Ataxia with oculomotor apraxia type 2 (AOA2) is characterized by onset of ataxia between age three and 30 years after initial normal development, axonal sensorimotor neuropathy, oculomotor apraxia, cerebellar atrophy, and elevated serum concentration of alpha-fetoprotein (AFP).

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.

Spinocerebellar ataxia-29 (SCA29) is an autosomal dominant neurologic disorder characterized by onset in infancy of delayed motor development and mild cognitive delay. Affected individuals develop a very slowly progressive or nonprogressive gait and limb ataxia associated with cerebellar atrophy on brain imaging. Additional variable features include nystagmus, dysarthria, and tremor (summary by Huang et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Autosomal recessive spastic paraplegia-46 (SPG46) is a neurodegenerative disorder characterized by onset in childhood of slowly progressive spastic paraplegia and cerebellar signs. Some patients have cognitive impairment, cataracts, and cerebral, cerebellar, and corpus callosum atrophy on brain imaging (summary by Boukhris et al., 2010 and Martin et al., 2013). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

Leukoencephalopathy-dystonia-motor neuropathy syndrome is a peroxisomal neurodegenerative disorder characterized by spasmodic torticollis, dystonic head tremor, intention tremor, nystagmus, hyposmia, and hypergonadotrophic hypogonadism with azoospermia. Slight cerebellar signs (left-sided intention tremor, balance and gait impairment) are also noted. Magnetic resonance imaging (MRI) shows bilateral hyperintense signals in the thalamus, butterfly-like lesions in the pons, and lesions in the occipital region, whereas nerve conduction studies of the lower extremities shows a predominantly motor and slight sensory neuropathy.

A rare genetic isolated dystonia with characteristics of adult-onset non-progressive focal cervical dystonia typically manifesting with torticollis and occasionally accompanied by mild head tremor and essential-type limb tremor.

Dystonia-24 is an autosomal dominant form of focal dystonia affecting the neck, laryngeal muscles, and muscles of the upper limbs (summary by Charlesworth et al., 2012).

Hereditary essential tremor-6 (ETM6) is an autosomal dominant neurologic disorder characterized by adult-onset kinetic and/or postural tremor usually affecting the upper limbs. Some patients may have involvement of the head, trunk, lower limbs, and/or voice. Additional neurologic features, such as cognitive impairment or pyramidal signs, are usually not observed. Brain imaging does not show cerebellar atrophy or leukodystrophy. Skin biopsy shows intranuclear eosinophilic inclusions in fibroblasts and sweat gland cells, which may be used for diagnosis. There is evidence of genetic anticipation, with progressive earlier age at onset in younger generations. In rare cases, the phenotype may convert to NIID over time (summary by Sun et al., 2020; Ng et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of hereditary essential tremor, see ETM1 (190300).

Focal segmental glomerulosclerosis and neurodevelopmental syndrome (FSGSNEDS) is characterized by global developmental delay and renal dysfunction manifest as proteinuria and nephrotic syndrome apparent from infancy or early childhood. Some patients present with renal disease, whereas others present with developmental delay and develop renal disease later in childhood. Renal biopsy shows focal segmental glomerulosclerosis (FSGS), but the course of the disease is variable: some patients have transient proteinuria and others require renal transplant. Neurodevelopmental features are also variable, with some patients having only mildly impaired intellectual development, and others having a severe developmental disorder associated with early-onset refractory seizures or epileptic encephalopathy. Additional features, including feeding difficulties, poor overall growth, and nonspecific dysmorphic facial features, are commonly observed (summary by Assoum et al., 2018 and Weng et al., 2021).

Myoclonic dystonia-34 (DYT34) is an autosomal dominant neurologic disorder characterized by childhood-onset dystonia primarily involving the hands and neck, with a fast tremor with superimposed myoclonus (Balint et al., 2020).

Autosomal recessive intellectual developmental disorder-77 (MRT77) is a nonsyndromic neurodevelopmental disorder characterized by global developmental delay with variably impaired cognitive development apparent from infancy. Affected individuals usually have delayed walking, sometimes with an unsteady gait, and may have poor speech and communication. Brain imaging is normal, and there are no additional significant neurologic abnormalities (Khoshbakht et al., 2021). Mutation in the CEP104 gene also causes a form of Joubert syndrome (JBTS25; 616781).

Spinocerebellar ataxia-50 (SCA50) is an autosomal dominant neurologic disorder characterized by cerebellar ataxia, oculomotor apraxia and other eye movement abnormalities, and cerebellar atrophy on brain imaging. Most patients develop symptoms as adults, although childhood onset has rarely been reported. Additional more variable features may include tremor, dysarthria, dysphagia, and cognitive impairment with executive dysfunction (Coutelier et al., 2022; Schoggl et al., 2022).