Typical nemaline myopathy

Typical nemaline myopathy (also called typical congenital nemaline myopathy or nemaline rod myopathy, typical form) is the most common form of nemaline myopathy, a group of inherited neuromuscular disorders characterized by the presence of rod-shaped structures called nemaline bodies (or nemaline rods) in skeletal muscle fibers. This form primarily affects the skeletal muscular system and, in some cases, the respiratory system. It is classified under congenital myopathies (ICD-10: G71.2) and is distinguished from the severe neonatal and milder childhood-onset forms by its intermediate clinical presentation. Typical nemaline myopathy usually presents in infancy or early childhood with generalized muscle weakness that predominantly affects proximal muscles, the trunk, and facial muscles. Affected individuals commonly exhibit hypotonia (reduced muscle tone), delayed motor milestones, facial weakness with a characteristic elongated face, a high-arched palate, nasal speech, and feeding difficulties. Weakness tends to be more prominent in flexor muscles of the neck and limb-girdle muscles. Respiratory muscle involvement can occur and may lead to respiratory insufficiency, which is a significant source of morbidity. Deep tendon reflexes are often reduced or absent. Most individuals with typical nemaline myopathy achieve independent ambulation, though motor abilities may be limited, and the clinical course is generally non-progressive or slowly progressive. There is currently no cure or disease-specific treatment for typical nemaline myopathy. Management is supportive and multidisciplinary, including physical therapy to maintain mobility and prevent contractures, respiratory monitoring and support (including non-invasive ventilation when needed), nutritional support for feeding difficulties, orthopedic interventions for scoliosis or foot deformities, and speech therapy. Genetic counseling is recommended for affected families. Multiple genes have been implicated, including NEB (nebulin), ACTA1 (skeletal muscle alpha-actin), TPM3, TPM2, TNNT1, CFL2, KBTBD13, KLHL40, KLHL41, and LMOD3, with mutations in NEB being the most common cause of the typical form.

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