Lower Motor Neurone (LMN) Syndrome

Lower Motor Neurone (LMN) Syndrome

A lower motor neurone (LMN) syndrome constitutes a constellation of motor signs resulting from damage to lower motor neurone pathways, i.e., from anterior horn cell distally, encompassing the motor roots, nerve plexuses, peripheral nerves, and neuromuscular junction. Following the standard order of neurological examination of the motor system, the signs include:

• Appearance:

muscle wasting; fasciculations (or "fibrillations") may be observed or induced, particularly if the pathology is at the level of the anterior horn cell

• Tone:

reduced tone (flaccidity, hypotonus), although this may simply reflect weakness

• Power:

weakness, often marked; depending on the precise pathological process, weakness often affects both flexor and extensor muscles equally (although this is not always the case)

• Coordination:

depending on the degree of weakness, it may not be possible to comment on the integrity or otherwise of coordination in LMN syndromes; in a pure LMN syndrome coordination will be normal

• Reflexes:

depressed (hyporeflexia) or absent (areflexia); plantar responses are flexor.
It is often possible to draw a clinical distinction between motor symptoms resulting from lower or upper motor neurone pathology and hence to formulate a differential diagnosis and direct investigations accordingly. Sensory features may also be present in LMN syndromes if the pathology affects sensory as well as motor roots, or both motor and sensory fibers in peripheral nerves.

 

Cross References

Areflexia; Fasciculation; Fibrillation; Flaccidity; Hyporeflexia; Hypotonia, Hypotonus; Neuropathy; Reflexes; Upper motor neurone (UMN) syndrome; Weakness