The Deep Tendon Reflex (DTR) is a rapid, involuntary muscle contraction that occurs immediately after a tendon is briefly stretched. By testing these reflexes, health professionals can determine the integrity of the neural pathways that control muscle movement.
The Underlying Biology of the Reflex Arc
The speed and involuntary nature of the DTR are possible because the response is routed through a dedicated neural pathway known as the reflex arc. This pathway is considered a monosynaptic reflex, involving only two neurons and one synapse within the spinal cord. The process begins when the tendon tap causes a sudden, slight stretch of the muscle fibers.
Specialized sensory receptors called muscle spindles, which are embedded within the muscle tissue, detect this change in length. The stretch activates Ia afferent sensory fibers connected to the muscle spindles, which rapidly transmit an electrical signal toward the spinal cord. This sensory neuron enters the gray matter of the spinal cord through the dorsal root. Once inside the spinal cord, the sensory neuron forms a direct, single synapse with an alpha motor neuron, the second neuron in the arc.
The alpha motor neuron, which provides the efferent or motor signal, immediately sends an impulse back out to the same muscle that was stretched. This quick communication causes the extrafusal muscle fibers to contract, resulting in the visible reflex action, such as the classic knee-jerk.
Performing the Deep Tendon Reflex Examination
Testing the deep tendon reflexes requires a specific technique using a specialized reflex hammer to deliver a quick, controlled tap to the tendon. The patient’s muscles must be completely relaxed for the reflex to be accurately elicited, as muscle tension can inhibit the response. The clinician gently supports the limb to ensure the muscle is in a neutral position before striking the tendon.
The most commonly tested reflexes provide insight into different segments of the spinal cord. Clinicians always compare the response on one side of the body to the corresponding reflex on the opposite side, as asymmetry often indicates a problem.
- The Patellar reflex, or knee-jerk, tests the L2-L4 spinal nerve roots.
- The Achilles reflex, at the ankle, assesses the S1 nerve root.
- The Biceps reflex targets the C5-C6 roots.
- The Triceps reflex evaluates C7-C8 nerve function.
Decoding the Results: The DTR Grading Scale
The intensity of the muscle contraction during a DTR test is quantified using a standardized, five-point clinical grading scale, typically ranging from 0 to 4+. A score of 2+ is considered the normal, expected response, described as a brisk but not exaggerated contraction. A complete absence of a response is graded as 0, while a diminished or weak response is graded as 1+ and is called hyporeflexia.
Hyporeflexia indicates a potential problem within the reflex arc itself, such as damage to the sensory neuron, motor neuron, or the muscle tissue. Conversely, a score of 3+ signifies a brisker-than-normal response, and 4+ is an exaggerated, hyperactive response that may include clonus, which is a rhythmic, involuntary muscle tremor. Both 3+ and 4+ responses are grouped under the term hyperreflexia, suggesting a lack of inhibitory control over the reflex arc.
A score of 0 or 4+ is generally considered abnormal and warrants further investigation into the nervous system.
Diagnostic Significance of Abnormal Reflexes
Abnormal DTR results help pinpoint the location of neurological dysfunction within the body. Hyporeflexia, or a diminished response, often suggests a problem in the Peripheral Nervous System, which includes the nerves outside the brain and spinal cord. This type of abnormality is associated with Lower Motor Neuron (LMN) lesions, such as those caused by peripheral neuropathy from conditions like diabetes or nerve root compression.
In contrast, hyperreflexia, or an exaggerated response, points to a disruption within the Central Nervous System. This finding is characteristic of damage to the Upper Motor Neurons (UMN), which are the nerve pathways that descend from the brain to the spinal cord. Conditions like a stroke, multiple sclerosis, or spinal cord injury above the level of the tested reflex can cause this loss of central inhibitory control, leading to an overactive muscle response.