Spinal muscular atrophy is a group of inherited diseases that cause progressive muscle degeneration and weakness, eventually leading to death.

Spinal muscular atrophy (SMA) is the second leading cause of neuromuscular disease. It is inherited as an autosomal recessive trait (a person must get the defective gene from both parents to be affected) and has an incidence of approximately 4 per 100,000 people.

In its most severe form (SMA type I, also called Werdnig-Hoffman disease), infants are born floppy with weak, thin muscles and feeding and breathing problems. Their lifespan seldom exceeds 2 to 3 years. Infants with SMA type II have less severe symptoms during early infancy, but they become progressively weaker with time. Survival time with type II is longer, but the disease kills most of those affected while they are still children.

SMA type III is the least severe form of the disease, and symptoms may not appear until the second year of life. Often, weakness is first noted in the shoulder muscles and proximal leg muscles. Weakness is progressive and will eventually become profound, but children with type III disease may survive into early adulthood.

Rarely, SMA may begin in adulthood. This is usually a milder form of the disease. This form may be inherited in an autosomal dominant (only one copy of the gene is needed for the disease to occur) or autosomal recessive manner.

Family history of spinal muscular atrophy is a risk factor for all types of the disorder.

In an infant:

• Floppy infant
• Very weak infant
• Little spontaneous movement
• Lack of head control
• Feeding difficulty
• Breathing difficulty
• Progressive weakness (older infant to toddler)

In a child:

• Nasal speech
• Worsening posture
• Frequent, increasingly severe respiratory infections

• A family history of neuromuscular disease
• Absent deep tendon reflexes
• Flaccid muscles
• Muscle fasciculation
• Fasciculation of tongue muscle
• Elevated CPK levels

• Muscle biopsy
• Electromyography
• MRI of the spine
• DNA testing to confirm diagnosis