How are motor neuron diseases (MNDs) classified?

Updated: Sep 06, 2019
  • Author: Carmel Armon, MD, MSc, MHS; Chief Editor: Nicholas Lorenzo, MD, MHCM, CPE, FAAPL  more...
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Motor neuron diseases (MNDs) are progressive degenerative diseases in which death of the cell bodies of motor neurons is the primary process. These should be distinguished from diseases in which primarily the axons of motor neurons are affected. The traditional classification of MNDs is according to the affected cell types, as follows:

  • Upper motor neurons alone - PLS

  • Lower motor neurons alone - Progressive muscular atrophy (PMA) and spinal muscular atrophies (SMAs)

  • Upper and lower motor neurons - ALS

ALS is the most common of the MNDs. In British-English–speaking areas, ALS is often called motor neurone disease (singular), but this chapter reserves the term MNDs (usually in plural form) as an umbrella term. Therefore, not every MND is ALS.

Patients with PLS occasionally have mild, nonspecific, and nonprogressive findings of denervation on electrodiagnostic testing. The severity of the denervation and reinnervation does not resemble that seen in ALS and does not justify these patients' being classified as having ALS. These patients may be concerned that their PLS eventually could evolve into ALS. Although absolute guarantees cannot be given, some measure of reassurance may be derived from the overall slow progression in these patients.

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