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Spastic hypertonia: mechanisms and measurement.

R T Katz¹, W Z Rymer

¹Rehabilitation Institute of Chicago, IL 60611-4496.

Archives of Physical Medicine and Rehabilitation

|February 1, 1989

Summary

Spastic hypertonia, a condition of increased muscle tone, may stem from a lower reflex threshold rather than increased reflex gain. Understanding this helps differentiate it from other upper motor neuron syndrome symptoms.

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Area of Science:

Neuroscience
Motor Control
Clinical Neurology

Background:

Spastic hypertonia is a key feature of the upper motor neuron syndrome, characterized by velocity-dependent increases in muscle tone.
Its underlying mechanisms, involving muscle or reflex properties, have been debated, particularly concerning reflex threshold and gain.
Previous understanding suggested increased reflex gain, but recent evidence points to altered reflex thresholds.

Purpose of the Study:

To clarify the mechanisms contributing to spastic hypertonia.
To differentiate between changes in stretch reflex threshold and reflex gain in spasticity.
To contextualize spastic hypertonia within the broader upper motor neuron syndrome.

Main Methods:

Review of clinical scales, biomechanical paradigms, pendulum models, and electrophysiologic studies.
Analysis of parameters affecting the stretch reflex, including set point (threshold) and gain.
Comparison of different methodologies for quantifying spastic hypertonia.

Main Results:

Recent investigations suggest spastic hypertonia may result from a decreased stretch reflex threshold.
This contrasts with earlier beliefs that an increased reflex gain was the primary cause.
Biomechanical methods show strong correlation with the clinical presentation of spastic hypertonia.

Conclusions:

Spastic hypertonia's primary mechanism may involve a reduced stretch reflex threshold.
Distinguishing between threshold and gain alterations is crucial for understanding spasticity.
Other upper motor neuron syndrome symptoms like weakness and loss of dexterity can be more debilitating than hypertonia.

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