Thought to be congenital but often only picked up in adulthood when symptoms develop.

Type 1 most common, where cerebellar tonsils protrude into spinal canal, potentially putting pressure on brainstem, spinal cord and cause obstruction to flow of cerebrospinal fluid.

Can be found incidentally. Symptoms however include:

• headache (especially occipital)
• neck pain
• numbness or paraesthesiae of fingers, arms, lower limbs
• Coordination problems, dizziness

Can be complicated by upper spinal syringomyelia.

Rarely familial.