1. EXAMPLES OF LESIONS
Upper Motor Neuron (UMN) Lesions
Involves the Corticospinal tracts from the Cortex to the synapse with the Anterior Horn Cell.
- Cerebral Palsy (Spastic Diplegia/Quadriplegia): Periventricular Leukomalacia (PVL) or HIE.
- Acute Transverse Myelitis: Inflammatory demyelination of the cord.
- Spinal Cord Compression: Pott’s Spine (Tuberculosis) or Spinal tumors (Ewing’s sarcoma metastasis).
- Leukodystrophies: Metachromatic Leukodystrophy (MLD), Adrenoleukodystrophy.
- Hereditary Spastic Paraparesis (HSP): Genetic degeneration of corticospinal tracts.
Lower Motor Neuron (LMN) Lesions
Involves the Anterior Horn Cell, Nerve Root, Peripheral Nerve, or Neuromuscular Junction.
- Spinal Muscular Atrophy (SMA): Anterior Horn Cell degeneration (Werdnig-Hoffmann).
- Poliomyelitis: Viral destruction of Anterior Horn Cells.
- Guillain-Barré Syndrome (GBS): Acute Inflammatory Demyelinating Polyneuropathy.
- Traumatic Neuritis: Injection palsy (Sciatic nerve injury) or Obstetric Brachial Plexus Injury (Erb’s Palsy).
- Hereditary Motor Sensory Neuropathy (HMSN): Charcot-Marie-Tooth disease.
2. CLINICAL DIFFERENTIATION
| Feature | Upper Motor Neuron (UMN) | Lower Motor Neuron (LMN) |
|---|---|---|
| Muscle Tone | Hypertonia (Spasticity): “Clasp-knife” type; velocity-dependent resistance. | Hypotonia (Flaccidity): Loss of tone due to loss of efferent supply. |
| Power/Weakness | Pyramidal Distribution: Affects functional groups (Extensors of arm, Flexors of leg). | Segmental/Focal: Affects specific muscles supplied by the damaged nerve/root. |
| Deep Tendon Reflexes | Exaggerated (Hyperreflexia): Loss of descending inhibition. Clonus may be present. | Diminished or Absent (Areflexia): Interruption of the reflex arc. |
| Superficial Reflexes | Absent: Abdominal, Cremasteric reflexes are lost. | Present: Unless the segmental nerve supplying the skin is involved. |
| Plantar Response | Extensor (Babinski Sign positive): Dorsiflexion of great toe, fanning of others. | Flexor or Absent (mute): No pathological extension. |
| Wasting (Atrophy) | Disuse Atrophy: Mild and occurs late (weeks/months). | Neurogenic Atrophy: Severe, rapid (days/weeks). |
| Fasciculations | Absent. | Present: Spontaneous twitching (classic in SMA/Polio). |