Spasticity and rigidity are both types of increased muscle tone, but they differ in their underlying causes and how they present clinically. Spasticity is velocity-dependent, resulting from damage to the corticoreticulospinal (pyramidal) tracts, while rigidity is not velocity-dependent and arises from dysfunction of extrapyramidal pathways, most commonly the basal ganglia.
Here's a more detailed breakdown:
Spasticity
- Cause: Damage to the corticoreticulospinal (pyramidal) tracts (upper motor neuron lesion).
- Mechanism: Characterized by hyperexcitability of the stretch reflex. This means that the muscles contract more strongly in response to being stretched.
- Velocity-Dependent: The resistance to passive movement increases with the speed of the movement. Faster stretching elicits a stronger muscle contraction. Think of it like trying to quickly bend an elbow affected by spasticity; it will feel more resistant than bending it slowly.
- Clasp-Knife Phenomenon: In some cases, initial resistance is followed by a sudden release of tension during passive movement.
- Distribution: Often affects specific muscle groups, such as the flexors in the upper limbs and extensors in the lower limbs.
- Associated Conditions: Common in conditions like stroke, cerebral palsy, multiple sclerosis, and spinal cord injury.
Rigidity
- Cause: Dysfunction of the extrapyramidal pathways, most commonly the basal ganglia. Lesions of the mesencephalon and spinal cord can also cause it.
- Mechanism: Increased resistance to passive movement throughout the range of motion.
- Not Velocity-Dependent: The resistance to passive movement remains relatively constant regardless of the speed of the movement. Whether you move the limb quickly or slowly, the resistance feels similar.
- Lead-Pipe Rigidity: A constant, uniform resistance to movement throughout the entire range.
- Cogwheel Rigidity: A ratchet-like jerking quality to the resistance, often felt during passive movement, which is caused by the superimposition of tremor on the hypertonia.
- Distribution: Can affect muscles on both sides of a joint equally.
- Associated Conditions: Commonly associated with Parkinson's disease and other parkinsonian syndromes.
Table Summarizing the Differences
| Feature | Spasticity | Rigidity |
|---|---|---|
| Cause | Pyramidal tract damage | Extrapyramidal pathway dysfunction (basal ganglia) |
| Velocity Dependence | Yes (velocity-dependent resistance) | No (not velocity-dependent) |
| Description of Resistance | Clasp-knife phenomenon possible | Lead-pipe or cogwheel rigidity |
| Underlying Mechanism | Hyperexcitability of the stretch reflex | Increased muscle tone throughout range of motion |
| Common Associations | Stroke, cerebral palsy, MS, spinal cord injury | Parkinson's disease |
In summary, spasticity and rigidity are both characterized by increased muscle tone but differ in their underlying pathophysiology, clinical presentation, and associated conditions. Spasticity involves velocity-dependent resistance due to pyramidal tract damage, while rigidity involves non-velocity-dependent resistance due to extrapyramidal pathway dysfunction.
