The deep tendon reflexes (DTR), particularly the tricep, patellar, and the Achilles reflex, were initially described by Wilhelm Heinrich Erb and Carl Friedrich Otto Westphal in adjacent articles simultaneously published in 1875, in the German Archives for Psychiatry and Nervous Disease. Since then, the Achilles reflex test also called the ankle jerk test has remained an essential component of the lower extremity neurologic exam. It is a myotatic (deep tendon) reflex that can be elicited easily with the use of a reflex hammer to assess the S1 nerve root of the lumbosacral plexus. As for all deep tendon reflexes, the Achilles reflex is then interpreted by grading the response using a 0 to 4 scale and by comparing to the contralateral limb and nearby reflexes. Examining the DTRs' such as the Achilles reflex provides insight into the function of the reflex arc and its' symmetry or lack thereof when compared to the contralateral side. Deep tendon reflexes remain an essential component of the neurologic exam that can guide further clinical decisions and differentiate between upper motor neuron and lower motor neuron pathology.
The Achilles tendon can be palpated just superior to the posterior aspect of the calcaneus. It is the strongest and largest tendon in the human body that attaches the triceps surae (soleus and gastrocnemius) muscle to the calcaneal tuberosity. The tendon is covered by a paratenon and not a tendon sheath. A paratenon is a thin fascial covering that decreases friction while serving as a conduit for blood vessels.
Innervation and Reflex Arc
The Achilles tendon is innervated primarily by the S1 and S2 nerve roots of the Tibial nerve. The Achilles tendon reflex is a stretch reflex, which refers to the involuntary contraction of a muscle in response to passive stretching. By tapping the tendon with a reflex hammer, a subsequent lengthening (stretch) in the triceps surae muscle occurs, causing the stretch receptors in the muscle spindle to be activated and propagate an excitatory stimulus. The stimulus is then carried by the Ia afferent neurons to the spinal cord, specifically to the cell bodies of the afferent neurons in the dorsal root ganglion. The afferent neurons then synapse on and activate an alpha motor neuron in the ventral horn of the spinal cord. The activated alpha motor neuron then transmits an efferent stimulus back to the neuromuscular junction of the agonist (triceps surae) muscle leading to a muscular contraction (reflex). To ensure that the agonist muscle (triceps surae) of the reflex has an unopposed contraction, the Ia afferent neurons simultaneously stimulate an inhibitory neuron in the spinal cord that prevents the contraction of the antagonist (tibialis anterior) muscle. Although spinal reflex arcs bypass the brain to provide a quick response, the descending corticospinal tracts dampen and refine the amplitude of the reflex arc. The S1 nerve root serves as the conduit for both the afferent and efferent impulses of the Achilles tendon reflex arc.
The deep tendon reflex arc is divided into the upper motor neuron (UMN) component and the lower motor neuron (LMN) component. The UMN component consists of the descending structures that transmit the dampening stimulus to the LMN at the ventral horn, such as the corticospinal tracts, brainstem, internal capsule, and cerebral cortex. The LMN structures are the ventral (anterior) horn of the spinal cord and the peripheral nerve that innervates the muscle. The LMN of the Achilles reflex consists of the ventral horn of the S1 nerve root and the tibial nerve.
The Achilles reflex test is performed as part of any complete physical exam, particularly when assessing the neurologic functions of the lower extremities. When pathology of the S1 nerve root is in question, the Achilles reflex test is the initial assessment of choice. It can aid in differentiating UMN pathology from LMN pathology as well as help in localizing the lesion. Additionally, it can provide information on peripheral nerve function which can be of utmost importance in certain patients populations such as those with diabetes mellitus and hypothyroidism.
The Achilles reflex can be elicited using reflex hammers. Modern-day reflex hammers are available in four different styles, Taylor hammer (Tomahawk), Queen Square hammer, Babinski hammer, and the Troemner hammer. However, no study has shown any hammer to be superior to the next, selection is based purely on preference and tradition of the institution. A practitioner holds a standard reflex hammer by the handle in a firm manner, typically with their dominant thumb and forefinger holding most of the weight with the flat part of the top toward the patient’s Achilles tendon area. Other, larger, and more advanced reflex hammers, like a pendulum hammer, can be used to assess the Achilles reflex as well.
There is minimal preparation needed to complete the Achilles reflex test. The patient should be relaxed, laying on an examination table and their Achilles tendon area well exposed from superior calf to plantar aspect of the foot. The practitioner should have washed hands, reflex hammer in hand and should be in an adequate position to perform the exam.
There are various ways to perform an Achilles reflex test. The traditional approach is to elicit the reflex with a "tendon tap" where the examiner strikes the Achilles tendon. The traditional "tendon tap" can be performed with the patient in a supine position, knee flexed, and hip externally rotated on the examination table. One hand is slightly dorsiflexing the foot from the plantar aspect, as the other hand strikes the Achilles tendon with the reflex hammer. This approach can also be performed with the leg dangling off of the examination table or with the leg elevated being held up by the examiner.
An alternative method of assessing the Achilles tendon reflex that may be of use, particularly for hospitalized patients with limited mobility, is the plantar strike. A plantar strike allows the examiner to assess the reflex while the patient remains supine with extended legs. The plantar strike method is performed by tapping the examiner’s fingers while they are placed on the patients' forefoot with slight tension causing dorsiflexion, a plantarflexion reflex should then be observed. The "plantar tap" has been shown to have 89% concordance with the traditional Achilles "tendon tap" regardless of the examiner's experience level. Furthermore, it has been shown that the plantar strike or the Achilles tendon strike with an elevated leg provides the most reliable results when examining elderly patients, as it is commonly difficult to elicit in this patient population. 
