Definition/Introduction

The neurological exam is a practice that dates back to the late 1800s. Joseph Babinski and Wilhelm Erb were the first to discover the clinical application of a neurological exam. The practice has since become standard procedure across the world. The importance of this exam stems from its ability to determine the integrity of both the central and peripheral nervous systems. Beyond this, the utilization of a neurological exam allows for the determination of whether pain in the extremities results from peripheral or central tissue.[1] Testing of reflexes within the context of a neurological exam should take place when patients present with a sensory, motor, or both forms of symptoms. In this instance, the results of reflex testing will help to distinguish between an upper and a lower motor neuron lesion as upper motor neuron lesions are associated with hyperreflexia and lower with hyporeflexia.[2]

If reflex testing indicates the possibility of a neural insult, then further testing should be conducted, inclusive of cranial nerve assessment, the brachioradialis reflex, clonus in the upper extremity, and Hoffman’s reflex.[1] To be herein discussed in greater detail is the brachioradialis reflex.

The brachioradialis muscle itself is a flexor of the forearm at the elbow that also participates in the supination and pronation of the forearm. It originates from the proximal two-thirds of the lateral supracondylar ridge of the humerus and inserts into the lateral aspect of the styloid process of the radius.[3] Due to its points of origin and insertion, the medial aspect of this muscle forms the lateral border of the cubital fossa.[4]

The brachioradialis reflex is a deep tendon reflex involving the C5 and C6 nerve roots through its innervation by the radial nerve. Babinski first introduced the testing of this reflex in 1910.[5] Elicitation of the reflex is done by briskly tapping the muscle-tendon, which results in sending afferent impulses from the muscle spindles to the spinal cord and then back through an efferent neuron to produce a muscle response. Concurrent to this process, the descending corticospinal fibers are activated, which results in the activation of opposing muscle groups that dampen the muscle jerk produced by the reflex. Testing of deep tendon reflexes such as the brachioradialis reflex serves a crucial role in both general practice and the hospital setting as 9% and 10-20% of cases, respectively, are neurological in origin.[2]

Muscle Involvement

The brachioradialis is superficial, on the radial side of the forearm within the superficial extensor compartment.[6] As previously mentioned, it forms the lateral border of the cubital fossa.[7] The origination of the muscle is from the front of the lateral intermuscular septum of the arm as well as from the upper two-thirds of the lateral supracondylar ridge of the humerus. The insertion of the muscle is on the lateral aspect of the lower end of the radius, just proximal to the styloid process.[7][6] Due to the path of the muscle, it transverses the elbow joint and works in flexing the elbow.[6] This action is contrary to what would be expected based upon the muscle’s origination in the posterior compartment of the forearm.[4]

Anatomical variations exist within the muscle; however, there have been studies to determine the most typical presentations of motor units. The results of one of such studies showed that motor units receive innervation by a mean of four distinct endplate zones separated by a minimum of 15 mm and a maximum of 55 mm along the proximal-distal axis. Further study showed that terminal waves were present in distally innervated motor units but not in proximally innervated motor units. This finding indicates that distal motor units have tendinous termination as compared to the intra-fascicular termination of the proximal motor units. Taken together, these two findings point to the brachioradialis muscle having a series-fibered architecture composed of overlapping bands of muscle fibers. It is this organization of the muscle fibers that determines the biomechanical action of the brachioradialis muscle along with its force-generating capacity and its neural control. One postulation for the series-fibered architecture observed in the brachioradialis muscle is the allowance it makes for a significant mechanical exertion while preserving short enough muscle fibers for the effective coupling of electrical and mechanical forces.[8]

Innervation

The input from C5 and C6 travels along the radial nerve to innervate the brachioradialis and cause the action of elbow flexion.[7] Variation exists in the branching pattern of the radial nerve however the most common pattern from proximal to distal is as follows: brachioradialis, extensor carpi radialis longus, superficial sensory, extensor carpi radialis brevis, supinator, extensor digitorum/extensor carpi ulnaris, extensor digiti minimi, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.[9] The basis of the proximal to distal order of innervation was on the mean shortest branch lengths.[10] Beyond variation in a branching pattern, variation also exists in the quantity and locations of muscle-entry points.[11] The knowledge of the anatomy of the radial nerve motor branches is of importance in surgery in the area, in neurorrhaphy, in nerve blocks, and when predicting the rate and sequence of muscle recovery post-trauma.[10] Regarding surgery, in particular, care must be taken when dissecting the brachioradialis muscle more proximal than 50 mm from the elbow as at this point, the extra-muscular branches of the radial nerve may be at risk.[11]

