The Deep Tendon Reflex (DTR), also known as the muscle stretch reflex, is a fundamental test conducted during a neurological exam. This involuntary, rapid response is an automatic contraction of a muscle following a sharp stretch of its tendon. Evaluating DTRs provides a simple yet powerful assessment of the nerve pathways, helping medical professionals quickly determine if there is an issue with the sensory nerve, the motor nerve, or the spinal cord segment where they connect.
The Physiology of the Deep Tendon Reflex
The DTR mechanism relies on the reflex arc, a sophisticated neural circuit that is the shortest pathway in the nervous system. This arc is typically monosynaptic, involving only two neurons and a single synapse within the spinal cord. The process begins when tapping a tendon stretches the attached muscle, activating specialized sensory receptors called muscle spindles.
The resulting impulse travels rapidly along a sensory neuron toward the spinal cord, where it directly synapses with an alpha motor neuron in the anterior horn. This direct connection ensures a quick response. The alpha motor neuron immediately transmits an efferent signal back to the muscle, causing the fibers to contract—the visible reflex response.
To ensure coordinated movement, the arc also activates an inhibitory interneuron that prevents the opposing muscle group from contracting (reciprocal inhibition). Since the pathway is contained within a specific spinal cord segment (e.g., the knee-jerk reflex uses L3 and L4 segments), its assessment provides precise information about the functional integrity of that spinal cord level and corresponding peripheral nerves.
Standard Assessment Techniques and Grading
The clinical assessment of DTRs is a standard, non-invasive procedure performed using a reflex hammer to mechanically induce muscle stretch. The technique requires the patient to be relaxed, and the examiner must strike the tendon briskly, ensuring the limb is positioned for a free swing.
Providers test several common sites:
- Biceps (C5-C6 spinal level)
- Triceps (C6-C7)
- Brachioradialis (C5-C6)
- Patellar or knee jerk (L3-L4)
- Achilles or ankle jerk (S1)
The reflex response is categorized using the standardized, five-point National Institute of Neurological Disorders and Stroke (NINDS) Myotatic Reflex Scale. This system ranges from 0 to 4+, with 2+ indicating a normal and expected response. A grade of 0 signifies a complete absence of a reflex, even with reinforcement techniques.
A 1+ response is diminished or sluggish. A 3+ is brisker than average, and a 4+ is the most exaggerated response, often accompanied by clonus (a repetitive, rhythmic muscle contraction).
The most significant finding is often an asymmetry between the two sides of the body. An asymmetrical finding, such as a 2+ on the right knee and a 0 or 3+ on the left, strongly indicates a localized neurological problem. If a reflex is absent or difficult to elicit, the provider may use reinforcement maneuvers, such as asking the patient to lock their fingers and pull, to temporarily increase reflex activity.
Clinical Significance of Abnormal Results
Results deviating from the normal 2+ grading fall into two categories: hyporeflexia (diminished or absent reflexes) and hyperreflexia (exaggerated or brisk reflexes). These patterns help localize potential damage within the nervous system.
Hyporeflexia (Grades 0 and 1+)
Hyporeflexia suggests a lesion within the reflex arc itself. This diminished response is a common sign of Lower Motor Neuron (LMN) pathology, involving the peripheral nerves or the anterior horn cells in the spinal cord. Conditions like peripheral neuropathy (often caused by diabetes) or spinal nerve root compression from a herniated disc frequently result in reduced or absent reflexes.
Hyperreflexia (Grades 3+ and 4+)
Hyperreflexia is generally associated with a loss of inhibitory control from the brain, a hallmark of Upper Motor Neuron (UMN) lesions. UMN lesions involve nerve pathways in the brain or spinal cord above the reflex arc. Damage to the descending corticospinal tract releases the spinal cord from inhibition, leading to an overactive reflex. Common UMN conditions manifesting with hyperreflexia include stroke, multiple sclerosis, and spinal cord injury above the tested reflex level. The presence of hyperreflexia alongside signs like muscle weakness and increased muscle tone (spasticity) helps pinpoint the location and nature of the neurological issue.