Today we want to explain the difference between spasticity and rigidity. Both conditions refer to hypertonia (that is, an abnormal increase in muscle tone), however, they are caused by different types of lesions.
What causes spasticity?
Spasticity is a type of condition that presents with tight and stiff muscles, exaggerated deep tendon reflexes, spasms, and cramps.
Spasticity is caused by lesions that affect the pyramidal tract, a direct pathway of the central nervous system responsible for voluntary muscle movements (such as flexing, tightening, or increasing muscle tone).
Specifically, spasticity is caused by an upper motor neuron lesion that originates in the motor cortex and reaches the spinal cord, along with a reduction in neuronal inhibition (damage to inhibitory interneurons).
The presence of excessive excitation due to the lack of an inhibitory mechanism leads to an exaggerated motor response (hypertonia), in which some muscles remain permanently contracted, mainly in the proximal and distal parts of the limbs, with a preference for the arms, hands, fingers, and knees.
The lesion of the pyramidal tract is caused by various conditions and pathologies that, among other sequelae, can also lead to spasticity:
- Traumatic brain injury
- Multiple sclerosis
- Childhood cerebral palsy
- Advanced encephalitis or meningitis
- Stroke
- Certain metabolic disorders such as adrenoleukodystrophy or phenylketonuria
What causes rigidity?
Rigidity, on the other hand, is caused by a lesion of the extrapyramidal tract. Specifically, it is an indirect motor pathway that connects with other nuclei, such as the basal ganglia. Rigidity also involves hypertonia; however, it differs from spasticity due to the increased muscle resistance to the passive movement of a body segment (arms, hands, fingers, and knees). It affects both flexor and extensor muscles.
Lesions of the basal ganglia, as seen in Parkinson’s disease, often cause rigidity along with other symptoms.
How do spasticity and rigidity differ clinically?
Clinically, spasticity is characterized by weakness, hypertonia, and an increased flexor tone that resists extension at the beginning of the movement and then suddenly gives way toward the end (the “clasp-knife” phenomenon). In other words, in cases of spasticity, a certain resistance is felt at the start of the movement, which then fades suddenly with varying intensity. For this reason, when moving a body segment (arms, hands, fingers, etc.) in a patient with spasticity, it is best to proceed slowly and gently.
On the other hand, in cases of rigidity due to extrapyramidal damage, there is a uniform and constant resistance to passive movement (known as lead-pipe rigidity) and/or intermittent resistance with a step-like release (known as cogwheel rigidity).
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