What is nemaline myopathy?
Nemaline myopathy is a rare genetic muscle disorder, and there are technically six different clinical subtypes of this disorder based on disease severity and age of onset. These subtypes range from severe, life-threatening congenital-onset (at birth) to less severe forms in childhood or adulthood, but all forms share characteristic symptoms of muscle weakness, diminished muscle tone, and reduced or absent reflexes. Fortunately, this muscle weakness is static (nonprogressive) over time.
What causes nemaline myopathy?
Nemaline myopathy is caused by the presence of fine, thread- or rod-like structures called “nemaline bodies.” These nemaline bodies consist of accumulations of muscle proteins, which are due to mutations in genes that encode protein components of muscle filament. Ten genes (most prominently, the NEB gene) have been found to contribute to these abnormality, which are inherited as either an autosomal dominant or autosomal recessive trait. However, 15-25 percent of nemaline myopathy cases are caused by mutations in the ACTA1 gene, which are spontaneous mutations and not inherited.
What are the symptoms of nemaline myopathy?
Both the age of onset and the severity of symptoms of nemaline myopathy vary greatly from patient to patient. The characteristic symptoms of the disorder listed above are most severe in the muscles of the face, neck, and those closest to the center of the body. In turn, since the facial muscles are involved, affected individuals may also develop distinctive facial features and complications such as an elongated face, a displaced jaw, a highly-arched palate, and difficulty speaking, swallowing, and/or breathing. Additionally, a delay in attaining motor milestones and abnormally fixed joints with a sunken chest and scoliosis are also common.
Other characteristic signs of nemaline myopathy are dependent on the specific clinical subtype of the disorder:
- Typical Congenital Nemaline Myopathy (most common): present at or shortly after birth; weakness of respiratory muscles, but less severe muscle weakness than the other forms, with most individuals being able to eventually walk independently
- Severe Congenital (Neonatal) Nemaline Myopathy: apparent at birth, with profound muscle weakness, severe hypotonia, and life-threatening complications, such as cardiomyopathy, fractures, and respiratory failure
- Intermediate Congenital Nemaline Myopathy: early development of contractures; mid-level severity
- Childhood-Onset Nemaline Myopathy: apparent between 10-20 years of age when individuals start to develop progressive muscle weakness and foot drop
- Adult-Onset Nemaline Myopathy (exceptionally rare): variable onset and severity: apparent between 20-50 years of age; rapidly progressing generalized muscle weakness
- Amish Nemaline Myopathy: identified in several related families within an Amish community; onset is shortly after birth with hypotonia, multiple contractures, and tremors, and then progressive muscle weakness, a severely deformed chest, muscle wasting, and life-threatening respiratory insufficiency
How is nemaline myopathy diagnosed?
A diagnosis of nemaline myopathy is made following a thorough clinical evaluation, a detailed patient and family history, and an identification of characteristic findings of the disease. A muscle biopsy to determine the presence of thread- or rod-like structures can confirm a diagnosis.
What are the available treatments for nemaline myopathy?
No specific cure nor treatment currently exists for nemaline myopathy, so therapy options are symptomatic and supportive. These could include:
- Low-impact exercise, massage, and stretching techniques
- Respiratory support, such as mechanical ventilation
- Speech therapy
- Orthopedic techniques, such as special braces, other devices, and/or surgical measures
Where can I find more information on nemaline myopathy?