Brachioradialis Reflex

Issues of Concern

Reflex Mechanism

A reflex occurs when a force, in this context a tap, stretches the muscle-tendon. Stretch receptors within the muscle spindle become activated through the stretching of the muscle. This action then initiates impulse conduction along a reflex arc consisting of an afferent signal to the spinal cord and an efferent signal to the muscle, which causes the muscle to twitch.[5] 1a afferent neurons carry the afferent portion of the arc to the dorsal root ganglion. Once in the spinal cord, the 1a afferent neurons directly synapse on an alpha motor neuron, which transmits the efferent impulse back to the brachioradialis muscle, resulting in a muscle twitch.[12]

Reflexes, however, are not solely dependent upon excitation; inhibition is also necessary for a reflex to occur. Specifically, a signal branch from the 1a afferent neuron to an inhibitory interneuron is activated to inhibit the opposing muscle group during the reflex. The descending corticospinal tract provides additional modulation of the monosynaptic junction of the 1a afferent neuron and the motor neuron, which, in most cases, dampens the reflex response. Due to the involvement of the descending corticospinal tract, a reflex divides into lower and upper motor neuron components. The lower motor neuron components include the peripheral nerves and spinal segment, whereas the upper motor neuron portion consists of the descending corticospinal tract. Injury to the lower or upper motor neuron portion can result in pathology.[12]

The foundation of reflexes is the propagation of vibration waves from the point of impact to the stretch receptors within the muscle spindle. Failure of this to occur means that either the muscle was unable to sense the impact or the muscle was unable to propagate the signal. In either situation, the result is a lack of muscle response as no sensory neuron, subsequent reflex arcs, or motor neurons were activated. One study exploring this phenomenon in the brachioradialis muscle found that a gamma efferent block created by procaine injection could decrease the excitability of muscle spindles. This finding is important when conducting procedures on the muscle itself.[13]

Testing

The evocation of deep tendon reflexes forms a crucial part of the neurological examination as it reveals information about the status of the portions of the nervous system that contribute to the reflexes tested.[12] Additionally, reflex testing is a part of the clinical examination for myelopathy. Specifically, this examination includes the Hoffman test, clonus, deep tendon reflex testing, Babinski sign, inverted supinator sign, hand withdrawal reflex testing, and suprapatellar quadriceps reflex testing.[14]

For the testing of the brachioradialis reflex, the examiner places the patient in a seated position. From there, the clinician uses his or her forearm to support the patient’s forearm in a slightly pronated position. The physician supports the patient’s forearm rather than asking the patient to maintain the position to achieve muscle relaxation. Once in position, the physician delivers a series of quick hits to the area of the styloid process of the radius at the point of brachioradialis insertion.[14]

Interpretation of the test depends upon muscle movement observed due to the striking of the brachioradialis tendon. The striking of the muscle tendon should produce flexion and supination of the forearm. In cases where it does so, that would be considered a negative test. Either finger flexion or slight elbow extension indicates a positive test.[14] If the test is positive due to finger flexion, a hyperactive finger jerk reflex is indicated, and if the test is positive due to slight elbow extension, then a hyperactive biceps reflex is indicated. There is no consensus regarding the significance of a positive brachioradialis reflex in asymptomatic individuals.[15]

Elicitation of the brachioradialis reflex can pose some difficulty due to the tendon's insertion point. Generally, however, the aim is to strike the tendon perpendicularly to the plane of the hammer. Again, this should occur in the area of the styloid process of the radius, which is the point of insertion of the brachioradialis tendon. Many tools for eliciting reflexes exist, including specialized and improvised hammers. There are 3 groups of specialized hammers: triangular in shape, T-shaped, and circular. For eliciting the brachioradialis reflex, there is no preference given to the type of hammer utilized other than ensuring that it has a flat edge to strike the tendon.[12]

Grading

Reflex responses are graded based on the amplitude. Numerous scales have been applied to the grading of reflexes; however, a commonly used scale is the NINDS Muscle Stretch Reflex Scale, which is empirically supported. This is a 4-point scale ranging from 0 to 4. A score of 0 indicates the reflex is absent. A score of 1 indicates that a trace response of the reflex is present with reinforcement. A score of 2 indicates that a reflex is present; however, its amplitude is within the lower half of the normal range. A present reflex that falls within the upper half of the normal range warrants a score of 3. An enhanced reflex, meaning that the response is greater than normal, is equivalent to a score of 4; this could include clonus if present.[12]

