An autosomal dominant type of Chiari malformation characterized by the protrusion of the cerebellar tonsils through the FORAMEN MAGNUM, defined radiologically as tonsillar descent of 5 mm or more. CM1 is associated with SYRINGOMYELIA in up to 80% of cases. May be asymptomatic or can cause headaches, ocular disturbances, otoneurologic disturbances, lower cranial nerve signs, CEREBELLAR ATAXIA, or MUSCLE SPASTICITY. Onset of symptoms is usually in the third decade of life. OMIM: 118420
