Deep tendon reflexes

Deep Tendon Reflexes (DTR), also known as muscle stretch reflexes, are a fundamental part of the neurological examination. They are involuntary responses to a sharp tap on the tendons of muscles that help in assessing the integrity of the nervous system. These reflexes are mediated by the spinal cord and are used to determine abnormalities in the central nervous system and peripheral nervous system.

Mechanism

The mechanism of deep tendon reflexes involves a simple neural pathway known as the monosynaptic reflex arc. When a muscle tendon is tapped, the muscle spindle fibers are stretched, sending a signal via afferent nerve fibers to the spinal cord. Here, the signal directly synapses with an efferent nerve fiber, which sends a signal back to the muscle, causing it to contract. This pathway does not require the involvement of the brain, although higher brain functions can modulate the reflex.

Clinical Significance

Deep tendon reflexes are an essential part of the neurological examination. They help in diagnosing and differentiating between upper and lower motor neuron lesions. Increased reflexes (hyperreflexia) may indicate an upper motor neuron lesion, such as in conditions like stroke or multiple sclerosis. Decreased reflexes (hyporeflexia) or absent reflexes may suggest a lower motor neuron issue, such as in spinal cord injury or peripheral neuropathy.

Commonly Tested Reflexes

Several deep tendon reflexes are commonly tested in a neurological examination, including:

• Biceps Reflex: Assesses the C5-C6 spinal nerves.
• Triceps Reflex: Assesses the C6-C7 spinal nerves.
• Brachioradialis Reflex: Assesses the C5-C6 spinal nerves.
• Patellar Reflex or Knee-jerk: Assesses the L2-L4 spinal nerves.
• Achilles Reflex: Assesses the S1-S2 spinal nerves.

Procedure

The examination of deep tendon reflexes involves using a reflex hammer to tap firmly on the tendon. The patient should be in a relaxed and comfortable position to ensure accurate assessment. The response is graded on a scale from 0 to 4, with 0 indicating no response and 4 indicating a very brisk response, possibly with clonus (a series of involuntary, rhythmic, muscular contractions and relaxations).

Grading Scale

• 0 - No response
• 1+ - Sluggish or diminished
• 2+ - Active or expected response
• 3+ - More brisk than expected, slightly hyperactive
• 4+ - Brisk, hyperactive, with clonus

Limitations

While deep tendon reflexes are a valuable diagnostic tool, they must be interpreted in the context of the overall neurological examination. Factors such as age, muscle mass, and level of relaxation can influence reflex responses. Additionally, reflexes can vary significantly among healthy individuals.