Blood Supply

The main blood supply of the brachioradialis muscle is the recurrent radial branch of the radial artery. This branch also provides blood flow to the supinator muscle. The recurrent radial branch comes off the radial artery just distal to the radial head. From that point, it crosses back up the arm to anastomose with the radial collateral branch from the deep brachial artery [6]. Surgeons exploit this blood supply to the brachioradialis muscle when using the proximal part of the muscle as a flap transposed to cover an exposed elbow. The blood supply, in this case, is preserved through the main vascular pedicle and smaller inconsistent branches of the radial recurrent artery.[7]

Issues of Concern

Reflex Mechanism

A reflex occurs when a force, in this context a tap, stretches the muscle-tendon. Through the stretching of the muscle, stretch receptors within the muscle spindle become activated. 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] The afferent portion of the arc is carried by 1a afferent neurons 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. Additional modulation of the monosynaptic junction of the 1a afferent neuron and the motor neuron is provided by the descending corticospinal tract, which in most cases, serves to dampen 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 either 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, nor subsequent reflex arcs or motor neurons were activated. One study which explored this phenomenon in the brachioradialis muscle found that a gamma efferent block created by procaine injection was able to decrease the excitability of muscle spindles. This finding is of importance 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, which contribute to the reflexes tested.[12] Additionally, reflex testing is a part of the clinical examination for myelopathy. Specifically, this examination includes Hoffman’s 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 relaxation of the muscle. 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 is dependent upon muscle movement observed as a result of 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. A positive test is indicated by either finger flexion or slight elbow extension.[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 insertion point of the tendon. 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. A myriad of tools for eliciting reflexes exists, including both specialized and improvised hammers. There are three 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 with which 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 four-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 though a lower motor neuron lesion, it also exaggerates all reflexes below that level. These reflexes undergo stimulation via an upper motor neuron and include the finger flexion reflexes caused by C8. In the case where C5 and C6 are intact yet neither the biceps nor the brachioradialis is capable of contraction, then 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 in an amount proportionate 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 the presence of hyperactive stretch reflexes caused by the presence of 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 inhibitory impulses from the cerebral cortex are disrupted, hence causing a state of hyperactive reflexes. When hyperreflexia affects the brachioradialis the wrist will be supinated and finger flexion would be observed. Wrist supination is the normal brachioradialis reflex however the finger flexion is abnormal in all circumstances.[5]

While the brachioradialis reflex is not in and of itself diagnostic, it can be used to rule in or out possible explanations for symptoms seen. Included within these possibilities are upper and lower motor neuron lesions as described above, as well as cervical spine dysfunction among others. In a study looking at 249 patients with symptoms associated with cervical spine dysfunction, and abnormal brachioradialis reflex was seen, through multivariate analysis, to be able to rule in cervical spine dysfunction when coupled with any two of the following signs: gait deviation, a positive Hoffman’s test, a positive Babinski test, or an 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 has been shown to be of importance, the significance of an abnormal reflex in asymptomatic individuals has yet to be determined. One studied 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 participant being 16 and the eldest 78. The brachioradialis reflex was found to be abnormal in 75 (27.6%) of the participants. 39% of the participants exhibiting the abnormal response exhibited it bilaterally, and 10% also had a positive Hoffman’s signs. No other signs of myelopathy were found in these participants. Due to these findings, the result was held 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. In one study an automated system was used 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 two responses.[15]

Nursing, Allied Health, and Interprofessional Team Interventions

The body of literature on the brachioradialis reflex and its testing largely consists of large and small randomized clinical trials (RCTs), giving it evidence levels of either one or two depending upon the clarity of the results of the study.[15][16][15][17] With the usage of deep tendon reflex testing within a neurological exam, proper interpretation of reflexes is necessary for the healthcare team to provide a correct assessment of the state of the reflex arc. The examiner prompts the brachioradialis reflex by tapping the brachioradialis muscle tendon at its point of insertion onto the styloid process of the radius. 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 points to hyperactive in either 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. Identification of a lesion if present can lead to improved patient care.