Umn vs lmn signs 5 mark qution medicine mbbs
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UMN vs LMN Signs - 5 Mark Answer
Anatomy Basis
The motor system has two neurons in series:
- Upper Motor Neuron (UMN): Cell body in motor cortex → travels via corticospinal (pyramidal) tract → synapses on anterior horn of spinal cord
- Lower Motor Neuron (LMN): Cell body in anterior horn of spinal cord → travels via peripheral nerve → ends at neuromuscular junction / skeletal muscle
Comparison Table (Most Important for Exam)
| Feature | UMN Lesion | LMN Lesion |
|---|---|---|
| Weakness | Yes (spastic paresis) | Yes (flaccid paralysis) |
| Muscle Tone | Increased (spasticity) | Decreased (flaccidity/hypotonia) |
| Reflexes (DTR) | Increased (hyperreflexia) | Decreased/Absent (hyporeflexia) |
| Atrophy | No (mild disuse only) | Yes (marked, early) |
| Fasciculations | Absent | Present |
| Plantar Reflex | Extensor (Babinski positive) | Flexor (normal) or absent |
| Clonus | Present | Absent |
| Hoffmann's Sign | Present (upper limb) | Absent |
Detailed Features
UMN Signs - "SHIRE"
- Spasticity - increased tone (clasp-knife rigidity)
- Hyperreflexia - brisk deep tendon reflexes
- Inverted plantar - Babinski sign (extensor plantar response)
- Reflexes abnormal - Hoffmann's sign, jaw jerk, sustained clonus
- Extensor posturing in severe cases
Mechanism: Loss of descending inhibitory pathways leads to increased excitability of anterior horn motor neurons → brisk reflexes + spasticity.
Important: In acute UMN lesions (e.g. spinal shock, acute stroke), there is initially flaccid paralysis with decreased tone and reflexes - spasticity develops hours to months later.
LMN Signs - "FAWDA"
- Fasciculations - spontaneous twitching of motor units (pathognomonic of LMN/anterior horn cell lesion)
- Atrophy - early, marked wasting due to denervation
- Weakness - flaccid type
- Decreased tone (flaccidity)
- Areflexia - absent deep tendon reflexes
Mechanism: Denervation of muscle fiber causes atrophy. Spontaneous firing of degenerating motor neurons causes fasciculations.
Pathological Reflexes in UMN Lesions
| Reflex | Method | UMN Response |
|---|---|---|
| Babinski sign | Stroking lateral sole of foot | Extension of big toe + fanning of others |
| Hoffmann's sign | Flicking distal middle finger | Flexion of thumb + index finger |
| Clonus | Sudden passive dorsiflexion of ankle | Rhythmic oscillations |
Causes
| UMN Lesion | LMN Lesion | |
|---|---|---|
| Location | Above anterior horn (brain/spinal cord) | At/below anterior horn (AHC, nerve root, peripheral nerve, NMJ) |
| Examples | Stroke, MS, tumor, cervical myelopathy, head injury | Polio, GBS, peripheral neuropathy, MND, disc herniation |
Mixed UMN + LMN
Amyotrophic Lateral Sclerosis (ALS / Motor Neuron Disease) - classic example with BOTH upper and lower motor neuron signs present simultaneously (hyperreflexia + fasciculations + atrophy).
Sources: Neuroanatomy through Clinical Cases, 3rd Ed. - Table 6.4; Ganong's Review of Medical Physiology, 26th Ed.
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