fourth nerve palsy, skew deviation, ocular dysmetria, ocular flutter, and various neuro-otologic abnormalities also occur.927

Acute comitant esotropia can be another presentation of Chiari I malformation. Ocular motor signs suggestive of this condition include an esotropia that is acquired after age 3, a distance deviation greater than the near deviation,692 A-pattern with bilateral superior oblique overaction, and nystagmus (particularly downbeat).409

Bixenman and Laguna92 described a 13-year-old girl who developed comitant esotropia who was successfully treated with strabismus surgery. Three years later, downbeat nystagmus developed, and Chiari I malformation was diagnosed with MR imaging. The nystagmus resolved, and the eyes remained aligned after neurosurgical decompression. Passo et al679 described a similar patient who was initially treated with strabismus surgery. After recurrence of esotropia and development of downbeat nystagmus, Chiari I malformation was diagnosed. In this patient, neurosurgical decompression of the posterior fossa restored ocular alignment and single binocular vision. On careful examination, other neurologic signs (downbeat nystagmus, headache, hydrocephalus) are often found.409 Subsequent case series have shown that suboccipital decompression often produces resolution of the esotropia.92,231,533,929 Although strabismus surgery initially restores horizontal alignment, recurrence of esotropia is common, and suboccipital decompression is often necessary for definitive treatment.692

In addition to comitant vertical strabismus, posterior fossa disease may also precipitate comitant strabismus that is purely horizontal. This prenuclear disorder is the horizontal analog of skew deviation. Like its vertical counterpart, horizontal skew deviation seems to be precipitated by a prenuclear perturbation of the ocular motor system.120 In light of recent reports, it would seem appropriate to expand our concept of skew deviation to include horizontal cases of acquired comitant esotropia that are increasingly recognized to accompany the Arnold–Chiari malformations and posterior fossa tumors in some children

An association between Chiari malformations and idiopathic intracranial hypertension (IIH) has recently been recognized. Disturbed CSF movement at the foramen magnum, with increased resistance to outflow and venous flow abnormalities resulting in venous hypertension are likely to be contributory risk factors for the development of IIH in this setting.501 Decompressive surgery for Chiari I malformation may lead to IIH in children.288 Conversely, bony decompression of the posterior fossa may produce resolution of IIHwhen both conditions coexist.904 Surgical intervention could cause changes in CSF circulation due to postoperative scarring, or CSF inflammation due to blood reabsorption. This imbalance in CSF circulation may lead to changes in the turgor or the brain paren-

chyma or an increase in resistance across the arachnoid villi leading to the postoperative development of IIH. Posterior cranial fossa decompression can produce clinical improvement in symptomatic patients.156 Lumboperitoneal shunting is well-recognized to result in Chiari malformations although these patients tend to be asymptomatic and rarely require treatment. Neuro-ophthalmologic symptoms and signs associated with Chiari I malformation often stabilize, improve, or resolve after suboccipital craniotomy.

Chiari II

The Chiari II malformation (also known as the Arnold–Chiari malformation) was until recently the most common of the Chiari malformations in the pediatric age group. However, the increased prevalence of in utero diagnosis (with ultrasound and alpha fetoprotein) leading to therapeutic abortion for Chiari II malformation, together with the increased clinical diagnosis of Chiari I with MR imaging, have rendered it less common than the Chiari I malformation. The Chiari II malformation is a highly complex malformation that is almost exclusively present in children with myelomeningocele. It can show any of the infratentorial features of Chiari I malformation, but it differs by involving supratentorial structures as well (Fig. 11.23).

Ninety percent of cases of Chiari II malformation occur in association with myelomeningocele and hydrocephalus. Conversely, all patients with myelomeningocele and hydrocephalus harbor a Chiari II malformation. Patients with Chiari II malformation have a reduced cerebellar volume, weight, and cell content.237,275,382 The small cerebellar size has been blamed on mechanical compression secondary to crowding, which is presumed to lead to the secondary ischemic changes and parenchymal cerebellar loss.771 However, this reduction in cerebellar volume is not uniform: the hemispheres shrink, but the vermis can expand vertically.771

