Chiari malformation (Arnold-Chiari malformation) is a congenital condition, in which there is abnormal development of the base of the skull and the cerebellum and lower brainstem. Females are three times more likely to harbour a Chiari malformation.
Four different forms are recognized, of which types 1 and 2 are by far the commonest.
Chiari 1 is characterised by downward displacement of the cerebellar tonsils through the foramen magnum, by greater than 5mm. The excess of tissue in the foramen magnum may result in altered CSF flows through this region, which may lead to characteristic headaches, syrinx formation in the cervical or thoracic spinal cord, and occasionally to brainstem or cranial nerve dysfunction.
A form of Chiari malformation may be acquired in patients who suffer an underlying hereditary disorder of connective tissue such as Ehlers-Danlos or Marfan’s syndrome, as these patients are susceptible to instabilities of the craniocervical junction.
Chiari 2 malformations are usually associated with other congenital abnormalities of the neuraxis, particularly lumbar or lumbosacral myelomeningocoele. The downward cerebellar herniation tends to include the vermis as well as the cerebellar tonsils, and hydrocephalus is frequently associated, as are brainstem structural anomalies.
Conditions associated with Chiari malformation may include hydrocephalus, syringomyelia, tethered cord syndrome, and scoliosis.
Symptoms that may be associated include headaches aggravated by Valsalva manoeuvres (yawning, crying, sneezing, straining), tinnitus, vertigo, difficulty swallowing, impaired gag reflex, and balance disturbance.
Treatment when required is surgical, using a procedure designed to increase the size of the foramen magnum and increase the volume of the dural sac at that point whereby aiming to restore normal CSF flows through the foramen magnum.
Treatment of associated conditions (e.g. hydrocephalus, tethered cord) may require separate procedures.