Adults with NM typically have a very slender physique. Causes
Nemaline myopathy is caused by mutations in one of at least 11 different genes. Nemaline myopathy is a clinically and genetically heterogeneous disorder and both autosomal dominant and autosomal recessive forms can occur. Diagnosis is made based upon clinical signs such as muscle weakness, absent or low deep tendon reflexes (hyporeflexia), and a high-arched palate,  along with electron-dense aggregates, called nemaline rods, being observed at the microscopic level within muscle fibers. Genetic confirmation through identification of a known genetic mutation in the patient is also an important component of diagnosis.The two most common gene mutations causing nemaline myopathy are found on NEB or ACTA1. Mutations of the NEB gene usually result in symptoms present at birth or beginning in early childhood. This mutation results in about 50% of affected nemaline myopathy patients. The most common inheritance pathway for those with mutations in NEB is autosomal recessive in which each parent carries one mutated copy along with one normal functioning copy of the gene, and they pass the mutated copy to their offspring. In some cases, occasionally with ACTA1 mutations, NM can be caused by an inheritance pattern of autosomal dominance. This mutation results in about 15 to 25 percent of NM cases. One reason why this is lower is because NM is associated with de novo mutations in ACTA1, occurring spontaneously in the egg or sperm.
When the condition is heritable, each pregnancy with the same partners has the same risk of passing the mutated genes to offspring. New mutations (de novo) can also occur causing NM and de novo mutations have been most often found to occur in the ACTA1 gene. MYPN is the last found gene related to NM The risk of all cases of nemaline myopathy is the same in males and females. The physical capabilities of a given person with NM do not correlate well either with genotype or with muscle pathology as observed in the biopsy. Mechanism
Muscle cells contract in complex mechanical and chemical processes. If any part of the process or structure is disrupted, dysfunction will likely result, as in the case of those with genetic variations. In those with nemaline myopathy, muscle contraction is adversely affected. At the electron microscopic level, rod-shaped components can often be seen in some of the muscle cells, and when seen, are diagnostic for the condition called nemaline rod myopathy. The presence of these rods is not itself causing muscle weakness; rather they appear as a result of something going wrong within the muscle fiber. There is no connection between the number of rods found in the muscle cells and the amount of weakness a person has. All of the different gene mutations leading to the condition called nemaline myopathy that have been found so far are in genes that encode different components of the sarcomere.