation) over the cervical spinal cord (see Fig 28-2 ). The Chiari I malformation is occasionally equated with herniation syndromes involving pressure coning and necrosis of the cerebellar tonsils. These two entities should remain distinct, since Chiari I represents a malformation and the other entity a result of increased pressure within the posterior fossa. Occasionally, hydromyelia or syringomyelia may be associated with the Chiari I malformation, as with the Chiari II malformation. However, myelomeningoceles are not associated with Chiari I malformations.

The rare Chiari III malformation represents a displacement of the cerebellum into an occipital encephalocele. Occipital encephaloceles can involve either the occipital lobes of the cerebral hemispheres or the posterior fossa elements. As mentioned previously, encephaloceles involving the posterior fossa elements, and the cerebellum in particular, frequently carry a poor prognosis.

The Dandy-Walker malformation, by the strictest definition, includes a dilated fourth ventricle, an anteriorly rotated cerebellar vermis that is partially or completely absent, and an enlarged posterior fossa with an elevated tentorial insertion. y The Dandy-Walker malformation is not a true cyst of the fourth ventricle but a cystlike dilation of that ventricle (see Fig.:...,.28:2 ).

Absence or partial absence of the cerebellar vermis is also a characteristic of Joubert's syndrome. This syndrome can be distinguished neuropathologically from the Dandy- Walker malformation by the absence of a cystically dilated fourth ventricle. Relatively few brains of patients who have been clinically diagnosed with Joubert's syndrome have been studied pathologically. One problem related to understanding the pathological findings in this syndrome is the lack of a clearly defined clinical spectrum. Patients described

as having Joubert's syndrome show a broad spectrum of clinical findings (see later), including variation between families, suggesting the possibility of more than one etiology.

Focal deficits primarily involving the sixth and seventh cranial nerves, whether unilateral or bilateral, have been variably classified as Mobius' syndrome. Although disagreements exist as to the definition of this syndrome, at the very least, bilateral deficits of the sixth and seventh cranial nerves justify the designation of Mobius' syndrome. Whether unilateral deficits involving one or multiple cranial nerves should be designated Mobius' syndrome as well remains controversial. Although very few cases have been studied pathologically, most data agree that destructive lesions, frequently assumed to be vascular in origin, are the basis for most cases of Mobius' syndrome. Whether congenital absence of cranial nerve nuclei exists as a distinct entity is likely but unknown.

Several clinical syndromes are believed to result from a form of miswiring in the nervous system that is derived from the establishment of aberrant neural connections. Duane's syndrome is one form and is discussed later. Another example of this phenomenon is the Marcus Gunn syndrome, in which patients present with activation of the levator palpebrae upon use of the muscles of mastication, such as suckling in an infant. Often there is congenital ptosis on the affected side. This syndrome is believed to result from misdirected neuronal connections between the third, fifth, and other cranial nerves.

Epidemiology and Risk Factors. Given the diverse group of anomalies that afflict the posterior fossa, it is impossible to give a concise estimate for the incidence or prevalence of this group of disorders. Some of the anomalies are found in almost constant association with other malformations. For example, the Chiari II malformation (discussed earlier with myelomeningoceles), is almost uniformly associated with lumbosacral spinal neural tube closure defects and thus the incidence parallels the incidence of myelomeningoceles. Other anomalies, like the Dandy- Walker malformation, may be found either as an isolated malformation, in conjunction with other malformations (e.g., cardiovascular), or as part of a syndrome (e.g., Meckel-Gruber syndrome and WWS). Risk factors also vary depending on the malformation in question. Mobius' syndrome, for example, is found more frequently among children born with congenital heart malformations than in isolation. Dandy-Walker malformations have been associated with maternal exposure to retinoic acid and warfarin. A complete discussion of the risk factors and associated syndromes involving the posterior fossa has been compiled. y

Clinical Features and Associated Disorders. The cerebellum may be diffusely small (hypoplasia) or absent (aplasia). These conditions are relatively rare, especially cerebellar aplasia. Cerebellar hypoplasia and aplasia may present with any combination of hypotonia, motor delays, cognitive delays, ataxia, head and truncal titubations, irregularities of speech rhythm, and nystagmus. However, clinically asymptomatic cases of cerebellar hypoplasia or even aplasia are well documented in the older literature.

