What is the Difference Between Rigidity and Spasticity?

Rigidity and spasticity are two distinct types of hypertonia that arise from different anatomical causes and have different characteristics. Here are the main differences between them:

Rigidity:

• Caused by dysfunction of extrapyramidal pathways, most commonly the basal ganglia, but also as a result of lesions of the mesencephalon and spinal cord.
• Seen in extrapyramidal lesions, such as Parkinson's disease, and other conditions like neuroleptant malignant syndrome or stiff man syndrome.
• Subtypes include cogwheel rigidity (Parkinson's) and lead pipe rigidity (neuroleptant malignant syndrome or stiff man syndrome).
• Affects antagonistic muscle groups equally.
• Plantar reflexes are normal.
• No specific associated neurological signs, such as tremor at rest or clonus.

Spasticity:

• Caused by lesions in the pyramidal tract (i.e., upper motor neurons), such as the corticospinal tract.
• Common causes include stroke, spinal cord compression, and motor neuron disease.
• Characterized by abnormally high muscle tone, which often asymmetrically affects antagonistic muscle groups.
• Velocity-dependent, meaning it is more noticeable with fast movements.
• Associated with specific neurological signs, such as tremor at rest, clonus, and extensor toe sign.
• Hyperactivity in muscle stretch reflexes.

Being able to differentiate between rigidity and spasticity is crucial for clinicians, as it provides important information about the underlying anatomical causes and helps guide appropriate treatment and management.

Comparative Table: Rigidity vs Spasticity

Here is a table comparing the differences between rigidity and spasticity:

In summary, rigidity is characterized by a constant resistance throughout the range of motion, while spasticity is velocity-dependent and occurs only during muscle stretch. Rigidity is usually present at rest and in all directions of movement, whereas spasticity is present in one direction (flexion or extension) and is length-dependent. Rigidity is typically associated with extrapyramidal lesions, while spasticity is caused by pyramidal tract lesions.