Spinal muscular atrophy (Werdnig-Hoffmann)

What it is

Spinal muscular atrophy refers to a group of autosomal recessive motor neuron diseases caused by apoptosis of lower motor neurons, that is congenital degeneration of the anterior horns of the spinal cord. Because only the anterior horn is affected there are LMN lesions only, with symmetric weakness. The inheritance is autosomal recessive, of a mutation in SMN1.

It belongs with the diseases of the anterior horn, alongside polio, and among the grey matter diseases it is the genetic one: anterior horn cell degeneration giving hypotonia in infants. SMA type 1 is called Werdnig-Hoffmann disease.

How it presents

Patients typically present during infancy or early childhood with progressive weakness, decreased reflexes, hypotonia and muscle atrophy. The picture that names it is the floppy baby with marked hypotonia, that is flaccid paralysis, and tongue fasciculations. Chest deformity, proximal muscle wasting and floppiness are the signs in the child.

The types are set apart by the age of onset and by what the child manages to do:

• SMA-1, Werdnig-Hoffmann disease, is the severe infantile-onset form, with onset 0-6 months, diminished foetal movements, and a child who does not sit.
• SMA-2, Kugelberg-Welander syndrome, is intermediate progressive SMA, with onset 6-18 months, in a child who can sit but never walks independently and survives to adulthood.
• SMA-3 is the mild juvenile progressive SMA, with variable onset after 18 months, difficulty running and stair-climbing, and frequent falls.
• SMA-4 is the mild adult progressive form.