Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting. It is usually diagnosed in infancy or early childhood and if left untreated it is the most common genetic cause of infant death. It may also appear later in life and then have a milder course of the disease. The common feature is progressive weakness of voluntary muscles, with arm, leg and respiratory muscles being affected first. Associated problems may include poor head control, difficulties swallowing, scoliosis, and joint contractures.The age of onset and the severity of symptoms form the basis of the traditional classification of spinal muscular atrophy into a number of types.Spinal muscular atrophy is due to an abnormality (mutation) in the SMN1 gene which encodes SMN, a protein necessary for survival of motor neurons. Loss of these neurons in the spinal cord prevents signalling between the brain and skeletal muscles. Another gene, SMN2, is considered a disease modifying gene, since usually the more the SMN2 copies, the milder is the disease course. The diagnosis of SMA is based on symptoms and confirmed by genetic testing.Usually, the mutation in the SMN1 gene is inherited from both parents in an autosomal recessive manner, although in around 2% of cases it occurs during early development (de novo).
Individuals with type 1 suffer chronic attacks in spite of ongoing medical therapy, while those with type 2 experience sudden, acute and even potentially life-threatening attacks even though otherwise their asthma seems well managed.When first defined by Margaret Turner-Warwick in 1977, the term brittle asthma was used specifically to describe type 1, but as studies into the phenotype were conducted the second type was also distinguished. Treatment
In addition to any issues of treatment compliance, and maximised corticosteroids (inhaled or oral) and beta agonist,  brittle asthma treatment also involves for type 1 additional subcutaneous injections of beta2 agonist and inhalation of long acting beta-adrenoceptor agonist,  whilst type 2 needs allergen avoidance and self-management approaches. Since catastrophic attacks are unpredictable in type 2, patients may display identification of the issue, such as a MedicAlert bracelet, and carry an epinephrine autoinjector. Epidemiology
The condition is rare. 1999s Difficult Asthma estimates a prevalence of approximately 0.05% brittle asthma sufferers among the asthmatic population. Though found in all ages, it is most commonly found in individuals between the ages of 18 and 55; it is present in both sexes, though type 1 has been diagnosed in three times as many women as men. Hospitalization is more frequent for type 1 than type 2. == References ==