The Dandy-Walker malformation is clinically heterogeneous, as alluded to earlier. Some cases exhibit other CNS abnormalities, such as agenesis of the corpus callosum, brain stem anomalies, and migrational abnormalities. Mental retardation is also variable and frequently correlates with the presence of associate anomalies. Patients without associated anomalies may have relatively normal intelligence and development. y Therefore, the evaluation of an individual with the Dandy-Walker malformation requires a careful search for such associated abnormalities. Ataxia and brain stem dysfunction are often the presenting features and are believed to result from local pressure effects on the brain stem and cerebellum. Owing to pressure fluctuations in the posterior fossa, these symptoms can occur intermittently. The initial presentation may occur as late as adulthood, with headache and progressive neurological deterioration. Although hydrocephalus is often included in the definition of the Dandy-Walker malformation, hydrocephalus is often not present at birth and may present in the first year of life as a secondary event. Although occasionally clear etiologies for the hydrocephalus, such as aqueductal stenosis, are identified, most cases show no clear obstruction of CSF flow. Lack of patency of the foramen of Magendie and the foramina of Lushka, once considered the pathogenesis of the malformation, is rare and unlikely to be the etiology of the hydrocephalus in most cases.

Joubert's syndrome is an autosomal recessive condition that presents with abnormalities of respiratory rate control in infancy (particularly episodic tachypnea and also respiratory pauses), rhythmical tongue protrusion, abnormal eye movements, ataxia, and mental retardation. y , y Structural eye abnormalities such as chorioretinal colobomata have been described. The degree of developmental delay can be variable and is not necessarily as severe as initially thought.

The clinical spectrum associated with the Chiari I malformation has broadened in recent years with the increasing use of MRI, the imaging modality of choice for the diagnosis of this condition. As opposed to the Chiari II malformation (see the previous discussion of neural tube closure defects), the Chiari I malformation usually becomes symptomatic in teenage to early adult years, although the initial presentation can also be in an infant or older adult. The initial complaint is often neck pain, at times presenting with torticollis or retrocollis. If cervical spinal cord syringomyelia co-exists, symmetrical, asymmetrical, or even totally unilateral arm pain can be the initial complaint. Neurological symptoms and signs also are dependent on the neural structures involved. Predominant impairment of the brain stem and cerebellar tonsils by either compression at the level of the foramen magnum or by extension of the syrinx into the brain stem (syringobulbia) can lead to a variety of brain stem and cerebellar signs. In the very young age group, sleep apnea, stridor, and feeding difficulties may be the presenting symptoms. Eye movement abnormalities are relatively common. Oscillopsia is reported by the patient, and a variety of forms of nystagmus can be seen. Downbeating nystagmus, especially when accentuated by lateral downgaze, should alert the examiner to a potential problem at the craniocervical/pontomedullary junction. Torsional nystagmus may also be present, especially in the presence of syringomyelia or syringobulbia. Lower cranial nerve impairment can lead to dysphagia and aspiration, which may first present as late as the eighth decade. When complicated

