What are the differences between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) lesions in terms of symptoms and treatment?

Upper Motor Neuron (UMN) vs Lower Motor Neuron (LMN) Lesions

The fundamental distinction is that UMN lesions produce spasticity, hyperreflexia, and extensor plantar responses, while LMN lesions cause flaccidity, hyporeflexia/areflexia, fasciculations, and muscle atrophy. 1, 2

Clinical Features of Lower Motor Neuron Lesions

Cardinal Signs

• Fasciculations are the most characteristic sign of LMN damage, appearing as spontaneous discharges of entire motor units that originate from the motor neuron or distally along the axon 2
• Muscle weakness and atrophy develop progressively due to denervation 2
• Hypotonia results from interruption of normal neural input 2
• Hyporeflexia or areflexia with diminished or absent deep tendon reflexes 2
• Flaccid paralysis characterized by decreased muscle tone with weakness 2

Pathophysiology

LMN lesions affect the cell bodies in the anterior horn of the spinal cord or the peripheral motor nerves themselves 3. The damage occurs at the level of the motor neuron cell body, its axon, or the neuromuscular junction 4.

Clinical Features of Upper Motor Neuron Lesions

Cardinal Signs

• Spasticity with increased muscle tone and velocity-dependent resistance 2
• Hyperreflexia manifesting as brisk or exaggerated deep tendon reflexes 2
• Clonus showing rhythmic muscle contractions in response to sudden, maintained stretch 2
• Extensor plantar response (Babinski sign) 4
• Weakness without significant atrophy (at least initially) 5

Pathophysiology

UMN lesions involve the corticospinal tract from the motor cortex through the internal capsule, brainstem, and spinal cord down to the anterior horn cells 1, 5. These lesions disrupt descending motor control pathways 4.

Diagnostic Approach

Electrodiagnostic Studies

Electromyography (EMG) and nerve conduction studies are essential for confirming LMN involvement. 1, 3

• LMN findings on EMG: Fibrillation potentials, positive sharp waves, fasciculations, and complex repetitive discharges indicating denervation 3, 2
• Nerve conduction studies in LMN disease: Normal or low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 3
• UMN assessment: F-wave and H-reflex studies show increased F/M amplitude ratios and disinhibition patterns even when clinical UMN signs are subtle 6

Neuroimaging

MRI of the brain without IV contrast is the optimal initial imaging modality for suspected motor neuron disease. 1

• UMN findings: Abnormal T2/FLAIR signal in the corticospinal tracts, particularly in the posterior limb of the internal capsule and cerebral peduncles 1
• LMN findings: 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, 3

Laboratory Testing

• Creatine phosphokinase (CK) should be measured when LMN involvement with weakness is identified, as it is significantly elevated (>1000 U/L) in conditions like Duchenne muscular dystrophy 4
• Rule out metabolic and infectious causes that can mimic motor neuron disease 3

Combined UMN and LMN Disease: Amyotrophic Lateral Sclerosis

ALS characteristically presents with both UMN signs (hypertonicity, hyperreflexia) and LMN signs (fasciculations, weakness, atrophy) affecting the same body regions. 1

Clinical Presentation

Motor manifestations begin focally in one body region (98% of patients), with both UMN and LMN signs maximal in the same region but independent in severity 7. The disease then spreads contiguously to adjacent body regions 7.

Diagnostic Criteria

The revised El Escorial criteria require: LMN signs, UMN signs, progressive spread within or to other regions, and absence of other disease processes 1.

Treatment Considerations

For ALS (Combined UMN/LMN Disease)

Riluzole 50 mg twice daily is the disease-modifying therapy that extends survival by approximately 60-90 days. 8

• Multidisciplinary care including neurology, pulmonology, nutrition, physical therapy, occupational therapy, speech-language pathology, social work, and palliative care improves survival and quality of life 1
• Non-invasive ventilation (NIV) for respiratory insufficiency improves median survival and quality of life 1

For Isolated LMN or UMN Syndromes

Treatment depends on the specific etiology identified. For example:

• SLE myelopathy with UMN signs: High-dose glucocorticoids combined with intravenous cyclophosphamide, initiated within hours for optimal outcomes 4
• Specific genetic forms of LMN disease: Submaximal, functional, and aerobic exercise while avoiding excessive resistive and eccentric exercise 3

Critical Pitfalls to Avoid

• Do not rely on clinical examination alone to establish anterior horn cell (LMN) involvement; electrodiagnostic studies are mandatory 3
• Sensory symptoms should prompt reconsideration of pure motor neuron disease, as sensory pathways are not involved in typical UMN or LMN lesions 1
• Maintain high suspicion for infections in patients with LMN disease, as they are at very high risk for pneumonia and respiratory failure 3
• Perform a second lumbar puncture if initial CSF cytology is negative but clinical suspicion for leptomeningeal disease remains high 4