UMN vs LMN Signs
Distinguishing upper motor neuron (UMN) from lower motor neuron (LMN) lesions is fundamental to neurological localisation. The patterns are opposite in almost every sign.
✦ The Mnemonic
"UMN: Upgoing toes, Up-tone, Up-reflexes; LMN: Everything Down"
UMN (cord/cortex) = spastic hypertonia + hyperreflexia · LMN (nerve/root) = flaccid hypotonia + hyporeflexia
Clinical Breakdown
UMN lesions are above the anterior horn cell — cortex, internal capsule, brainstem, or spinal cord. LMN lesions affect the anterior horn cell itself, the nerve root, or the peripheral nerve. Knowing the level determines which system is affected.
Amyotrophic lateral sclerosis (ALS/MND): mixed UMN and LMN signs in the same limb — the pathognomonic finding of motor neuron disease. Brisk reflexes in a wasted, fasciculating limb is the clinical hallmark. Differentiating ALS from pure LMN disease (Kennedy disease, multifocal motor neuropathy) has prognostic importance.
Pyramidal distribution of weakness: in UMN lesions, weakness follows a pyramidal pattern — the anti-gravity muscles are weaker. In the arm: extensors weaker (patient cannot extend the wrist/fingers against gravity). In the leg: flexors weaker (patient cannot flex the knee against gravity). This explains the hemiplegic gait (circumduction).
⭐ Clinical Pearl
Clonus: rhythmic involuntary contractions elicited by sudden passive dorsiflexion of the ankle. Sustained clonus (>5 beats) indicates UMN disease. Unsustained clonus can be normal in anxious patients. Clonus at the wrist or patella similarly indicates upper motor neuron involvement.