How to differentiate and manage Lower Motor Neuron (LMN) vs Upper Motor Neuron (UMN) lesions in a patient with neurological symptoms?

Differentiating and Managing LMN vs UMN Lesions

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

Clinical Differentiation

Lower Motor Neuron Signs

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

Upper Motor Neuron Signs

• Spasticity with increased muscle tone and velocity-dependent resistance 1, 2
• Hyperreflexia manifesting as brisk or exaggerated deep tendon reflexes 1, 2
• Clonus showing rhythmic muscle contractions in response to sudden, maintained stretch 1, 2
• Extensor plantar response (Babinski sign) 1
• No fasciculations or muscle atrophy (these are exclusively LMN findings) 1

Diagnostic Approach

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
• Nerve conduction studies in LMN disease: Normal or low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 1
• EMG can distinguish between peripheral and central nervous system disorders affecting motor function 3

Neuroimaging

• MRI brain without IV contrast is the optimal initial imaging modality for suspected motor neuron disease 1, 4
• 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, 4, 5
• LMN findings on spine MRI: May show abnormal T2/STIR signal in the anterior horns ("snake eyes" appearance), though this is not specific and may appear late 1, 4

Laboratory Testing

• Creatine phosphokinase (CK) should be measured when LMN involvement with weakness is identified, as it is significantly elevated in conditions like muscular dystrophy 1
• Consider thyroid function tests and electrolytes to exclude metabolic causes 2

Anatomic Localization

Peripheral vs Central Disorders

• Peripheral neurological disorders (LMN) affect efferent lower motor neurons, afferent/sensory neurons, neuromuscular junctions, or muscles in myopathies 3
• Central neurological disorders (UMN) affect the firing rates of motor neurons, upper motor neurons, or central sensory pathways in the spinal cord, brainstem, or brain 3
• Vocal fold paralysis can be caused by either traumatic peripheral nerve injuries or central disorders such as brainstem stroke 3

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
• Do not miss mixed presentations: In diseases like ALS, both UMN and LMN signs coexist, with motor manifestations beginning focally in one body region and spreading contiguously 6, 7
• Focal muscle weakness or bulbar onset at baseline are most suggestive of eventual LMN involvement in patients initially presenting with only UMN signs 8
• Weight loss, reduced forced vital capacity, and progressive limb weakness predict LMN dysfunction development in patients with initially pure UMN presentations 8

Management Considerations

Multidisciplinary Care

• Multidisciplinary care including neurology, pulmonology, nutrition, physical therapy, occupational therapy, speech-language pathology, social work, and palliative care improves survival and quality of life in motor neuron diseases 1, 4

Respiratory Management

• Non-invasive ventilation (NIV) for respiratory insufficiency improves median survival and quality of life 1, 4
• Maintain high suspicion for infections in patients with LMN disease, as they are at very high risk for pneumonia and respiratory failure 1

Specific Interventions

• For SLE myelopathy with UMN signs: High-dose glucocorticoids combined with intravenous cyclophosphamide, initiated within hours for optimal outcomes 1
• For genetic forms of LMN disease: Submaximal, functional, and aerobic exercise while avoiding excessive resistive and eccentric exercise 1