What is the difference between upper motor neuron pathology and lower motor neuron pathology?

Understanding Upper vs. Lower Motor Neuron Pathology

Upper motor neuron (UMN) pathology affects neurons in the brain and spinal cord that control voluntary movement, while lower motor neuron (LMN) pathology affects neurons in the spinal cord's anterior horn and peripheral nerves that directly innervate muscles—the key distinction is that UMN lesions cause spasticity and hyperreflexia, whereas LMN lesions cause flaccidity and hyporeflexia. 1, 2

Anatomical Location

Upper Motor Neurons:

• UMN cell bodies reside in the motor cortex of the brain and send axons down through the corticospinal tract to synapse with lower motor neurons in the spinal cord's anterior horn 1, 3
• These neurons are part of the central nervous system and control voluntary movement initiation and resting muscle tone 3
• The corticospinal tract descends through the internal capsule, cerebral peduncles, and brainstem before reaching the spinal cord 4, 5

Lower Motor Neurons:

• LMN cell bodies are located in the anterior horn of the spinal cord (also called anterior horn cells) 6
• Their axons exit the spinal cord as peripheral nerves and directly innervate skeletal muscle fibers 1
• Each LMN and all the muscle fibers it innervates constitute a "motor unit" 1

Clinical Signs: The Critical Difference

Upper Motor Neuron Signs (think "increased" or "exaggerated"):

• Spasticity: velocity-dependent increase in muscle tone with resistance to passive movement 1, 2
• Hyperreflexia: brisk or exaggerated deep tendon reflexes 1, 2
• Clonus: rhythmic muscle contractions in response to sudden, maintained stretch 2
• Extensor plantar response (Babinski sign): upgoing toe when the sole of the foot is stroked 3
• Weakness without muscle atrophy (at least initially, since the muscle itself is not denervated) 3

Lower Motor Neuron Signs (think "decreased" or "absent"):

• Flaccid paralysis: decreased muscle tone with weakness 1, 2
• Hyporeflexia or areflexia: diminished or absent deep tendon reflexes 1, 2, 7
• Muscle atrophy: progressive loss of muscle bulk due to denervation 1, 2
• Fasciculations: visible spontaneous muscle twitches that look like "raindrops on a tin roof" on EMG, representing spontaneous motor unit discharges 1, 2
• Fibrillation potentials: spontaneous discharges of individual muscle fibers seen on EMG, indicating denervation 1

Why These Differences Occur

Upper Motor Neuron Pathology:

• Loss of descending inhibitory control from the brain leads to uninhibited spinal reflexes, causing hyperreflexia and spasticity 3
• The muscle itself remains innervated by intact lower motor neurons, so no atrophy occurs initially 3

Lower Motor Neuron Pathology:

• Direct loss of nerve supply to muscles causes denervation, leading to muscle atrophy and weakness 1, 2
• Without functioning motor neurons, reflex arcs cannot be completed, causing hyporeflexia or areflexia 1, 2
• Dying motor neurons generate spontaneous discharges, producing fasciculations 1, 2

Diagnostic Approach

Clinical Examination:

• Assess muscle tone by passively moving limbs—increased tone suggests UMN, decreased tone suggests LMN 1
• Test deep tendon reflexes—hyperreflexia indicates UMN, hyporeflexia/areflexia indicates LMN 1, 2
• Look for muscle atrophy and fasciculations, which indicate LMN involvement 1, 2
• Check for Babinski sign (extensor plantar response), which indicates UMN pathology 3

Electrodiagnostic Studies:

• EMG and nerve conduction studies are essential for confirming LMN involvement 1, 6, 4
• EMG in LMN disorders shows fibrillation potentials, positive sharp waves, and fasciculations 1, 2
• EMG cannot directly assess UMN function but helps exclude LMN pathology 8

Neuroimaging:

• MRI of the brain without contrast is optimal for suspected UMN disorders, showing T2/FLAIR hyperintensity along the corticospinal tracts 4, 5
• Spinal MRI may show "snake eyes" appearance (T2 hyperintensity in anterior horns) in some LMN disorders like ALS 4, 5

Laboratory Testing:

• Measure creatine phosphokinase (CK), which may be markedly elevated (>1000 U/L) in muscular dystrophies, a type of LMN disorder 1

Common Disease Examples

Pure Upper Motor Neuron Disorders:

• Primary lateral sclerosis: affects only UMN, causing progressive spasticity and weakness without atrophy 4, 7
• Hereditary spastic paraplegia: genetic disorder causing progressive leg spasticity 3
• Multiple sclerosis with corticospinal tract involvement 3, 5

Pure Lower Motor Neuron Disorders:

• Spinal muscular atrophy: genetic disorder causing anterior horn cell degeneration 6, 4
• Progressive muscular atrophy: affects only LMN, causing progressive weakness and atrophy 4
• Poliomyelitis and post-polio syndrome: viral destruction of anterior horn cells 6
• Peripheral neuropathies (though these affect nerves outside the spinal cord, not anterior horn cells) 6

Mixed Upper and Lower Motor Neuron Disorders:

• Amyotrophic lateral sclerosis (ALS): the most common motor neuron disease, affecting both UMN and LMN simultaneously 4, 9, 3
• ALS patients show both spasticity/hyperreflexia (UMN) and fasciculations/atrophy (LMN) 4, 7, 10

Critical Clinical Pitfalls

• Don't confuse anterior horn cell disorders with peripheral neuropathies: Anterior horn cell disease (like spinal muscular atrophy) is technically a CNS disorder affecting LMN cell bodies, while peripheral neuropathy affects the peripheral nerves themselves 6
• Sensory symptoms point away from pure motor neuron disease: If sensory deficits are present, consider alternative diagnoses like peripheral neuropathy or spinal cord compression 4
• UMN signs can be masked by severe LMN involvement: In advanced ALS, profound muscle atrophy may make it difficult to elicit hyperreflexia, even though UMN pathology is present 7, 8
• Fasciculations are the hallmark LMN sign: When you see fasciculations, think LMN pathology first—they are considered a "red flag" for disorders like ALS or spinal muscular atrophy 2
• Weight loss in motor neuron disease suggests LMN involvement: Even when controlled for dysphagia and muscle atrophy, weight loss is more prominent in patients with LMN signs 7