SELENON Gene

Rigid spine muscular dystrophy (RSMD) is a form of congenital muscular dystrophy. Disorders in this group cause muscle weakness and wasting (atrophy) beginning very early in life. In particular, RSMD involves weakness of the muscles of the torso and neck (axial muscles). Other characteristic features include spine stiffness and serious breathing problems.

In RSMD, muscle weakness is often apparent at birth or within the first few months of life. Affected infants can have poor head control and weak muscle tone (hypotonia), which may delay the development of motor skills such as crawling or walking. Over time, muscles surrounding the spine atrophy, and the joints of the spine develop deformities called contractures that restrict movement. The neck and back become stiff and rigid, and affected children have limited ability to move their heads up and down or side to side. Affected children eventually develop an abnormal curvature of the spine (scoliosis). In some people with RSMD, muscles in the inner thighs also atrophy, although it does not impair the ability to walk.

A characteristic feature of RSMD is breathing difficulty (respiratory insufficiency) due to restricted movement of the torso and weakness of the diaphragm, which is the muscle that separates the abdomen from the chest cavity. The breathing problems, which tend to occur only at night, can be life-threatening. Many affected individuals require a machine to help them breathe (mechanical ventilation) during sleep.

The combination of features characteristic of RSMD, particularly axial muscle weakness, spine rigidity, and respiratory insufficiency, is sometimes referred to as rigid spine syndrome. While these features occur on their own in RSMD, they can also occur along with additional signs and symptoms in other muscle disorders. The features of rigid spine syndrome typically appear at a younger age in people with RSMD than in those with other muscle disorders.

Congenital fiber-type disproportion is a condition that primarily affects skeletal muscles, which are muscles used for movement. People with this condition typically experience muscle weakness (myopathy), particularly in the muscles of the shoulders, upper arms, hips, and thighs. Weakness can also affect the muscles of the face and muscles that control eye movement (ophthalmoplegia), sometimes causing droopy eyelids (ptosis). Individuals with congenital fiber-type disproportion generally have a long face, a high arch in the roof of the mouth (high-arched palate), and crowded teeth.

Individuals with congenital fiber-type disproportion may have joint deformities (contractures) and an abnormally curved lower back (lordosis) or a spine that curves to the side (scoliosis). Approximately 30 percent of people with this disorder experience mild to severe breathing problems related to weakness of muscles needed for breathing. Some people who experience these breathing problems require use of a machine to help regulate their breathing at night (noninvasive mechanical ventilation), and occasionally during the day as well. About 30 percent of affected individuals have difficulty swallowing due to muscle weakness in the throat. Rarely, people with this condition have a weakened and enlarged heart muscle (dilated cardiomyopathy).

The severity of congenital fiber-type disproportion varies widely. It is estimated that up to 25 percent of affected individuals experience severe muscle weakness at birth and die in infancy or childhood. Others have only mild muscle weakness that becomes apparent in adulthood. Most often, the signs and symptoms of this condition appear by age 1. The first signs of this condition are usually decreased muscle tone (hypotonia) and muscle weakness. In most cases, muscle weakness does not worsen over time, and in some instances it may improve. Although motor skills such as standing and walking may be delayed, many affected children eventually learn to walk. These individuals often have less stamina than their peers, but they remain active. Rarely, people with this condition have a progressive decline in muscle strength over time. These individuals may lose the ability to walk and require wheelchair assistance.