Tendon reflexes

Definition

• A reflex is an involuntary and nearly instantaneous movement in response to a stimulus.
• It is an automatic response to a stimulus that does not receive or need conscious thought as it occurs through a reflex arc.
• Reflex arcs act on an impulse before that impulse reaches the brain. It can be
  • Monosynaptic contain only two neurons, a sensory and a motor neuron, including Patellar reflex and Achilles reflex.
  • Polysynaptic contain multiple inter-neurons that interface between the sensory and motor neurons in the reflex pathway.

The tendon reflex (deep tendon reflex, muscle stretch reflex, myotatic reflex) is a monosynaptic proprioceptive reflex

Types

1. Superficial reflexes: Plantar response, abdominal reflex, cremastic reflex, corneal reflex
2. Deep reflexes: Biceps, Brachioradialis, Triceps, Knee jerk, and ankle jerk.

Deep Tendon (muscle stretch) Reflexes

Evaluates afferent nerves, synaptic connections within the spinal cord, motor nerves, and descending motor pathways.

• Lower motor neuron lesions (eg GBS) depress reflexes
• Upper motor neuron lesions (eg MS) increase the reflexes.

Reflexes tests

Reflexes tested include the following:

• Biceps (C5 and C6)
• Radial brachialis (C6)
• Triceps (C7)
• Distal finger flexors (C8)
• Quadriceps knee jerk (L4)
• Ankle jerk (S1)
• Jaw jerk (Trigeminal nerve)

Grading

1. Arefleia
2. Hyporeflexia
3. 2+ Normal
4. 3+ Super-normal (Hyper reflexia)
5. 4+ Clonus: Repetitive shortening of the muscle after a single stimulation

Pathologic reflexes

• Pathologic reflexes (eg, Babinski, rooting, grasp) are reversions to primitive responses and indicate loss of cortical inhibition.

Other reflexes

Clonus (rhythmic, rapid alternation of muscle contraction and relaxation caused by sudden, passive tendon stretching) testing is done by rapid dorsiflexion of the foot at the ankle. Sustained clonus indicates an upper motor neuron disorder.

Significance of Superficial reflexes in Physiotherapy

• Abdominal reflex-is stimulated by stroking around the abdomen which helps in determining the level of CNS lesion.
• Cremastic reflex-It is elicited when the inner part of thigh is stroked in males (Geigel reflex is the counterpart in females). This reflex helps in evaluation of acute scrotal pain and assessing for testicular torsion(associated with loss of reflex).
• Corneal reflex-It is elicited by gentle stroking on the cornea with a cotton swab. This reflex mainly helps in knowing the damage peripherally to either the trigeminal nerve (V) or facial nerve (VII) nerve will disrupt the corneal blink circuit.
• Plantar reflex-abnormal reflex indicates metabolic or structural abnormality in the corticospinal system upstream from the segmental reflex.

Patellar reflex

• Also called the knee reflex or knee-jerk
• It is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord.

Mechanism

• Striking of the patellar tendon with a reflex hammer just below the patella stretches the muscle spindle in the quadriceps muscle. This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of L3 or L4 in the spinal cord, completely independent of higher centres.
• From there, an alpha motor neuron conducts an efferent impulse back to the quadriceps femoris muscle, triggering contraction.
• This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick.
• There is a latency of around 18 ms between stretch of the patellar tendon and the beginning of contraction of the quadriceps femoris muscle.
• This is a reflex of proprioception which helps maintain posture and balance, allowing to keep one's balance with little effort or conscious thought.
• It is a clinical and classic example of the monosynaptic reflex arc since there is no inter-neuron in the pathway leading to contraction of the quadriceps muscle.
• Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord. However, there is an inhibitory interneuron used to relax the antagonistic hamstring muscle (reciprocal innervation).
• This test of a basic automatic reflex may be influenced by the patient consciously inhibiting or exaggerating the response; the doctor may use the Jendrassik maneuver in order to ensure a more valid reflex test.

Clinical significance

• After the tap of a hammer, the leg is normally extended once and comes to rest. The absence or decrease of this reflex is problematic, and known as Westphal's sign.
• This reflex may be diminished or absent in lower motor neuron lesions and during sleep.
• Multiple oscillation of the leg (pendular reflex) following the tap may be a sign of cerebellar diseases.
• Exaggerated (brisk) deep tendon reflexes such as this can be found in upper motor neuron lesions, hyperthyroidism, anxiety or nervousness.
• The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord.
• Patellar reflex is often tested in infants to test the nervous system.

