For a general discussion of autosomal dominant spinocerebellar ataxia (SCA), see SCA1 (164400).

Spinocerebellar ataxia-25 (SCA25) is an autosomal dominant neurologic disorder characterized by the onset of lower limb ataxia resulting in gait difficulties in the first few decades of life, although later onset has been reported. Affected individuals often have upper limb involvement, dysarthria, scoliosis, abnormal eye movements, and sensory neuropathy with decreased reflexes. Some patients have sensorineural hearing loss. Brain imaging shows cerebellar atrophy. There is incomplete penetrance and variable expressivity, even within families (Barbier et al., 2022). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Spinocerebellar ataxia type 14 (SCA14) is characterized by slowly progressive cerebellar ataxia, dysarthria, and nystagmus. Axial myoclonus, cognitive impairment, tremor, and sensory loss may also be observed. Parkinsonian features including rigidity and tremor have been described in some families. Findings seen in other ataxia disorders (e.g., dysphagia, dysphonia) may also occur in SCA14. The average age of onset is in the 30s, with a range from childhood to the seventh decade. Life span is not shortened.

Rare disease with manifestations of action tremor associated with relatively mild cerebellar ataxia. Associated pyramidal and extrapyramidal signs and dementia have been reported. Prevalence is unknown. Approximately 40 families have been reported. The pathogenesis seems to be related to a toxic effect at the RNA level as it is caused by a CAG expansion at the 5'' end of the PPP2R2B gene on chromosome 5q31-5q32.

ADCY5 dyskinesia is a hyperkinetic movement disorder (more prominent in the face and arms than the legs) characterized by infantile to late-adolescent onset of chorea, athetosis, dystonia, myoclonus, or a combination of these. To date, affected individuals have had overlapping (but not identical) manifestations with wide-ranging severity. The facial movements are typically periorbital and perioral. The dyskinesia is prone to episodic or paroxysmal exacerbation lasting minutes to hours, and may occur during sleep. Precipitating factors in some persons have included emotional stress, intercurrent illness, sneezing, or caffeine; in others, no precipitating factors have been identified. In some children, severe infantile axial hypotonia results in gross motor delays accompanied by chorea, sometimes with language delays. The overall tendency is for the abnormal movements to stabilize in early middle age, at which point they may improve in some individuals; less commonly, the abnormal movements are slowly progressive, increasing in severity and frequency.

Intellectual developmental disorder with muscle tone abnormalities and distal skeletal defects (IDDMDS) is an autosomal recessive disorder characterized by global developmental delay apparent in infancy manifest as speech delay and late walking by a few years. Affected individuals have hypertonia or, more rarely, hypotonia; a notable common feature is facial myokymia with corresponding EMG findings. Additional features include distal skeletal defects such as joint contractures, hypo- or areflexia, and hernia (Marafi et al., 2022).