UMN lesions

Damage to motor pathways anywhere from the precentral gyrus of the frontal cortex, to the internal capsule, brainstem, cord, anterior horn cells in the cord.

UMN lesions affect muscle groups, not individual muscles.

Weakness is typically 'pyramidal' in distribution.

• UL extensors are weak.
• LL flexors are weak.
• No muscle wasting.
• Loss of fine finger movements may be greater than expected from the overall grade of weakness.
• Spasticity in stronger muscles. Spasticity = increased tone that is velocity-dependent (in rigidity, increased tone is independent of velocity - it is constant throughout passive movement) and non-uniform (resistance to passive movement that can suddenly be overcome - clasp-knife feel).
• Hyperreflexia (reflexes are brisk).

The reflexes in UMN lesions:
- Babinski sign positive (plantars are upgoing).
- Clonus.
- Hoffman's reflex positive. Neck extension is said to increase sensitivity of this test.

NB: UMN lesions can mimic LMN lesions in the first few hours before the spasticity and hyperreflexia develop.