Mitochondrial DNA (mtDNA)-associated Leigh syndrome and NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) are part of a continuum of progressive neurodegenerative disorders caused by abnormalities of mitochondrial energy generation. Leigh syndrome (or subacute necrotizing encephalomyelopathy) is characterized by onset of symptoms typically between ages three and 12 months, often following a viral infection. Decompensation (often with elevated lactate levels in blood and/or CSF) during an intercurrent illness is typically associated with psychomotor retardation or regression. Neurologic features include hypotonia, spasticity, movement disorders (including chorea), cerebellar ataxia, and peripheral neuropathy. Extraneurologic manifestations may include hypertrophic cardiomyopathy. About 50% of affected individuals die by age three years, most often as a result of respiratory or cardiac failure. NARP is characterized by proximal neurogenic muscle weakness with sensory neuropathy, ataxia, and pigmentary retinopathy. Onset of symptoms, particularly ataxia and learning difficulties, is often in early childhood. Individuals with NARP can be relatively stable for many years, but may suffer episodic deterioration, often in association with viral illnesses.

Beare-Stevenson cutis gyrata syndrome (BSTVS) is an autosomal dominant condition characterized by the furrowed skin disorder of cutis gyrata, acanthosis nigricans, craniosynostosis, craniofacial dysmorphism, digital anomalies, umbilical and anogenital abnormalities, and early death (summary by Przylepa et al., 1996).

Microhydranencephaly (MHAC) is a severe neurodevelopmental defect characterized by extreme microcephaly, profound motor and mental retardation, spasticity, and incomplete cerebral formation. Radiologic studies show gross dilation of the ventricles resulting from the absence of cerebral hemispheres or severe delay in their development, as well as hypoplasia of the corpus callosum, cerebellum, and brainstem (summary by Guven et al., 2012).

Autosomal dominant primary hypertrophic osteoarthropathy (PHOAD) is characterized by 3 major features: digital clubbing, periostosis, and pachydermia. Patients may also experience joint swelling and pain, and some have reported gastrointestinal symptoms, including watery diarrhea. Males are more commonly affected, and more severely affected, than females (Lee et al., 2016; Xu et al., 2021). Touraine et al. (1935) recognized pachydermoperiostosis (PDP) as a familial disorder with 3 presentations or forms: a complete form with periostosis and pachydermia, an incomplete form without pachydermia, and a forme fruste with pachydermia and minimal skeletal changes. Genetic Heterogeneity Autosomal recessive forms of PHO have been reported (see 259100), including PHOAR2E (614441), which is also caused by mutation in the SLCO2A1 gene.