Spinal Muscular Atrophy (SMA) Types I, II, and III belong to a group of hereditary diseases that cause weakness and wasting of the voluntary muscles in the arms and legs of infants and children. The disorders are caused by an abnormal or missing gene known as the survival motor neuron gene 1 (SMN1), which is responsible for the production of a protein essential to motor neurons.
Without this protein, lower motor neurons in the spinal cord degenerate and die. The type of SMA (I, II, or III) is determined by the age of onset and the severity of symptoms. Type I (also known as Werdnig-Hoffman disease, or infantile-onset SMA) is evident at birth or within the first few months. Symptoms include floppy limbs and trunk, feeble movements of the arms and legs, swallowing and feeding difficulties, and impaired breathing. Type II (the intermediate form) usually begins 6 and 18 months of age.
Legs tend to be more impaired than arms. Children with Type II may able to sit and some may be able to stand or walk with help. Symptoms of Type III (also called Kugelberg-Welander disease) appear between 2 and 17 years of age and include difficulty running, climbing steps, or rising from a chair.
The lower extremities are most often affected. Complications include scoliosis and chronic shortening of muscles or tendons around joints.
The prognosis is poor for babies with SMA Type I. Most die within the first two years. For children with SMA Type II, the prognosis for life expectancy or for independent standing or walking roughly correlates with how old they are when they first begin to experience symptoms – older children tend to have less severe symptoms
Life expectancy is reduced but some individuals live into adolescence or young adulthood. Individuals with SMA type III may be prone to respiratory infections but with care may have a normal lifespan.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications.
In December 2016 the U.S. Food and Drug Administration approved nusinersen (Spinraza ™) as the first drug approved to treat children and adults with spinal muscular atrophy. The drug is administered by intrathecal injection into the fluid surrounding the spinal cord. It is designed to increase production of the full-length SMN protein, which is critical for the maintenance of motor neurons.
Muscle relaxants such as baclofen, tizanidine, and the benzodiazepines may reduce spasticity. Botulinum toxin may be used to treat jaw spasms or drooling. Excessive saliva can be treated with amitriptyline, glycopyolate, and atropine or by botulinum injections into the salivary glands. Antidepressants may be helpful in treating depression.
Physical therapy, occupational therapy, and rehabilitation may help to improve posture, prevent joint immobility, and slow muscle weakness and atrophy. Stretching and strengthening exercises may help reduce spasticity, increase range of motion, and keeps circulation flowing. Some individuals require additional therapy for speech, chewing, and swallowing difficulties. Applying heat may relieve muscle pain. Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs may help some people retain independence.
Proper nutrition and a balanced diet are essential to maintaining weight and strength. People who cannot chew or swallow may require insertion of a feeding tube. Non-invasive ventilation at night can prevent apnea in sleep, and some individuals may also require assisted ventilation due to muscle weakness in the neck, throat, and chest during daytime.