Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

GDAP1-related hereditary motor and sensory neuropathy (GDAP1-HMSN) is a peripheral neuropathy (also known as a subtype of Charcot-Marie-Tooth disease) that typically affects the lower extremities earlier and more severely than the upper extremities. As the neuropathy progresses, the distal upper extremities also become severely affected. Proximal muscles can also become weak. Age at onset ranges from infancy to early childhood. In most cases, disease progression causes disabilities within the first or second decade of life. At the end of the second decade, most individuals are wheelchair bound. Disease progression varies considerably even within the same family. The neuropathy can be either of the demyelinating type with reduced nerve conduction velocities or the axonal type with normal nerve conduction velocities. Vocal cord paresis is common. Intelligence is normal. Life expectancy is usually normal, but on occasion may be reduced because of secondary complications.

A rare congenital muscular dystrophy characterised by neonatal hypotonia, life-threatening respiratory failure and feeding difficulties, furthermore by delayed motor development, severe muscle weakness predominantly affecting axial muscles (leading to poor head control, rigid cervical spine, and severe scoliosis), generalised joint laxity with no or mild contractures, as well as dry skin with follicular hyperkeratosis. Serum creatine kinase is normal or slightly elevated. Muscle biopsy shows fibre size variability, rounded fibres with mild increase of endomysial connective tissue and adipose replacement, abundant minicore lesions, increase of centrally located nuclei, angular fibres and cap lesions.

Facioscapulohumeral muscular dystrophy-3 (FSHD3) is a digenic muscle disorder characterized by adult onset of proximal muscle weakness affecting the face, neck, scapular muscles, and upper and lower limbs. Muscle involvement is usually asymmetric, and other muscle groups may become involved with progression of the disease (summary by Hamanaka et al., 2020). For a discussion of genetic heterogeneity of FSHD, see FSHD1 (158900).

Inclusion body myopathy and brain white matter abnormalities (IBMWMA) is an autosomal dominant adult-onset disorder characterized predominantly by proximal limb girdle muscle weakness affecting the lower and upper limbs and resulting in gait difficulties and scapular winging. Additional features may include dysarthria, dysphagia, low back pain, and hyporeflexia. EMG is consistent with a myopathic process, although neuropathic findings have also been shown. Muscle biopsy shows fiber type variation, internal nuclei, rimmed vacuoles, and cytoplasmic protein aggregates or inclusions. Serum creatine kinase is usually elevated. Cognitive impairment or frontotemporal dementia occurs in some patients. The disorder is slowly progressive; some patients become wheelchair-bound after many years. Rare patients with this mutation develop ALS; some have both myopathy and ALS. Brain imaging shows white matter abnormalities using diffusion tensor imaging. The disorder is classified as multisystem proteinopathy-6 (MSP6) due to the characteristic disease mechanism of protein misfolding and abnormal tissue deposition (summary by Leoni et al., 2021).

Juvenile amyotrophic lateral sclerosis-27 (ALS27) is an autosomal dominant disorder characterized by early childhood-onset lower extremity spasticity manifesting as toe walking and gait abnormalities, followed by progressive lower motor neuron-mediated weakness without sensory signs or symptoms (Mohassel et al., 2021). For a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).