Spinal muscular atrophy type 2 (SMA II) is an inherited condition that affects the muscles. It is characterized primarily by progressive muscle weakness that develops in children between ages 6 and 12 months. Affected children can sit without support; however, they cannot stand or walk unaided. Other signs and symptoms may include a tremor of the fingers, breathing issues, feeding difficulties and skeletal abnormalities (such as scoliosis and hip dislocation). SMA II is caused by mutations in the SMN1 gene and is inherited in an autosomal recessive manner. Treatment is based on the symptoms in each person.

Symptoms:

The signs and symptoms of spinal muscular atrophy type 2 (SMA II) typically become apparent between 6 and 12 months of age. Poor muscle tone may be noticed at birth or within the first few months of life. Affected children may initially slowly gain some motor milestones. However, the highest motor milestone attained is generally the ability to sit independently, and this milestone is often lost by the mid-teens. People with SMA II are not able to stand or walk unaided. Other signs and symptoms may include a tremor of the fingers, breathing issues, feeding difficulties and skeletal abnormalities (such as scoliosis and hip dislocation).

Causes:

Spinal muscular atrophy type 2 (SMA II) is caused by changes (mutations) in the SMN1 gene. Extra copies of the SMN2 gene affect how severe the condition is. These genes encode a protein that is important for the normal functioning of certain nerve cells (called motor neurons) which help control muscle movements. Mutations in the SMN1 gene lead to reduced levels of this protein and the death of motor neurons. This in turn causes the characteristic signs and symptoms associated with SMA II.

The protein made by additional copies of SMN2 can compensate for some of the protein lost due to mutations in SMN1. Affected people who have extra copies of SMN2 may, therefore, have milder symptoms and develop the condition later in life.

Diagnosis:

A diagnosis of spinal muscular atrophy (SMA) is first suspected based on the presence of characteristic signs and symptoms. Identifying mutations in the SMN1 gene confirms the diagnosis.^[1]

Classifying the type of SMA is based on the age of onset and the maximum function attained. Classisfying SMA as type 2 is based on:

• onset of muscle weakness usually after age six months; ability to sit independently achieved when placed in a sitting position
• finger trembling – almost invariably present
• low muscle tone (flaccidity)
• absence of tendon reflexes in approximately 70% of individuals
• average intellectual skills during the formative years and above average by adolescence

The treatment of spinal muscular atrophy type 2 (SMA II) is based on the symptoms present. Physical therapy, occupational therapy and assistive devices (walkers, wheelchairs, etc) are generally recommended to help encourage maximum mobility and independence. These interventions may also prevent or delay scoliosis and abnormal contractions of the muscles and tendons. Some children with SMA II may have difficulty eating enough calories to maintain a normal weight. In these cases, nutritional counseling and/or a feeding tube may become necessary. Breathing problems are also common in people affected by SMA II and may require the use of BiPAP machines or other methods of respiratory support. Some affected children may require surgery to treat scoliosis or severe cases of hip dislocation.