Absence of Reflex

Compression of the C5 and C6 spinal nerves results in the loss of contraction of both the biceps and the brachioradialis muscles. As a result, the patient can lose their brachioradialis. While the lesion at C5 to C6 eliminates the brachioradialis reflex through a lower motor neuron lesion, it also exaggerates all reflexes below that level. These reflexes undergo stimulation via an upper motor neuron, including the finger flexion reflexes caused by C8. If C5 and C6 are intact, yet neither the biceps nor the brachioradialis can contract, there must be a lesion to the anterior horn cell unless the damage affects the reflex arc. If the damage is to the anterior horns rather than the spinal nerves, then the reflex contraction is reduced proportionately to the reduction seen in muscle power.[13]

Clinical Significance

Brachioradialis reflex testing can constitute an important portion of the neurological exam. For one, in cases where the patient has hyperreflexia of the brachioradialis, it may point to hyperactive stretch reflexes caused by an upper motor lesion.[16] Under normal function, the cerebral cortex sends inhibitory impulses to the spinal cord to dampen reflexes. In the case of hyperreflexia, however, the cerebral cortex's inhibitory impulses are disrupted, causing a state of hyperactive reflexes. When hyperreflexia affects the brachioradialis, the wrist is supinated, and finger flexion is observed. Wrist supination is the normal brachioradialis reflex; however, finger flexion is abnormal in all circumstances.[5]

While the brachioradialis reflex is not in and of itself diagnostic, it can rule in or out possible explanations for symptoms seen. These possibilities include upper and lower motor neuron lesions, as described above, and cervical spine dysfunction, among others. In a study looking at 249 patients with symptoms associated with cervical spine dysfunction, an abnormal brachioradialis reflex was seen, through multivariate analysis, to be able to rule in cervical spine dysfunction when coupled with any 2 of the following signs: gait deviation, a positive Hoffman test, a positive Babinski test, or age greater than 45 years old.[17] This finding offers an example of the clinical importance of the brachioradialis reflex.

While the testing of the brachioradialis reflex is important, the significance of an abnormal reflex in asymptomatic individuals has yet to be determined. One study aimed at answering this was done over the course of 6 months and involved 277 asymptomatic, neurologically normal individuals. The mean age of the participants was 27 years old, with the youngest being 16 and the eldest 78. The brachioradialis reflex was abnormal in 75 (27.6%) participants. 39% of the participants exhibiting the abnormal response exhibited it bilaterally, and 10% also had positive Hoffman signs. No other signs of myelopathy were found in these participants. Due to these findings, the result was that an abnormal brachioradialis reflex, when present in isolation, is potentially a variation of a normal clinical response.[15]

As both the biceps brachii and the brachioradialis muscle receive innervation from C5 and C6 and work to flex the elbow, studies have been conducted to separate the effects of their respective reflexes. One study used an automated system to create controlled passive movement stretch reflexes in patients with upper motor lesions. Analysis of the results showed that the brachioradialis has an earlier and larger stretch response.[16] This is important in being able to differentiate the responses.[15]

Nursing, Allied Health, and Interprofessional Team Interventions

The literature on the brachioradialis reflex and its testing largely consists of large and small randomized clinical trials.[15][16][15][17] With deep tendon reflex testing within a neurological exam, proper interpretation of reflexes is necessary for the healthcare team to assess the reflex arc's state correctly. The examiner prompts the brachioradialis reflex by tapping the brachioradialis muscle tendon at its insertion point onto the radius's styloid process. A normal reflex would produce flexion and supination of the forearm.[14] This action must then be graded using a scale such as the NINDS Muscle Stretch Reflex Scale, which classifies reflexes on a scale from 0 to 4.[12] Abnormal responses to the tapping of the brachioradialis muscle include finger flexion and slight elbow extension. The presence of either indicates hyperactivity in the finger jerk or biceps reflex, respectively. Through testing the brachioradialis reflex and appropriate grading, the healthcare team can determine the condition of a patient with regards to C5 and C6 nerve roots, the integrity of the brachioradialis muscle, and the integrity of the corticospinal tract innervation to that level. Identifying a lesion, if present, can improve patient care.