Neuromuscular disorders that present in the newborn period with hypotonia and weakness are caused by a variety of conditions that affect the central nervous system (brain or spinal cord), peripheral nervous system, or skeletal muscle. Conditions that affect the anterior horn cells of the spinal cord are listed in the table (table 1).
This topic will review clinical aspects of spinal muscular atrophy.
Spinal muscular atrophy (SMA) disorders are characterized by degeneration of the anterior horn cells in the spinal cord and motor nuclei in the lower brainstem. These diseases are classified as types 1 through 4 depending upon the age of onset and clinical course.
Classification — SMA type 1, also known as infantile spinal muscular atrophy or Werdnig-Hoffmann disease, is the most common and severe type of SMA. It typically presents in the neonatal period. However, mothers of affected patients may recognize a decrease or loss of fetal movement in late pregnancy. Some experts classify prenatal onset as SMA type 0 [1,2]. In these neonatal forms, symptoms progress rapidly, and the majority of infants die before one year of age from respiratory failure [3,4]. Nevertheless, long-term survivors have been reported [5-7]. This is perhaps due, in part, to advances in the care of chronic respiratory insufficiency and to more aggressive care. (See 'Management' below.)
SMA 2 (intermediate form) and SMA 3 (mild form; Kugelberg-Welander disease) have a later onset and a less severe course [8,9]. SMA 2 presents between 3 and 15 months of age, whereas SMA 3, the least severe, typically presents with signs of weakness at or after one year of age and progresses to a chronic course. In a study of children and adolescents with SMA 2 and SMA 3, muscle strength was reduced to a variable extent . Although the muscle weakness affected motor function, walking, transfer from lying or sitting to the standing position, and stair-climbing were possible in some children. The outcome depends primarily upon the severity of muscle weakness at presentation rather than the age of onset, but earlier onset tends to correlate with greater weakness .