•Type I
–Cerebellar tonsils extend through foramen magnum
into
cervical spinal canal (chronic tonsillar herniation)
–40-75% associated with cervical
cord syringomyelia
–Usually becomes symptomatic in
teen to early adult years
•Type II
–Arnold-Chiari malformation
–Cerebellar vermis and hemispheres extend through
foramen magnum into
cervical spinal canal
–Associated anomalies of
brainstem (100%)
–Associated myelomeningocele
(95%)
•Type III
–Cerebellar encephalocele
–Very rare
•Type IV
–Cerebellar aplasia/absence
–May be secondary destructive
lesion