Nemaline myopathy (NM)

Overview

Nemaline myopathy (NM), sometimes called rod myopathy, is named for thread- or rod-like structures that form inside muscle cells. It's a congenital myopathy — a genetic muscle condition present from birth or infancy — causing weakness alongside breathing and feeding difficulties.

Symptoms

How NM presents depends heavily on the specific type. The mildest, adult-onset form appears in adolescence or adulthood with widespread weakness and sometimes muscle pain. A childhood-onset form typically appears between ages 8 and 15 with weakness concentrated in the calves. The most common form, typical congenital NM, appears at birth or in infancy with weakness, low muscle tone, delayed motor milestones, and breathing or feeding difficulties.

An intermediate congenital form shares those early features but progresses more severely, with earlier joint contractures and a greater likelihood of needing ventilator support. Severe congenital NM is life-threatening, with very low muscle tone, little spontaneous movement, and breathing and feeding difficulties needing respiratory support from birth. A form specific to the Amish community also exists, with low muscle tone, stiff joints, tremor and breathing problems from birth, and is similarly life-threatening.

Inheritance and cause

NM is caused by changes in genes that build the proteins responsible for muscle tone and movement; when faulty, these lead to the rod-like structures that disrupt normal muscle fibre shape and function. At least 12 genes are known to cause NM, most commonly ACTA1 and NEB, with others including TPM2, TPM3, KBTBD13, CFL2, KLHL40, KLHL41, LMOD3, MYPN, TNNT1, TNNT3 and MYO18B. Inheritance can be autosomal dominant (one changed copy from an affected parent) or autosomal recessive (a changed copy from each of two carrier parents); rarely, the change arises fresh with no family history.

Getting a diagnosis

A GP refers to a neurologist. Diagnosis combines clinical assessment with genetic blood testing for the known NM genes — once identified in one family member, the same test can confirm the diagnosis in relatives and identify unaffected carriers. A muscle biopsy, showing the distinctive rod-like structures under the microscope, is used if genetic testing doesn't give a clear answer.

Management and outlook

A multidisciplinary team led by a neurologist is central to care; the most severe forms need hospital-based support for breathing and feeding, and as a general pattern, earlier onset tends to mean more significant symptoms.

Breathing problems are common and need regular monitoring, with overnight sleep studies checking for nocturnal hypoventilation, which non-invasive ventilation can treat. Swallowing difficulties are managed with dietitian input — adjusting food texture, adding supplements, or, in severe cases, a feeding tube (gastrostomy) for adequate nutrition and hydration.

Physiotherapy and moderate exercise (swimming and walking are commonly suggested) help maintain strength, mobility and breathing capacity, and slow the progression of scoliosis and contractures. Some genetic forms carry a small added risk of heart involvement, worth checking with a cardiologist (ECG, echocardiogram); physiotherapists can also advise on walking aids or a wheelchair as needs change.