Spinocerebellar ataxia-21 (SCA21) is an autosomal dominant neurologic disorder characterized by onset in the first decades of life of slowly progressive cerebellar ataxia, which is associated with cognitive impairment in most patients (summary by Delplanque et al., 2014). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Neurodevelopmental disorder with eye movement abnormalities and ataxia (NEDEMA) is characterized by global developmental delay apparent from infancy. Affected individuals show delayed walking with an unsteady gait, variably impaired intellectual development, learning disabilities, and speech difficulties. Abnormal eye movements, which are often noted in early childhood, include opsoclonus, nystagmus, and strabismus. Some patients have seizures, which may be refractory (Lu et al., 2022).