DTR

Deep tendon reflexes or, more accurately, the 'muscle stretch reflex' can aid in evaluating neurologic disease affecting

• Afferent nerves,
• Spinal cord synaptic connections
• Motor nerves
• Descending motor pathways.

Proper technique and interpretation of results are crucial in achieving a proper distinction between upper and lower motor neuron pathologic processes such as MS, ALS, spinal cord injuries, and spinal muscular atrophies, with the presence of hyporeflexia or hyperreflexia considered a 'hard sign' of neurologic dysfunction.

Five primary DTRs

Biceps Reflex

• Muscle involved: biceps brachii
• Nerve supply: musculocutaneous
• Segmental innervation: C5-C6

Brachioradialis Reflex

• Muscle involved: brachioradialis
• Nerve supply: radial
• Segmental innervation: C5-C6

Triceps Reflex

• Muscle involved: triceps brachii
• Nerve supply: radial
• Segmental innervation: C7-C8

Patellar Reflex (knee-jerk)

• Muscle involved: quadriceps femoris
• Nerve supply: femoral
• Segmental innervation: L2-L4

Achilles Reflex (ankle-jerk)

• Muscles involved: gastrocnemius, soleus
• Nerve supply: tibial
• Segmental innervation: S1-S2

Grading

National Institute of Neurological Disorders and Stroke (NINDS) grading of deep tendon reflexes.

• 0: Reflex absent
• 1: Reflex small, less than normal, includes a trace response or a response brought out only with reinforcement
• 2: Reflex in the lower half of a normal range
• 3: Reflex in the upper half of a normal range
• 4: Reflex enhanced, more than normal, includes clonus if present, which optionally can be noted in an added verbal description of the reflex

Mechanism

A reflex arc is an involuntary pathway by which the stimulus to a tendon elicits a muscle response. It is considered a monosynaptic reflex as only two neurons are involved; a sensory and a motor neuron, with a single synapse between them. The sensory neuron provides the afferent component and consists of a cell body that lies in the dorsal root ganglion (DRG) and innervates the muscle or Golgi tendon organ associated with the muscle. The motor neuron provides the efferent component and consists of an alpha motoneuron located in the anterior horn of the spinal cord. The pathway initiates in the muscle spindle, a proprioceptive organ. The muscle spindle comprises intrafusal fibers engulfed by a connective tissue capsule responsible for detecting muscle stretch. The muscle spindle is present within the muscle in between the extrafusal fibers.[7]

The mechanism of eliciting a deep tendon reflex in a patient involves tendons, muscles, and the reflex arc. Tapping the appropriate tendon causes passive stretch of the associated muscle. The stretch of the muscle fiber is detected by the muscle spindle located within the muscle fibers. The muscle spindle is a sensory proprioceptor responsible for identifying the length of the muscle fibers, composed of intrafusal fibers that do not contract. The Ia afferent sensory fibers in the muscle spindles produce action potentials in response to the stretch. These Ia afferent fibers go to the spinal cord at the dorsal root and monosynaptically stimulate the alpha motor neuron that goes to the homonymous muscle extrafusal fibers. Glutamate is the neurotransmitter at the central synapse. The extrafusal muscle fibers then generate a contraction to resist this stretch. When the muscle contraction occurs, the muscle spindle decreases the action potential firing frequency, and the reflex is extinguished.

The antagonistic muscle is inhibited during the reflex while the agonist muscle contracts.[8] This action occurs polysynaptically through the Ia inhibitory interneuron, which inhibits alpha motor neurons to the antagonistic muscle. For example, during the knee jerk reflex, the hamstring muscles are inhibited and relaxed while the quadriceps muscles are stimulated and contract. Within the muscle spindle, the gamma motor neuron causes the tightening or relaxing of intrafusal muscle fibers to regulate the sensitivity of the muscle spindle and the reflex's response. This is mediated by acetylcholine.

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Related Testing

Testing the DTR is quick and easy but requires a proper technique for a reliable response. The examiner taps the muscle's tendon with an examination hammer, and the response is observed and graded. For effective and reliable results, the patient should be as relaxed as possible. If the patient thinks about the test or has a rigid posture, its integrity is limited.