by syringobulbia, asymmetrical cranial nerve involvement up to cranial nerve V may be present. Imbalance and vertigo with truncal ataxia may indicate impairment of the vestibulocerebellar and spinocerebellar systems. With the co-existence of cervical hydrosyringomyelia, this picture becomes even more complicated. Forty to seventy-five percent of cases of Chiari I malformation have an associated syringomyelia, depending on the series and method of ascertainment. y When the patient is presenting with symptoms referable to the spinal cord, syringomyelia can be demonstrated in over 90 percent of cases. The most common location of maximal expansion is C4 to C6, though the cavity can expand up and down the entire spinal cord. The cavity often is centrally or slightly asymmetrically located, thus first impairing crossing fibers in the anterior spinal commissure. Because these fibers carry sensory information predominantly for pain and temperature, and the posterior columns can be relatively spared, a dissociated sensory loss may result; pain and temperature sensation may be disturbed in a capelike distribution over the shoulders and down the arms, while light touch and joint position remain relatively unaffected. Compromise of the anterior horn cells may result in patchy weakness, amyotrophy, and loss of reflexes in the arms. These findings are often asymmetrical or even unilateral. With expansion of the cavity, additional symptoms, including long tract symptoms, can become apparent. Another important manifestation of syringomyelia, especially in childhood, is progressive scoliosis, sometimes the earliest sign of a syringomyelia. Skeletal anomalies of the skull base can be associated with the Chiari I malformation as well, most frequently basilar impression (see later). Other skeletal anomalies of the craniocervical junction associated with the Chiari I malformation include the Klippel-Feil anomaly and atlanto-occipital assimilation.

In contrast to these cerebellar syndromes, the brain stem can also be the primary area affected. In its broadest definition, Mobius' syndrome refers to a combination of bilateral symmetrical and asymmetrical facial weakness associated with abnormalities of horizontal gaze, most often deficiencies of abduction. Other cranial nerve functions can be impaired as well, especially those of cranial nerves IX, XII, V, and occasionally, III. The impairment of lower cranial nerves can lead to early feeding difficulties, problems with swallowing, and atrophy of the tongue that is frequently unilateral. Neurosensory deafness may also be associated. A variety of somatic anomalies may be present in patients with Mobius' syndrome.y At least 50 percent of patients have limb anomalies. Thirty percent have talipes, and 20 percent exhibit hypoplasia of digits, transverse terminal defects, or syndactyly. The Poland anomaly and the Klippel-Feil anomaly can also be associated with the condition. Concurrent congenital heart disease, urinary tract anomalies, and hypogonadism or hypogenitalism are less frequent. Mild mental retardation is reported in 10 to 15 percent of patients with Mobius' syndrome; however, psychomotor development can also be slowed because of communication and social interaction problems resulting from the cranial nerve palsies. Most cases of Mobius' syndrome appear to occur sporadically and may in some cases indicate a disruptive sequence. The association with Poland anomaly is sometimes referred to as Poland-Mobius syndrome and may be familial.

Another syndrome involving cranial nerve nuclei is Duane's syndrome. This syndrome consists primarily of limited abduction with widening of the palpebral fissure on attempted abduction and globe retraction on attempted adduction. It is due to deficient abducens innervation to the lateral rectus with compensatory innervation by the third nerve. Again, most of the cases that have been described have been sporadic; however, familial occurrences have been seen in the context of a number of anomalies such as limb anomalies, eye anomalies, and deafness.^ The combination of Duane's anomaly, Klippel-Feil anomaly, and perceptive deafness has been referred to as Wildervanck (cervico-oculo-acoustic) syndrome. It is of uncertain inheritance. The Marcus Gunn's (jaw-winking) syndrome consists of unilateral ptosis, with elevation of the ptotic lid to a position higher than the opposite side on opening of the mouth, especially with deviation of the mouth to the opposite side. There are a few families with a suggestion of a dominant mode of inheritance with incomplete penetrance.

Differential Diagnosis. Cases of congenital cerebellar hypoplasia have been reported as autosomal recessive and probably also with X-linked inheritance. y Cerebellar hypoplasia and pontocerebellar hypoplasia have been associated with a number of other syndromes. y Together, these findings indicate that cerebellar hypoplasia is genetically heterogeneous. A recently recognized group of metabolic disorders of protein glycosylation (carbohydrate-deficient- glycoprotein syndromes) can also present with a picture of neonatal olivo-ponto-cerebellar atrophy. y Prenatal cytomegalovirus infection also needs to be investigated in infants with congenital cerebellar hypoplasia or aplasia.

