Like many of the congenital myopathies, the features of nemaline myopathy tend to be more severe course if the disease initially presents in infancy.  A more moderate expression of the disease is expected if it presents in childhood.  There is a wide clinical spectrum seen in adult onset forms of the disease, ranging from asymptomatic to severe progressive disease.

Pathology 

Diagnosis of nemaline myopathy still depends on muscle pathology.  The defining feature is the presence of nemaline rods, which appear as very small granules within a muscle fiber, which can be visualized with trichrome staining. The granules often collect into aggregates.  The two images below are both Gomori Trichrome stained sections of skeletal muscle, on which nemaline rods are clearly seen (click to enlarge):

Rods are seen more often in type 1 fibers.  In nemaline myopathy, there may be an increased proportion of type 1 muscle fibers (also seen Central Core Disease - refer to that section for a more detailed explanation).

Defects in any one of three specific genes are associated with nemaline myopathy.  These genes are tropomyosin-3, alpha-actin, and nebulin.  Both autosomal dominant and autosomal recessive forms exist.  

There is no correlation between disease severity and the abundance of rods seen on microscopy.

Prognosis and Management:

When nemaline myopathy occurs in infancy, about 20% of infants die within the first year of life.  Others may stabilize or improve.

Management is supportive.

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.

Prayson, R.A., and Goldblum, J.R. (Eds.) (2005). Neuropathology. Elsevier Churchill Livingstone, Philadelphia. (ISBN:0443066582)  Pp. 537-581.