Scapuloperoneal spinal muscular atrophy (SPSMA) is a rare, inherited neuromuscular disorder affecting the nerve cells that control muscle movement. As a form of spinal muscular atrophy, it is distinguished by progressive weakness and wasting in the muscles of the shoulder blades and lower legs. The onset of symptoms can range from birth to early childhood, though some individuals may notice milder symptoms in adulthood.
Defining Characteristics and Symptoms
The “scapulo” component of the name refers to the shoulder girdle. Weakness in the muscles that stabilize the scapula (shoulder blade) is a primary sign of the disorder. This often leads to a “winged scapula,” where the shoulder blades protrude from the back when raising the arms or pushing against a wall. This protrusion occurs because the muscles that normally hold the scapula flat against the rib cage have atrophied.
The “peroneal” aspect relates to the peroneal muscles on the outer side of the lower leg, which are responsible for lifting the foot and ankle. When these muscles weaken, it results in foot drop, where an individual has difficulty lifting the front part of the foot. To compensate, people with foot drop often adopt a “steppage” gait, lifting their knee higher than normal to clear the foot from the ground.
Beyond weakness in the shoulders and lower legs, other symptoms can vary among individuals. Some may experience weakness in the small muscles of the hands and wasting of the pectoral muscles. In some cases, vocal cord paralysis may occur, leading to a hoarse voice or breathing difficulties. The specific presentation and severity can differ significantly from person to person, even within the same family.
Genetic Origins and Inheritance
SPSMA is a genetic condition most commonly caused by mutations in the TRPV4 gene, located on chromosome 12. The TRPV4 gene provides instructions for making a protein that forms a channel, allowing calcium ions to flow into cells. This channel is involved in various cellular functions, including responding to physical stimuli and maintaining osmotic pressure.
Mutations in the TRPV4 gene alter the function of this channel, leading to the progressive loss of motor neurons, which are the nerve cells in the spinal cord that control muscle movement. While the precise way these mutations cause nerve cell degeneration is still under investigation, it results in the muscle weakness and atrophy seen in SPSMA. The condition is passed down through an autosomal dominant inheritance pattern.
Autosomal dominant inheritance means a person only needs to inherit one copy of the mutated gene from one parent to develop the disorder. A parent with the TRPV4 mutation has a 50% chance of passing the gene to each child. It is also possible for the condition to arise from a new (de novo) mutation in an individual with no family history of the disorder.
The Diagnostic Process
The diagnostic process begins with a clinical evaluation. A physician will take a detailed family history to identify any pattern of similar symptoms across generations. This is followed by a physical examination to assess muscle strength and identify the specific pattern of weakness, such as winged scapula and foot drop, along with any muscle wasting.
To investigate the muscle weakness further, physicians use electrodiagnostic tests like electromyography (EMG) and nerve conduction studies (NCS). An EMG records a muscle’s electrical activity to show if the weakness is caused by a problem with the muscle or the nerve supplying it. An NCS measures how well electrical signals travel along a nerve, helping to confirm the issue originates from the motor neurons.
Genetic testing provides a definitive diagnosis. A blood sample is analyzed for a mutation in the TRPV4 gene or other genes associated with scapuloperoneal syndromes. Identifying a specific mutation distinguishes SPSMA from other neuromuscular disorders with similar symptoms, such as certain types of muscular dystrophy or Charcot-Marie-Tooth disease.
Management and Therapeutic Approaches
There is no cure for SPSMA, so management focuses on supportive care to maintain function and quality of life. Physical and occupational therapy are primary components of this approach. Physical therapists design exercise programs to maintain muscle strength and joint flexibility, while occupational therapists help individuals adapt daily activities and conserve energy.
Assistive devices play a large part in managing symptoms, particularly foot drop. Ankle-foot orthoses (AFOs) are braces worn on the lower leg and foot to provide stability and prevent the foot from dragging. These devices can improve gait, reduce the risk of falls, and as the condition progresses, other mobility aids like canes or wheelchairs may be recommended.
Regular monitoring by a healthcare team helps manage the condition long-term. This includes tracking the progression of muscle weakness and addressing complications. For instance, individuals with vocal cord weakness may need a specialist for voice and swallowing issues. In some cases, respiratory function is monitored if breathing muscles are affected.
Long-Term Outlook and Progression
SPSMA is a slowly progressive disorder, but the rate at which symptoms worsen varies greatly. Some individuals experience only mild weakness throughout their lives, while others face more significant functional limitations over time. This variability can be seen even within the same family, where one person might have a severe, early-onset form while another has a milder presentation in adulthood.
The progression often occurs over decades, allowing individuals time to adapt to changes in mobility. The disease can sometimes appear more severe in successive generations, a phenomenon known as genetic anticipation. Life expectancy is not significantly affected by SPSMA itself.