Because the Dandy-Walker malformation is a feature of a considerable number of syndromes, y evaluation of the internal organs, particularly the heart and kidneys, as well as a skeletal and muscular assessments is essential once a Dandy-Walker malformation is recognized. Differential diagnostic considerations have to include conditions that can cause cystlike expansions in the posterior fossa. These include arachnoid cysts, mega-cisterna magna, and cystic tumors. MRI is the imaging modality of choice given the problems of visualizing the posterior fossa by CT scanning. True cysts, like arachnoid or ependymal cysts, in the posterior fossa may mimic a Dandy-Walker malformation. However, in contrast with the enlarged fourth ventricle associated with Dandy-Walker malformations, the fourth ventricle is compressed in the presence of a true cyst.

In Joubert's syndrome, there is no cystic dilatation in the posterior fossa, so that the cerebellar hemispheres appear to lie closely opposed without an intervening vermis. A similar cerebellar picture of agenesis of the cerebellar vermis without a posterior fossa cyst has been seen in several other syndromes, including the oro-facio-digital syndrome type VI of Papp-Varadi, as well as other syndromes with vermal hypoplasia and agenesis. y Patients with oro-facio- digital syndrome type VI of Papp-Varadi often have an unusual V-shaped partially duplicated metacarpal on hand radiographs and sometimes hamartomatous nodules under the tongue can be found. An unequivocal distinction between these syndromes is not always possible.

The differential diagnosis in the Chiari I malformation

with syringomyelia is broad given the protean nature of the possible clinical symptoms. Multiple sclerosis, spinal muscular atrophy, amyotrophic lateral sclerosis, spinocerebellar ataxias, mononeuropathy multiplex, cervical disc and degenerative disease, and a variety of other disorders of the spinal cord and cerebellum can be confused with the condition on clinical grounds. Imaging resolves the majority of these differential diagnostic points. The situation can be more difficult in patients with prominent neurological symptoms but only borderline abnormal findings on imaging. Alternative diagnoses have to be considered seriously in that scenario. A cavity in the spinal cord can also have a traumatic (hematomyelia), inflammatory (necrotizing myelopathy), metabolic (Leigh's disease), or neoplastic (astrocytoma and ependymoma) basis. Of the neurocutaneous disorders, neurofibromatosis type 1 and von Hippel- Lindau disease can be associated with a syrinx. Demonstration of a flow void within the cavity by MRI can be helpful to show continuity with the CSF spaces.

Mobius' syndrome must be differentiated from neuromuscular diseases with a prominent component of facial weakness, especially those with involvement of the extraocular muscles. Centronuclear myopathy in particular should always be considered because of the congenital presentation in the X-linked form. Mitochondrial myopathies, myasthenic syndromes, and, given facial involvement alone, myotonic dystrophy and facioscapulohumeral dystrophy as well as some of the structural congenital myopathies need to be considered. In cases in which the time of onset symptoms is unclear, a multitude of acquired causes for multiple cranial nerve palsies has to be considered. These causes may include trauma to the skull base and cranial nerves; infection of the meninges, skull base, and cavernous sinus; inflammation of the nerves, brain stem, or cavernous sinus; and tumors of the skull base and meningeal neoplastic spread.

Moaubius' syndrome is distinguished from Duane's syndrome, in most cases, by the sparing of the face and lower cranial nerve nuclei in patients with Duane's syndrome. Duane's syndrome itself needs to be differentiated from other causes of abducens palsy in childhood, in which the elaborate co-innervatory phenomena of Duane's syndrome usually are not seen. More bizarre associated eye movement abnormalities can be seen in Duane's syndrome and may confuse the picture. The globe retraction can sometimes be difficult to appreciate, especially in infants.