Additionally, the Achilles reflex can be elicited in a standing patient using a pendulum hammer. Examing a standing patient allows for the assessment of the reflex arc during postural control, as it is known that reflex excitability varies during functional motor commands. Reflex arc assessment during muscular engagement such as when standing provides the opportunity to assess the entirety of the spinal reflex arc. However, it is more commonly used in the research setting.
Occasionally, reinforcement of deep tendon reflexes can be performed to elicit a reflex that might not be observed otherwise in that patient. The Jendrassik maneuver is a commonly employed reinforcement technique in which the patient is asked to hook their hands together with flexed fingers while trying to pull apart as strongly as possible. The use of the Jendrassik maneuver was shown to make the Achilles reflex appear in 70% of normal elderly patients that had an absent reflex prior to reinforcement.
As previously mentioned, the Achilles reflex test is interpreted and graded based on reflex amplitude using a scale such as, the NINDS Myotatic Reflex Scale. The NINDS scale uses a 0 to 4 grading system:
Additionally, a plus (+) or a minus (-) can be added to the number grade to identify an in-between grade response. A grading of 5 can also be used to denote a sustained clonus of >10 rhythmic oscillations after briskly stretching a hyperreflexic muscle, usually performed in the Achilles tendon.
Interpretation of Observed Reflex
When examining deep tendon reflexes, it is of utmost importance to compare each reflex response to the corresponding contralateral side. A numeric grading scale remains a subjective quantifier that depends on the examiners' pre-conceived idea of what a specific grade should be. However, the observation of an asymmetric reflex should be observed across all examiners regardless of their individual assignment of a grade. Hyporeflexia, hyperreflexia, and asymmetric responses are abnormal and should prompt further examination.
Hyporeflexia is usually indicative of LMN disease, that is, of the peripheral nerve or the anterior (ventral) horn of the spinal cord for that particular level. Disease or disruption of the LMN causes a diminished or absent reflex as there is no transmission of the stimulus to the effector muscle. Additional features of LMN disease are the presence of flaccidity or diminished muscle tone, weakness, fasciculations, and muscular atrophy. The causes of LMN disease include acquired peripheral neuropathies, hereditary causes, or structural damage to the LMN.  Peripheral neuropathies are the most common cause of absent or diminished reflexes. Physiologic derangements caused by diabetes mellitus, hypothyroidism, uremia, vitamin or electrolyte deficiencies, and toxins such as lead or arsenic can commonly present with hyporeflexia. Diminished or absent Achilles tendon reflex is commonly seen in patients with hypothyroidism or diabetes mellitus. Furthermore, the absence of the Achilles tendon reflex has been shown to have 72.1% sensitivity and 90.6% specificity in identifying peripheral neuropathy in elderly patients. Medications such as isoniazid, vincristine, and diphenylhydantoin are also known to cause hyporeflexia. There are multiple hereditary and immune-related causes of hyporeflexia such as and not limited to spinobulbar muscular atrophy, spinal muscular atrophy, distal hereditary motor neuropathies, and Guillian-Barre syndrome. Structural damage to the LMN components of the tibial nerve can occur with neural foraminal stenosis or a herniated intervertebral disc at the L5-S1 level that compresses the S1 nerve root.
Hyperreflexia is indicative of UMN disease. Damage to any of the UMN components (corticospinal tracts, brainstem, internal capsule, and cerebral cortex) leads to a hyperactive deep tendon reflex due to disinhibition of the spinal reflex arc. As previously described, the upper motor neurons have an essential role in dampening the reflex response. In addition to hyperreflexia, UMN lesions present with spasticity or increased muscle tone, weakness, as well as various pathological reflexes such as the Babinski sign, clonus, Hoffman's sign, and radiation of reflexes to other muscle groups. Causes of UMN damage can be due to anoxic brain injury, brain trauma or spinal cord trauma, cerebrovascular accidents, neurodegenerative disorders (amyotrophic lateral sclerosis, primary lateral sclerosis, multiple sclerosis), spinal cord compression or malignancy to name a few.
The Achilles tendon reflex is a simple maneuver that is part of any lower extremity neurologic exam. It can be performed relatively easily both in the clinic and in-patient environment. However, differences in technique and experience level can lead to varying intra-patient results. Various case-control studies have been published that compare the traditional Achilles tendon tap to the plantar tap as well as the reproducibility across different experience levels (Level 3). When comparing the two Achilles reflex methods, it was shown that the traditional Achilles tendon tap had less inter-observer and intra-observer reliability. The plantar tap method had greater reproducibility. When comparing senior-level physicians to junior level, the plantar tap had significant inter-observer reliability (p < 0.001). The intra-observer reliability was not significant in the senior level practitioners but did show a significant difference for the junior level physicians (p < 0.05). Additionally, 84% of ankle jerks would have initially been called absent if final year medical students would not have performed a plantar tap method for confirmation.
The ubiquitous nature of the Achilles reflex test and its use by senior physicians, junior physicians, medical students, nurses, and physician assistants, necessitate a reliable confirmatory alternative. Considering the interdisciplinary structure of patient care teams with varying levels of experience, it is prudent to consider the use of the plantar tap method. Particularly for patient populations that you may suspect a diminished or altered reflex response such as the elderly, diabetic, and hypothyroid patients.
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