Reflex testing is a non-invasive procedure used to evaluate the integrity of the nervous system. A reflex is an involuntary, rapid response to a stimulus, serving as the body’s fundamental protective mechanism. These automatic responses allow the body to react quickly to potential danger, such as withdrawing a hand from a hot surface, without conscious thought. Testing reflexes is a routine part of any neurological examination, providing clues about the location and type of potential nerve damage or disease.
The Basic Reflex Arc
The speed and automatic nature of a reflex is possible because the nerve impulse follows a specific pathway known as the reflex arc. This arc consists of five distinct components that allow the signal to complete its circuit without requiring processing by the brain. The process begins with a sensory receptor, which detects the initial stimulus, such as the stretch in a tendon caused by a tap.
The signal then travels along a sensory, or afferent, neuron toward the central nervous system, carrying the message of the detected change. The integration center, typically located within the spinal cord, receives this input and processes it almost instantaneously. For simple reflexes, the sensory neuron directly connects to the motor neuron, bypassing any further complex processing.
From the integration center, a motor, or efferent, neuron carries the command signal away from the spinal cord to the muscle or gland. This effector is the final component of the arc, which executes the involuntary response. In the case of a deep tendon reflex, the effector is the muscle that contracts, causing the limb to jerk.
Common Techniques for Reflex Testing
The most frequently assessed reflexes are the deep tendon reflexes (DTRs), which test the spinal cord segments controlling specific muscle groups. These tests involve a specialized reflex hammer delivering a quick, precise tap to a tendon, causing the muscle to momentarily stretch. Proper patient relaxation is paramount, as muscle tension can suppress the reflex response, leading to a false or diminished result.
The patellar reflex, or knee-jerk reflex, tests the L2, L3, and L4 nerve roots in the lumbar spine. The patient is seated with their legs dangling freely, allowing the quadriceps muscle to be relaxed and slightly stretched. The examiner strikes the patellar tendon, located just beneath the kneecap. A normal response is a brief, brisk extension of the lower leg.
The Achilles reflex tests the S1 nerve root and is elicited by tapping the Achilles tendon just above the heel. The patient may be seated with the foot in slight dorsiflexion or kneeling with their feet hanging over the edge. The normal reaction is a slight, quick plantar flexion (the foot pointing downward). If a DTR is difficult to elicit, a reinforcing technique like the Jendrassik maneuver may be used to distract the patient and remove inhibitory signals.
The Plantar reflex, or Babinski sign, is a superficial reflex tested by stroking the sole of the foot. The examiner uses a blunt object to trace a path starting from the heel, moving up the lateral side of the sole, and curving inward across the ball of the foot. The normal adult response is plantar flexion, where all the toes curl downward.
Interpreting the Reflex Grading Scale
To standardize the observation of reflex strength, clinicians use a numerical grading system, most commonly the 0 to 4+ scale. This scale provides a common language for describing the intensity of the muscle contraction and is crucial for tracking neurological changes over time. A grade of 0 indicates a completely absent reflex, even with reinforcement, which is always considered abnormal.
A grade of 1+ describes a diminished or trace response that is noticeably weaker than average, sometimes considered a low-normal finding. The 2+ grade represents the expected, average, or normal reflex response, characterized by a brisk but controlled muscle contraction. This is the goal for a healthy, functioning reflex arc.
The 3+ grade is designated for a reflex that is brisker than average, which can sometimes be a normal variant in an anxious person but may also signal pathology. A grade of 4+ is reserved for a hyperactive response that is very brisk and often accompanied by clonus, a rhythmic, involuntary oscillation of the joint. The presence of clonus is almost always a sign of a neurological disorder. Asymmetry, where the reflex response differs significantly between the left and right sides, is often the most telling finding.
Clinical Significance of Abnormal Findings
An abnormal reflex grade provides an important diagnostic distinction between two major types of neurological damage: those affecting the upper motor neurons (UMN) and those affecting the lower motor neurons (LMN). The terms hyporeflexia and hyperreflexia point to the likely location of the problem within the nervous system.
Hyporeflexia (grades 0 or 1+) suggests a problem with the LMN component of the arc. This occurs when the pathway carrying the signal to the muscle is damaged, such as in peripheral neuropathy, nerve root compression, or muscle disease. In these cases, the signal cannot efficiently reach the effector muscle, resulting in a weak or absent contraction.
Conversely, hyperreflexia (grades 3+ or 4+) typically points to an issue with the UMN system. UMNs originate in the brain and descend to the spinal cord, exerting an inhibitory influence on the reflex arcs below them. Damage from conditions like a stroke, spinal cord injury, or multiple sclerosis removes this inhibitory control, causing the spinal cord reflex to become exaggerated and overactive.
A positive Babinski sign in an adult, characterized by the abnormal extension and fanning of the toes, is a clear sign of UMN damage. This finding indicates a lesion within the corticospinal tract, the main pathway connecting the brain’s motor cortex to the spinal cord. While this response is normal in infants due to an immature nervous system, its presence in an adult indicates serious central nervous system dysfunction.