Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is characterized by progressive cerebellar ataxia and variable findings including pyramidal signs, a dystonic-rigid extrapyramidal syndrome, significant peripheral amyotrophy and generalized areflexia, progressive external ophthalmoplegia, action-induced facial and lingual fasciculations, and bulging eyes. Neurologic findings tend to evolve as the disorder progresses.

Spinocerebellar ataxia type 1 (SCA1) is characterized by progressive cerebellar ataxia, dysarthria, and eventual deterioration of bulbar functions. Early in the disease, affected individuals may have gait disturbance, slurred speech, difficulty with balance, brisk deep tendon reflexes, hypermetric saccades, nystagmus, and mild dysphagia. Later signs include slowing of saccadic velocity, development of upgaze palsy, dysmetria, dysdiadochokinesia, and hypotonia. In advanced stages, muscle atrophy, decreased deep tendon reflexes, loss of proprioception, cognitive impairment (e.g., frontal executive dysfunction, impaired verbal memory), chorea, dystonia, and bulbar dysfunction are seen. Onset is typically in the third or fourth decade, although childhood onset and late-adult onset have been reported. Those with onset after age 60 years may manifest a pure cerebellar phenotype. Interval from onset to death varies from ten to 30 years; individuals with juvenile onset show more rapid progression and more severe disease. Anticipation is observed. An axonal sensory neuropathy detected by electrophysiologic testing is common; brain imaging typically shows cerebellar and brain stem atrophy.

Spinocerebellar ataxia type 2 (SCA2) is characterized by progressive cerebellar ataxia, including nystagmus, slow saccadic eye movements, and in some individuals, ophthalmoparesis or parkinsonism. Pyramidal findings are present; deep tendon reflexes are brisk early on and absent later in the course. Age of onset is typically in the fourth decade with a ten- to 15-year disease duration.

A very rare subtype of autosomal dominant cerebellar ataxia type 3 with characteristics of late-onset and slowly progressive cerebellar signs (gait ataxia) and eye movement abnormalities. To date, only 23 affected patients have been described from one American family of Norwegian descent. Disease onset occurs between the ages of 26-60. A candidate gene has recently been identified as the eukaryotic translation elongation factor 2 (EEF2) gene, located on chromosome 19p13.3. Inherited autosomal dominantly.

STXBP1 encephalopathy with epilepsy is characterized by early-onset developmental delay, intellectual disability or cognitive dysfunction, and epilepsy. The median age of onset of seizures is six weeks (range: 1 day to 13 years). Seizure types can include infantile spasms; generalized tonic-clonic, clonic, or tonic seizures; and myoclonic, atonic, absence, and focal seizures. EEG abnormalities can include focal epileptic activity, burst suppression, hypsarrhythmia, or generalized spike-and-slow waves. Other neurologic findings include abnormal tone, movement disorders (especially ataxia and dystonia), and behavioral issues and autism spectrum disorder. Feeding difficulties are common.

POLR3-related leukodystrophy, a hypomyelinating leukodystrophy with specific features on brain MRI, is characterized by varying combinations of four major clinical findings: Neurologic dysfunction, typically predominated by motor dysfunction (progressive cerebellar dysfunction, and to a lesser extent extrapyramidal [i.e., dystonia], pyramidal [i.e., spasticity] and cognitive dysfunctions). Abnormal dentition (delayed dentition, hypodontia, oligodontia, and abnormally placed or shaped teeth). Endocrine abnormalities such as short stature (in ~50% of individuals) with or without growth hormone deficiency, and more commonly, hypogonadotropic hypogonadism manifesting as delayed, arrested, or absent puberty. Ocular abnormality in the form of myopia, typically progressing over several years and becoming severe. POLR3-related leukodystrophy and 4H leukodystrophy are the two recognized terms for five previously described overlapping clinical phenotypes (initially described as distinct entities before their molecular basis was known). These include: Hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); Ataxia, delayed dentition, and hypomyelination (ADDH); Tremor-ataxia with central hypomyelination (TACH); Leukodystrophy with oligodontia (LO); Hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Age of onset is typically in early childhood but later-onset cases have also been reported. An infant with Wiedemann-Rautenstrauch syndrome (neonatal progeroid syndrome) was recently reported to have pathogenic variants in POLR3A on exome sequencing. Confirmation of this as a very severe form of POLR3-related leukodystrophy awaits replication in other individuals with a clinical diagnosis of Wiedemann-Rautenstrauch syndrome.

The phenotypic spectrum associated with biallelic RFC1 AAGGG repeat expansion encompasses a range including (1) typical cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS); (2) cerebellar, sensory, and vestibular impairment; (3) more limited phenotypes involving predominantly or exclusively one of the systems involved in balance control; (4) autonomic dysfunction; and (5) cough. Onset begins after age 35 years. In a retrospective study of 100 affected individuals after ten years of disease duration, two thirds had clinical features of CANVAS; 16 had a complex sensory ataxia with cerebellar or vestibular involvement; and 15 had a sensory neuropathy as the only clinically detectable manifestation.