Muscle Stretch Reflexes

The muscle stretch reflex, also known as the myotatic reflex, is a reflex arc that causes a muscle to contract in response to its length being changed. This reflex helps to maintain muscle tone and support the body’s posture.

To perform a muscle stretch reflex, the chosen tendon is normally struck directly and smartly with the reflex hammer. An exception is the biceps reflex, which is best tested by tapping the thumb, which has been placed over the tendon. The limb to be tested should be relaxed and in a flexed or semiflexed position.

The Jendrassik maneuver can be used during testing to enhance a muscle reflex that is difficult to elicit:

• For the upper extremity reflexes, the patient is asked to cross the ankles and then to isometrically attempt to abduct the legs.
• For the lower extremity reflexes, the patient is asked to interlock the fingers and then to isometrically attempt to pull the elbows apart.

See Also: Reflex Testing

Muscle Stretch Reflex Scales

Two muscle stretch reflex scales can be used to grade a reflex:

1. National Institute of Neurological Disorders
2. Stroke (NINDS) scale and the Mayo Clinic scale.

NINDS scale

The NINDS scale uses the following five-point grading system:

1. absent (areflexia). The absence of a reflex signifies an interruption of the reflex arc. ” 1, slight and less than normal (hyporeflexia).
2. in the lower half of normal range.
3. in the upper half of normal range (brisk).
4. enhanced and more than normal (hyperreflexive).

One of the problems with the NINDS scale is that it does not have a separate category for normal, making it necessary to choose between a low normal or a high normal.

Mayo Clinic Scale

The Mayo Clinic uses the following nine point scale:

1. Absent: –4
2. Just elicitable: –3
3. Low: –2
4. Moderately low: –1
5. Normal: 0
6. Brisk: +1
7. Very brisk: +2
8. Exhaustible clonus: +3
9. Continuous clonus: +4

An absent or exaggerated reflex is significant only when it is associated with one of the following:

1. The reflex is unusually brisk compared with reflexes from a higher spinal level.
2. The exaggerated reflexes are associated with other findings of the UMN disease.
3. The absent reflexes are associated with other findings of LMN disease.
4. The reflex amplitude is asymmetric. Reflex asymmetry has more pathologic significance than the absolute activity of the reflex—a bilateral patella reflex of 3 is less significant than a 3 on the left and a 2 on the right. Additionally, in cases where the reflex findings are symmetrical, but either elevated or depressed, further investigation is required. For example, a patient presenting with symmetrically brisk patella tendon and Achilles stretch reflexes, while simultaneously having absent stretch reflexes in the upper extremity, requires further investigation (this is a typical finding with amyotrophic lateral sclerosis or Lou Gehrig disease, a mixed UMN and LMN pathology).

Muscle Reflex Testing Findings

The findings from the muscle reflex testing can occur as a generalized, or local, phenomenon:

1. Generalized hyporeflexia. The causes of generalized hyporeflexia run the gamut from neurologic disease, chromosomal metabolic conditions, and hypothyroidism to schizophrenia and anxiety.
2. Nongeneralized hyporeflexia. Generally, an asymmetrically depressed or absent reflex is suggestive of pathology that is impacting the reflex arc directly, such as a LMN lesion or sensory paresis, which may be segmental (root), multisegmental (cauda equina), or nonsegmental (peripheral nerve). Nongeneralized hyporeflexia can result from peripheral neuropathy, spinal nerve root compression, and cauda equina syndrome. It is thus important to test more than one reflex and to evaluate the information gleaned from the examination, before reaching a conclusion as to the relevance of the findings.

In those situations demonstrating an elevated or brisk reflex, the CNS’s normal role of integrating reflexes may have been disrupted, indicating an UMN lesion, such as a brain stem or cerebral impairment, spinal cord compression, or a neurologic disease. However, the distinction has to be made between a brisk reflex and the one that is hyperreflexive.

True neurological hyperreflexia contains a clonic component and is suggestive of CNS (UMN) impairment. The clinician also should note any additional recruitment that occurs during the reflex contraction of the target. A brisk reflex is a normal finding, provided that it is not masking a hyperreflexia caused by an incorrect testing technique. Unlike hyperreflexia, a brisk reflex does not have a clonic component.

As with hyporeflexia, the clinician should assess more than one reflex before coming to a conclusion about a hyperreflexia. The presence of an UMN impairment can be confirmed by the presence of the pathologic reflexes (see next section).

References & More

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