Annual incidence of nemaline myopathy is estimated to be 1 in 50,000 live births based on a study from Finland. The exact overall incidence and prevalence is not known. However, this disorder seems to be more common in those in the Amish community.

Nemaline myopathy is most often inherited in an autosomal recessive pattern, meaning both parents must have a copy of the mutated gene. Less often, the condition is inherited in an autosomal dominant pattern (only one copy of the altered gene is needed in each cell) or are completely sporadic (first occurrence in a family).

There are ten genes that, if mutated, can cause nemaline myopathy. Mutations in ACTA1 (alpha actin) gene have been found to cause 15-25 percent of the occurrences, most of which are sporadic. Mutations in the NEB (nebulin) gene have been found to cause 50% of nemaline myopathy, most of which are inherited as autosomal recessive trait. Other genes that can cause nemaline myopathy when mutated are the TPM2, TPM3, TNNT1, CFL2, KBTBD13, KLHL40, KLHL41 and LMOD3 genes.

Symptoms are dependent on age of onset and severity. Common symptoms of nemaline myopathy include hypotonia or poor muscle tone and muscle weakness, especially in face, neck and upper arms and legs. Many people also experience difficulty with breathing, swallowing and speech. Young children diagnosed may have a delay in walking or be unable to walk. Specific symptoms depend on age of onset and severity, which can vary greatly among people with this disorder. Currently, there are six different subsets of nemaline myopathy.

Nemaline myopathy types 1 to 3 are present at birth or before in the developing fetus stage. The symptoms of muscle weakness and hypotonia can be severe so the patients have very limited spontaneous movements. Nasal feeding tube due to swallowing difficulties and ventilator due to breathing difficulties are often required.

The most common type, typical congenital form (type 4), develops usually during the first year of life. Symptoms are less severe, but the children are often susceptible to respiratory tract and pulmonary infections. Muscle weakness and delayed motor development are also to be expected. Muscle weakness may not get worse or may get worse very slowly.

Type 5 is present in childhood and is mainly characterized by muscle weakness in hands and feet, while type 6 is an adult-onset form, whose first symptom is sometimes heart failure. Type 6 nemaline myopathy is characterized by general muscle weakness, muscle pain and rapid progression of symptoms.

A diagnosis of nemaline myopathy can be made thorough clinical evaluation, a detailed patient and family history and identification of characteristic physical findings. A muscle biopsy can confirm presence of rod-bodies. Electromyography can be used to further characterize the muscle weakness.

A muscle biopsy is done by using a needle to extract a small sample of muscle cells. Under a microscope, the sample can be checked for presence of rod-like bodies. However, rod-like bodies can also indicate other unrelated diseases, so it is impossible to also rely on other factors like identification of physical findings.

There is currently not a cure for nemaline myopathy, and treatments are directed at alleviating symptoms.

Immediate medical attention is required for newborns with types 1-3 nemaline myopathy. This include the use of a ventilator and feeding tube. Because of the risk of respiratory tract infections, bacterial and viral pneumonia vaccinations should be included.

For children with nemaline myopathy, specialists such as a pediatric neurologist and pediatric orthopedic surgeon aid the patient with physical functions affected by muscle weakness. As well, speech therapists might be required. Parents should establish contact with a habilitation team early on in child’s life. The same applies to adults who develop nemaline myopathy. Genetic counseling, as well, is recommended for affected individuals and their families.

Braces, manual or electric wheelchair, and surgical procedures are required in some cases, because some have impaired joint mobility.

An assessment of cardiac functions is recommended. The risk of cardiac abnormalities is small, but there are cases where they are an effect of nemaline myopathy.  

Individuals with typical nemaline myopathy tend to have a good prognosis. While they are delayed in their ability to walk, they usually gain that ability and live normal, active adult lives. The more severe forms of nemaline myopathy cause breathing problems and lung infections which can be fatal in early childhood.

Nemaline Myopathy. Genetics Home Reference website. Available at: https://ghr.nlm.nih.gov/condition/nemaline-myopathy

Nemaline Myopathy. National Organization for Rare Disorders website. Available at: http://rarediseases.org/rare-diseases/nemaline-myopathy/

Nemaline Myopathy. OrphaNet website. Available at:

http://www.orpha.net/consor/cgi-bin/OC_Exp.php?Lng=GB&Expert=607

Nemaline Myopathy. GeneReviews website. Available at:  http://www.ncbi.nlm.nih.gov/books/NBK1288/

Nemaline Myopathy. Socialsyrelsen website. Available at: http://www.socialstyrelsen.se/rarediseases/nemalinemyopathy