Spinal muscular atrophy (SMA) is a hereditary neuromuscular disorder characterized by degeneration of spinal cord motor neurons resulting in muscle weakness. SMALED shows autosomal dominant inheritance with muscle weakness predominantly affecting the proximal lower extremities (Harms et al., 2010). The most common form of SMA (see, e.g., SMA1, 253300) shows autosomal recessive inheritance and is due to mutation in the SMN1 gene (600354) on chromosome 5q. Genetic Heterogeneity of Lower Extremity-Predominant Spinal Muscular Atrophy See also SMALED2A (615290) and SMALED2B (618291), both of which are caused by mutation in the BICD2 gene (609797) on chromosome 9q22. SMALED2A and SMALED2B differ in age at onset and severity, with SMALED2B being more severe.

Age-related macular degeneration-3 (ARMD3) is characterized by numerous small round yellow lesions visible at the temporal edge of the macula. Larger, less distinct yellow areas near the center of the macula are also observed, which represent areas of pigment epithelial detachment (Stone et al., 2004). For a phenotypic description and a discussion of genetic heterogeneity of age-related macular degeneration, see 603075.

Peripheral neuropathy-myopathy-hoarseness-hearing loss syndrome is a rare, syndromic genetic deafness characterized by a combination of muscle weakness, chronic neuropathic and myopathic features, hoarseness and sensorineural hearing loss. A wide range of disease onset and severity has been reported even within the same family.

Autosomal dominant distal hereditary motor neuronopathy-12 (HMND12) is a neurologic disorder characterized by onset in the first or second decade of distal muscle weakness and atrophy, primarily affecting the intrinsic hand muscles, but also affecting the lower legs, resulting in abnormal gait and pes cavus (summary by Beetz et al., 2012). For a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).

SMALED2A is an autosomal dominant form of spinal muscular atrophy characterized by early childhood onset of muscle weakness and atrophy predominantly affecting the proximal and distal muscles of the lower extremity, although some patients may show upper extremity involvement. The disorder results in delayed walking, waddling gait, difficulty walking, and loss of distal reflexes. Some patients may have foot deformities or hyperlordosis, and some show mild upper motor signs, such as spasticity. Sensation, bulbar function, and cognitive function are preserved. The disorder shows very slow progression throughout life (summary by Oates et al., 2013). For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

Neurodevelopmental disorder with poor growth and behavioral abnormalities (NEDGBA) is an autosomal recessive disorder characterized by global developmental delay, moderately to severely impaired intellectual development, often with absent speech, and behavioral abnormalities, including hyperactivity, short attention span, and ADHD. Affected individuals show failure to thrive with poor overall growth; some have microcephaly. Additional features may include nonspecific facial dysmorphism, hypotonia, and feeding difficulties (Vogt et al., 2022; Meng et al., 2023).