Vertical expansion of the cerebellar vermis is an important neuroimaging sign as patients who do not show vertical expansion have a small vermis and tend to show eye movement abnormalities, while those with vertical expansion do not.771 Patients are usually diagnosed at birth with myelomeningocele and develop hydrocephalus shortly after its repair. After the repair of the myelomeningocele, the clinical presentation to be expected from the underlying Chiari II malformation as well as the associated hydrocephalus, lower cranial nerve palsies, and syringomyelia, may differ according to the age of the child. Patients younger than 6 months tend to present with stridor, apnea, and/or dysphagia (feeding difficulty), while children older than 3 years of age tend to present with hemiparesis, quadriparesis, oscillopsia, nystagmus, or opisthotonos.74,401 Chiari II malformation accounts

probably representing a form of alternating skew deviation on lateral gaze.378,380 Pathological studies on patients with myelomeningocele and Chiari II malformation have shown disorganized brainstem nuclei,342 a feature that may explain the propensity of these children to show this type of skew deviation. Other reported abnormalities include internuclear ophthalmoplegia,27,635,951 defective smooth pursuit and optokinetic nystagmus, periodic alternating nystagmus,844 and periodic alternating gaze deviation.

Chiari III

This is an exceedingly rare malformation in which the contents of the posterior fossa (cerebellum +/− brainstem) herniate through a cervical spina bifida cystica at the level of C1–C2. Hydrocephalus is a regular feature of this malformation.

The Dandy–Walker Malformation

These disorders, called Dandy–Walker malformation, Dandy–Walker variant, and mega cisterna magna, are considered to represent a continuum of developmental anomalies and are collectively designated as the Dandy–Walker complex (Fig. 11.24).56 The Dandy–Walker malformation is classically characterized by the neuropathologic triad of

(1) complete or partial agenesis of the cerebellar vermis, involving the cortex and deep cerebellar nuclei; (2) a greatly expanded, cystic, fourth ventricle; and (3) an enlarged posterior fossa with upward displacement of the lateral sinuses, tentorium, and torcula, and subsequently modified.155,877

An occipital encephalocele is also occasionallypresent.47,89 The Dandy–Walker malformation accounts for 2–4% of cases of hydrocephalus in children. The Dandy–Walker variant shows the above findings but with a normal-sized posterior fossa. It is more common than the true Dandy–Walker malformation and comprises about a third of posterior fossa malformations. Hydrocephalus is uncommon at birth but develops in 75% of cases by 3 months of age, and is present in 90% of patients at the time of diagnosis.47,653 The Dandy– Walker syndrome must be distinguished from mega cisterna magna (retrocerebellar arachnoid pouch), a cystic malformation wherein the posterior fossa is enlarged secondary to enlarged cisterna magna, but the cerebellar vermis and the fourth ventricle is normal.

Most cases of the Dandy–Walker malformation are diagnosed in the first year of life, and most of these are diagnosed at birth. The major signs and symptoms are those of hydrocephalus as well as associated developmental delay and failure to thrive. Some features are more characteristic of Dandy–

Fig. 11.24  Dandy–Walker cyst. MR image shows replacement of most of posterior fossa contents with large cyst. Note attenuation of brainstem

Walker malformation than other causes of hydrocephalus, such as a large occiput with a higher-than-normal inion and the predisposition of patients to have recurrent attacks of pallor, ataxia and, occasionally, sudden respiratory arrest. Hydrocephalus is infrequently present at birth but appears by 3 months of age in over 75% of patients. Some patients may remain asymptomatic throughout life, while others may require shunting.497

Dandy–Walker syndrome must be distinguished from arachnoid cysts of the fourth ventricular roof. Although the cerebellum is essential in adults for control of many aspects of ocular motility, eye movement abnormalities in children with Dandy–Walker syndrome are often mild or absent, suggesting that other parts of the brain may be capable of taking over these roles in the developing nervous system.523 Children with congenital cerebellar disorders such as Dandy–Walker malformation are said be less affected than adults with lesions at similar locations.375,483,523,598 Despite the large cerebellar defect, patients may show only mild saccadic dysmetria, and eye movements may be normal,523 perhaps reflecting preservation of the ponto-mesencephalic junction in these patients.572

Despite being addressed extensively in the literature, the Dandy–Walker malformation remains poorly understood.680 The disorder seems to the result of a genetic predisposition, because the recurrence rate for siblings is 6%.623 The Dandy–