Chiari malformation

PATHOGENESIS

Several hypotheses have been proposed to explain the pathological findings of these malformations. Gardner suggested that downward pressure from hydrocephalus played an important role in displacing the posterior fossa structures and, when associated with a patent central canal, explained the high incidence of syringomyelia (page 387). Others supposed that traction from a tethered spinal cord (dysraphism), or a CSF leak through a myelocele into the amniotic sac in fetal life resulted in caudal displacement of the posterior fossa structures. Of these theories, none provides an entirely satisfactory explanation; a more realistic view attributes the hindbrain deformity to maldevelopment during early fetal life. This would explain the presence of other developmental anomalies.

CLINICAL PRESENTATION

Depends on age

Severe type II (or III) deformities present with respiratory difficulties and lower cranial nerve palsies. Death may result from aspiration pneumonia infancy ^ or apnoeic attacks, or from complications of associated malformations, e.g. spina bifida. In milder forms, nystagmus (horizontal), retrocollis (neck extension) and spasticity predominate.

(With increasing age, gait ataxia may become evident. Features of an associated syringomyelia - dissociated sensory loss and spastic quadraparesis often contribute to the clinical problems.

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Only patients with a type I or a mild type II deformity present in adult life -

Occipital headaches are induced by coughing or straining

Nystagmus - downbeat (on looking down)

or rotatory (on lateral gaze)

may result from medullary compression or from an associated syringomyelia (see page 387).

Ataxia

Spastic quadraparesis

Progression may eventually lead to severe bulbar symptoms - lower cranial nerve palsies, respiratory difficulties.

INVESTIGATIONS

Magnetic resonance imaging (MRI) is the investigation of choice. T1 weighted sagittal and axial scans most clearly demonstrate cerebellar ectopia and the presence or absence of an associated syringomyelia.

Chiari malformation (with associated syringomyelia)

Chiari malformation (with associated syringomyelia)

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