Evaluation. Imaging the posterior fossa is best done with MRI. The relationship between the cerebellar vermis and fourth ventricle and cisterna magna needs particular attention. Diagnosis of cerebellar and or pontine hypoplasias can be difficult in mild cases and, strictly speaking, requires age-matched controls. As mentioned earlier, for any unusual posterior fossa anomaly, screening for additional malformations may lead to a syndrome diagnosis. Ultrasound of the heart and internal organs, a skeletal radiographic survey, and an ophthalmological evaluation are recommended. In cases of young patients with Joubert's syndrome, an overnight sleep study may alert the physician to respiratory irregularities and apnea spells. In cases of hydrocephalus associated with the Dandy-Walker malformation, evaluation should follow the guidelines outlined in the section on hydrocephalus.

Diagnosis of the Chiari I malformation as well as of syringomyelia relies on clinical suspicion as well as on adequate imaging. As mentioned earlier, the imaging modality of choice is MRI, especially sagittal images. The now routine inclusion of these images in any MRI study of the head has led to the discovery of a number of asymptomatic Chiari I malformations and asymptomatic cases of syringomyelia. It has also become apparent that the clinical picture is broader and more varied then the classic clinical syndromes described. The question frequently arises about the significance of low-lying tonsils that are discovered incidentially on a cranial MRI examination. The tonsils normally retract upward with age, so their location must be interpreted in an age-dependent context. The cut-off in the first 10 years of life appears to be 6 mm below the level of the foramen magnum; from 10 to 30 years, it would be 5 mm; and after that, 4 mm. Herniations greater than 12 mm are almost invariably symptomatic. On the other hand, about 30 percent of individuals with significant displacements of 5 to 10 mm are asymptomatic. y Similarly, syrinx cavities can be large with minimal clinical symptoms.

A patient with one of the brainstem syndromes needs to be evaluated in an effort to rule out disorders that may mimic the syndrome (see differential diagnosis), and also to define the extent of the abnormalities and establish the presence of any associated malformations. In patients with Mobius' syndrome, this may require (in addition to a careful neurological and ophthalmological examination) brain stem auditory evoked responses; creatine-kinase determination; imaging of the brain, posterior fossa, and skull base by MRI; a skeletal radiographic survey; and echocardiography.

Management and Prognosis. Treatment of the Chiari I complex consists mainly of decompressing the structures trapped in the foramen magnum. Surgical approaches for pediatric^ and adults patients are comparable. Most often, a suboccipital craniectomy, with or without dural patch grafting, and cervical laminectomies are performed, but anterior approaches may become necessary in selected cases to achieve satisfactory decompression. Occasionally, very large syrinx cavities require drainage by fenestration or shunting into the subarachnoid space. In general, the decompressive surgery at the foramen magnum is the more successful of the two,^1 and co-existing syrinx cavities may even shrink or resolve postoperatively without further intervention. Signs and symptoms referable to brain stem compression respond better to surgery then symptoms related to the spinal cord, even when syrinx decompression is included in the operation. Major pre-existing sensory or motor deficits are poor prognosticators for functional recovery. y

Treatment of the Dandy-Walker malformation is also treated surgically by shunting of the hydrocephalus and the posterior fossa cyst via a double ventriculoperitoneal and cystoperitoneal shunt.y This intervention may allow for normal mental development in the absence of additional CNS malformations.

The respiratory abnormalities associated with Joubert's syndrome usually improve after infancy. If significant and life-threatening apneic periods occur, home ventilation may be used successfully during the initial high-risk period. In

case of severe feeding difficulties, the placement of a gastric tube may be appropriate.

Although disabilities resulting from the Mobius' syndrome can initially be very significant, including feeding difficulties and speech impairments, these problems tend to improve with time. Therefore, temporary placement of a feeding tube may be indicated in severe cases. The facial immobility can cause a significant social handicap and needs to be addressed in the management plan for such patients.

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