To properly elicit a response, the proper tools are required. In use today are multiple reflex hammers, typically with a weight of 80 to 140 grams. With the appropriate technique, any reflex hammer can be utilized.[9]

• The Taylor hammer
• The Krauss hammer
• The Troemmer hammer
• The Berliner hammer
• The Babinski reflex hammer
• The Rabiner reflex hammer
• The Dejerine reflex hammer
• The Queen Square reflex hammer

Proper technique includes specific attention to the following:

1. Amount of hammer force used to obtain contraction
2. Velocity of the contraction
3. Strength of the contraction
4. Duration of the contraction
5. Duration of the relaxation phase
6. Response of other muscles not tested[4]

Testing the deep tendon reflexes

Biceps Reflex

Support the patient's forearm by resting it on the forearm of the examiner, with the arm midway between flexion and extension. The examiner's thumb should be firmly placed over the biceps tendon, with fingers curled around the elbow. Tap briskly. The forearm should flex at the elbow.

Triceps Reflex

Support the patient's forearm by resting on the forearm of the examiner, with the arm midway between flexion and extension. The triceps insertion should be located on the olecranon. Tap just above the insertion. The forearm should extend.

Brachioradialis Reflex

Support the patient's arm at the elbow and identify the brachioradialis tendon at the wrist. Its insertion is located at the base of the styloid process of the radius, about 1 cm lateral to the radial artery. The thumb of the hand supporting the elbow of the patient should be placed on the biceps tendon. Tap the brachioradialis tendon with the other hand. The brachioradialis reflex will show flexion and supination of the forearm. The finger jerk reflex may also be elicited by this maneuver and will show flexion of the fingers.

Knee Jerk

The patient's legs should swing freely on the side of the examination table, with the examiner placing one hand on the quadriceps. Tap the patellar tendon and look for quadriceps contraction and extension of the leg at the knee.

Ankle Jerk

The patient's legs should swing freely on the side of the examination table, with the examiner placing one hand underneath the sole of the patient's food and dorsiflexing it slightly. Tap the Achilles tendon just above its insertion on the calcaneus. The foot should plantarflex in response.

When testing lower extremity reflexes such as the patellar or ankle reflex, if no visible response is initially observed, the examiner may then use the Jendrassik maneuver. This maneuver consists of the patient clenching their teeth, flexing their elbows, and tightly interlocking both sets of fingers. The patient is instructed to pull their hands apart while keeping them interlocked. This maneuver causes voluntary upper motor neuron innervation, which counters some of the descending inhibition sent by the brain to the lower motor neuron reflex arc. It also prevents conscious inhibition of the reflex by the patient, as they focus more on the maneuver and less on the examiner.[10]

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Pathophysiology

Hyperactive deep tendon reflexes are seen in upper motor neuron lesions. Pathologically, hyperactive DTRs may be the earliest sign of corticospinal tract abnormalities or other descending pathways influencing the reflex arc due to a suprasegmental lesion, which means a lesion above the level of the spinal reflex pathways. Hyperactive DTRs can be evoked by a much lighter tendon tap than normal, have short latency, and reflex muscle activity may be seen in motor neuron pools of synergistic muscles. For example, a tendon tap to the biceps brachii may elicit wrist pronation.[11]

Hypoactive or absent deep tendon reflexes are seen in lower motor neuron lesions. Hypoactive DTRs may be seen in disease states such as hypothyroidism, hypothermia, cerebellar dysfunction, or beta-blockade.

Absent DTRs indicate a lesion within the reflex arc. An absent reflex + sensory loss in the nerve distribution of the reflex indicates the presence of a lesion involving the afferent arc of the reflex, either the nerve or dorsal horn. An absent reflex + paralysis, fasciculations, and muscle atrophy indicate the presence of a lesion involving the efferent arc, either the anterior horn cells, efferent nerve, or both. Peripheral neuropathy is the most common cause of areflexia and is typically caused by diabetes, alcoholism, uremia, vitamin deficiencies, amyloidosis, or toxins.[12]

While a bilateral absent ankle jerk usually indicates peripheral neuropathy, cauda equina syndrome can also elicit this finding.[13] Specific peripheral nerve injuries can also lead to decreased or absent DTRs. A musculocutaneous nerve injury can affect the biceps reflex, and a radial nerve injury can affect the triceps or brachioradialis reflex, depending on the anatomical area of damage in the nerve. Femoral nerve lesions can affect the patellar reflex, and tibial nerve lesions can affect the ankle reflex.

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Clinical Significance

DTR is used to assess the integrity of the motor system and provides information on the condition of UMN and LMN

• Hypoactive/ Absent reflex: Lower motor neuron disease (Nerve roots or peripheral nerves).
• Hyperactive reflex: Upper motor neuron (Brain, Brainstem, or Spinal cord).
• Severe chronic cases, usually associated with spasticity, clonus, which is caused by a lesion in the descending motor neurons
• Clonus is commonly seen in patients with stroke, spinal cord injury, cerebral palsy, or MS and can also occur after ingesting large amounts of serotonergic agents