The Cerebellar Dysplasia of Chiari II Malformation as Revealed by Eye Movements

Abstract

Chiari type II malformation (CII) is one of the most prevalent congenital deformities of the hindbrain (brainstem and cerebellum). There is early mechanical compression on the developing hindbrain in CII, as well as additional effects caused by the accompanying hydrocephalus.1 One prevailing theory is that the deformity of CII occurs secondary to mechanical compression of hindbrain structures caused by a small posterior fossa.2 The small size of the posterior fossa in CII is proposed to be caused by cerebrospinal fluid (CSF) leak through the spinal defect in spina bifida meningomyelocele, which always accompanies CII.2 This leak prevents distension of the ABSTRACT: Introduction: Chiari type II malformation (CII) is a developmental deformity of the hindbrain. We have previously reported that many patients with CII have impaired smooth pursuit, while few make inaccurate saccades or have an abnormal vestibuloocular reflex. In contrast, saccadic adaptation and visual fixation are normal. In this report, we correlate results from several eye movement studies with neuroimaging in CII. We present a model for structural changes within the cerebellum in CII. Methods: Saccades, smooth pursuit, the vestibulo-ocular reflex, and visual fixation were recorded in 21 patients with CII, aged 8-19 years and 39 age-matched controls, using an infrared eye tracker. Qualitative and quantitative MRI data were correlated with eye movements in 19 CII patients and 28 controls. Results: Nine patients with CII had abnormal eye movements. Smooth pursuit gain was subnormal in eight, saccadic accuracy abnormal in four, and vestibulo-ocular reflex gain abnormal in three. None had fixation instability. Patients with CII had a significantly smaller cerebellar volume than controls, and those with normal eye motion had an expanded midsagittal vermis compared to controls. However, patients with abnormal eye movements had a smaller (non-expanded) midsagittal vermis area, posterior fossa area and medial cerebellar volumes than CII patients with normal eye movements. Conclusions: The deformity of CII affects the structure and function of the cerebellum selectively and differently in those with abnormal eye movements. We propose that the vermis can expand when compressed within a small posterior fossa in some CII patients, thus sparing its ocular motor functions. RÉSUMÉ: Mouvements oculaires révélant une dysplasie cérébelleuse dans la malformation de Chiari de type II.Contexte : La malformation de Chiari de type II (CII) est un défaut du développement du cerveau postérieur. Nous avons déjà rapporté que les patients porteurs de CII ont une atteinte de la poursuite oculaire lente et qu'un petit nombre ont des saccades imprécises ou un réflexe vestibulo-oculaire anormal. Par contre, l'adaptation saccadique et la fixation visuelle sont normales. Dans cet article, nous établissons la corrélation entre les résultats des études de mouvements oculaires et la neuroimagerie dans le CII. Nous présentons également un modèle de changements structurels dans le cerve...

The Cerebellum in Children with Spina Bifida and Chiari II Malformation: Quantitative Volumetrics by Region

“…While cerebellar development is atypical in SBM, it is not yet clear whether the cerebellar configuration described here is unique to this condition. And while regional cerebellar expansion has been associated with enhanced ocular motor function in SBM [57], the significance of the enlarged anterior cerebellar lobe remains to be established.…”

“…Variation in regional cerebellar size may have functional significance for eye movements. Ocular motor function in individuals with SBM and an expanded midsagittal vermis is comparable to that of controls and better than that in individuals with SBM and a normal, non-expanded midsagittal vermis [57]. Our ongoing studies are investigating the functional significance of anomalous cerebellar development in SBM.…”

Age, plasticity, and homeostasis in childhood brain disorders

“…The small posterior fossa in this condition is caused by cerebrospinal fluid leak through the spinal defect that prevents distension of the embryonic ventricular system (McLone & Knepper, 1989), which prevents expansion of the posterior fossa and reconfigures the cerebellum (Juranek et al, 2010). Hyperdevelopment of the midsagittal vermis in spina bifida meningomyelocele is associated with sparing of ocular motor function (Salman, Dennis, & Sharpe, 2009). …”

Cerebellar Motor Function in Spina Bifida Meningomyelocele

“…In individuals with SBM, ocular motor function is correlated with two alternative patterns of cerebellar development [100]. The first involves reduction in total and lateral cerebellar volumes, expansion of the midsagittal cerebellar vermis area including vermis lobules VI and VII (which are important for saccade accuracy), and relative preservation of the medial cerebellar volume.…”

“…This pattern is associated with impairment of the same ocular motor functions of saccadic accuracy and smooth pursuit. In SBM, the smooth pursuit system is affected most by variations in vermis midsagittal area or cerebellar volume, likely because the flocculi, part of the inferior posterior lobules, are most affected by volume reductions [100]. …”