Spinal Muscular Atrophy (SMA) is a neuromuscular disease that causes the degeneration of motor neurons at the base of the brain and along the anterior horn of the spinal cord, causing muscle weakness.
The four main SMA forms differ for onset age, severity, development and transmission mode, but they all affect skeletal muscles of the trunk and limbs ; in general, muscles closer to the centre of the body are more affected than those further away:
- SMA I (Werdnig-Hoffmann disease): the most severe of SMA known forms, it occurs within the 6th months of age and generally has a poor prognosis of about two years.
- SMA II is an intermediate form of the disease that usually occurs between the 7th and 18th month of life. Affected children are able to maintain a sitting position and can stand up when helped, but are not able to walk.
- SMA III (Kugelberg-Welander disease) is the least severe form of SMA, occurs after the 18th month of life and has a variable symptomatology .
- SMA IV (Adult form) is the form that occurs in adulthood, usually after the fifth year of life, and the first symptoms develop during the second and third decades of life, allowing the patient to walk.
SMA is caused by a mutation of the Survival Motor Neuron (SMN), a gene expressed in all tissues. This mutation leads to motor neuron degeneration as it affects the electrical and chemical impulses directed to muscles and that are necessary for their normal functioning.
In its prevailing form SMA is an autosomal recessive disease: the defective gene is located on an autosome, and the disorder becomes manifest only when a person inherits two copies of the defective gene (one from each parent), while it remains silent when only one gene is mutated. Each set of healthy carrier parents has a 25% chance of having an affected child – that is one possibility out of 4.
SMA causes muscle weakness; it initially affects only the muscles closer to the trunk, then it extends towards peripheral areas, hindering activities such as running, walking, swallowing, and in severe cases standing upright and breathing.
In the most severe forms the weakness of respiratory muscles, together with the stiffening of thorax tissues, causes a progressive lung function impairment as well as the accumulation of secretions due to ineffective cough, which poses serious risks of lung infections.
Similarly, an acute infection increases lung secretions resulting in atelectasis and increased breathing struggle: this may lead to a disparity between ventilation and perfusion resulting, again, in respiratory failure.
Cures and Therapy
Even though this disease cannot be cured yet, targeted multidisciplinary treatments can greatly improve the patient’s life quality. The focus falls on various aspects:
- Mobility: physiotherapy and orthopaedic devices can help patients to maintain their walking ability longer and correct, along with surgical treatments, scoliosis and spinal deformities.
- Breathing: the key word here is infection prevention. In order to do this, it is necessary that airways are regularly cleared from secretions. The worsening of the disease may lead to respiratory problems that require the support of mechanical ventilation, which can be both invasive using endotracheal tube or non-invasive (NIV) via nasal or facial masks.