Upper Motor Neurone (UMN) Syndrome
Upper Motor Neurone (UMN) Syndrome
An upper motor neurone (UMN) syndrome constitutes a constellation of motor signs resulting from damage to upper motor neurone pathways, i.e., proximal to the anterior horn cell. These may be termed "pyramidal signs", but since there are several descending motor pathways (e.g., corticospinal, reticulospinal, vestibulospinal), of which the pyramidal or corticospinal pathway is just one, "upper motor neurone syndrome" is preferable. "Long tract signs" may be a more accurate term, often used interchangeably with "pyramidal signs." The syndrome may be variable in its clinical features but common elements, following the standard order of neurological examination of the motor system, include:
- Appearance:
usually normal, but there may be wasting in chronic UMN syndromes, but this is usually not as evident as in lower motor neurone syndromes; contractures may be evident in chronically spastic limbs
- Tone:
hypertonus, with spasticity, clasp-knife phenomenon and sustained clonus
- Power:
weakness, often in a so-called pyramidal distribution (i.e., affecting extensors more than flexors in the upper limb, and flexors more than extensors in the lower limb); despite its clinical utility, the term pyramidal is, however, a misnomer (see Weakness)
- Coordination:
depending on the degree of weakness, it may not be possible to comment on the integrity of coordination in UMN syndromes; in a pure UMN syndrome coordination will be normal, but syndromes with both ataxia and UMN features do occur (e.g., spinocerebellar syndromes, ataxic hemiparesis syndromes)
- Reflexes:
limb hyperreflexia, sometimes with additional reflexes indicative of corticospinal tract involvement (Hoffmann’s sign, Trömner’s sign, crossed adductor reflex); Babinski’s sign (extensor plantar response); cutaneous reflexes (abdominal, cremasteric) are lost.
The most reliable ("hardest") signs of UMN syndrome are increased tone, clonus, and upgoing plantar responses.
The clinical phenomena comprising the upper motor neurone syndrome may be classified as "positive" and "negative" depending on whether they reflect increased or decreased activity in neural pathways:
- Positive:
Exaggerated stretch/tendon reflexes, flexor spasms Clonus
Autonomic hyperreflexia Contractures
- Negative:
Muscle weakness Loss of dexterity
These features help to differentiate UMN from LMN syndromes, although clinically the distinction is not always easy to make: a "pyramidal" pattern of weakness may occur in LMN syndromes (e.g., Guillain-Barré syndrome) and acute UMN syndromes may cause flaccidity and areflexia (e.g., "spinal shock").
References
Barnes MP, Johnson GR (eds.). Upper motor neurone syndrome and spasticity. Clinical management and neurophysiology. Cambridge: CUP, 2001
Cross References
Abdominal reflexes; Ataxic hemiparesis; Babinski’s sign (1); Clasp-knife phenomenon; Clonus; Contracture; Cremasteric reflex; Hoffmann’s sign; Hyperreflexia; Hypertonia, Hypertonus; Lower motor neurone (LMN) syndrome; Pseudobulbar palsy; Spasticity; Trömner’s sign; Weakness
