• Congenital abnormalities of the brain characterised by:
o Extension of a tongue of cerebellar tissue into cervical spinal canal
o Caudal displacement of the medulla and inferior part of the 4th ventricle into the cervical spinal canal
o Frequent association with syringomyelia (or another spinal developmental abnormality)
• Incidence ~0.6% of the population
• Signs of increased intracranial pressure
o Headache
- usually occipital
- Worse with coughing, positional change or Valsalva
- ?Headache with exertion – unclear association
• Progressive cerebellar ataxia
• Progressive spastic quadriparesis
• Downbeating nystagmus
• Syndrome of cervical syringomyelia (segmental amyotrophy and sensory loss in upper limbs)
• Disorders of the lower cranial nerves
• Can be exacerbated by prolonged hyperextension
• Risk from trauma (e.g. contact sports) is unclear – there are case reports of sudden deterioration, however very low/no incidence of such in registries