How do you differentiate upper motor neuron (UMN) lesions from lower motor neuron (LMN) lesions?

Differentiating Upper Motor Neuron (UMN) from Lower Motor Neuron (LMN) Lesions

Upper motor neuron lesions produce spasticity, hyperreflexia, clonus, and extensor plantar responses (Babinski sign), while lower motor neuron lesions cause flaccid paralysis, hyporeflexia or areflexia, fasciculations, and progressive muscle atrophy. 1, 2

Clinical Examination Findings

Lower Motor Neuron Signs

• Fasciculations are the most characteristic sign of LMN damage, appearing as spontaneous discharges of entire motor units that sound like "raindrops on a tin roof" 1, 3
• Flaccid paralysis with decreased muscle tone and weakness 1, 2
• Hyporeflexia or areflexia with diminished or absent deep tendon reflexes 1, 2, 3
• Progressive muscle weakness and atrophy due to denervation 1, 2
• Hypotonia resulting from interruption of normal neural input 3

Upper Motor Neuron Signs

• Spasticity with increased muscle tone and velocity-dependent resistance to passive movement 1, 2, 4
• Hyperreflexia manifesting as brisk or exaggerated deep tendon reflexes 1, 2, 3
• Clonus showing rhythmic muscle contractions in response to sudden, maintained stretch 1, 2, 3
• Extensor plantar response (Babinski sign) 1, 2, 4
• Clasp-knife phenomenon and flexor/extensor spasms 4

Diagnostic Testing Algorithm

Electrodiagnostic Studies (Mandatory for LMN Confirmation)

Do not rely on clinical examination alone to establish LMN involvement; electrodiagnostic studies are mandatory. 1

• EMG findings in LMN disease: Fibrillation potentials, positive sharp waves, fasciculations, and complex repetitive discharges indicating denervation 1, 2, 3
• Nerve conduction studies in LMN disease: Normal or low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 1, 2
• Motor unit firing patterns in UMN disease: Decreased coefficient of variation of firing rate, indicating reduced physiological modulation in patients with spasticity 5

Neuroimaging

• MRI brain without IV contrast is the optimal initial imaging modality for suspected motor neuron disease 1, 2
• UMN findings on MRI: Abnormal T2/FLAIR signal in the corticospinal tracts, particularly in the posterior limb of the internal capsule and cerebral peduncles 1, 2
• LMN findings on spine MRI: Abnormal T2/STIR signal in the anterior horns ("snake eyes" appearance), though this is not specific and may appear late 1, 2

Laboratory Testing

• Creatine phosphokinase (CK) should be measured when LMN involvement with weakness is identified, as it may be significantly elevated in certain conditions 1, 2, 3
• Thyroid function tests and electrolytes should be considered to exclude metabolic causes 2

Critical Pitfalls to Avoid

• Sensory symptoms should prompt reconsideration of pure motor neuron disease, as sensory pathways are not involved in typical UMN or LMN lesions 1
• UMN signs may be difficult to elicit in patients with severe LMN muscle wasting, as the pathophysiological abnormalities from combined damage can mask classical UMN signs 6
• In ALS specifically, both UMN and LMN signs are maximal in the same body region but are independent of each other in severity and distribution 7
• Motor unit firing variability tends to be greater in diseases affecting the LMN, while it is decreased in UMN lesions with spasticity 5

Distinguishing Features in Mixed Presentations

When both UMN and LMN signs coexist (as in ALS), motor neuron degeneration is a focal process at both levels that begins in the same peripheral body region and advances contiguously but separately at each level 7. The clinical syndrome depends more on the location, extent, and time since the lesion occurred than on the specific pathology 4.