Spinal muscular atrophy

What it is

Spinal muscular atrophy is a group of autosomal recessive motor neuron diseases caused by apoptosis of lower motor neurons, that is degeneration of the anterior horn cells. It is a congenital degeneration of the anterior horns of the spinal cord, so the lesions are LMN only and the weakness is symmetric. The inherited mutation is in SMN1, and type 1 is called Werdnig-Hoffmann disease.

How it presents

The infant is a floppy baby with marked hypotonia and flaccid paralysis, and the tongue shows fasciculations. Presentation is during infancy or early childhood, with progressive weakness, decreased reflex, hypotonia and muscle atrophy. Three signs are named in such a child: chest deformity, proximal muscle wasting and floppiness.

Four types are set out by onset and by what the child can do.

• Type 1, Werdnig-Hoffmann disease, is the severe infantile onset form: onset 0 to 6 months, diminished foetal movements, a child who does not sit, and death from respiratory failure within 12 months.
• Type 2, named Kugelberg-Welander syndrome here, is the intermediate progressive form: onset 6 to 18 months, a child who can sit but never walks independently, and survival to adulthood.
• Type 3 is the mild juvenile progressive form: variable onset after 18 months, difficulty running and stair-climbing, and frequent falls.
• Type 4 is the mild adult progressive form.