Deep Tendon Reflexes and Plantar Response
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Direct answer
Deep tendon reflexes are graded from zero to four and test the whole reflex arc from afferent nerve to efferent motor neuron. Brisk reflexes with clonus and an extensor plantar response indicate an upper motor neuron lesion, while absent reflexes point to a lower motor neuron or peripheral problem. The normal plantar response is flexor and downgoing, and extension of the great toe is pathological after infancy.
What you must remember
- Upper limb root values: biceps C5 and C6, brachioradialis C6 and triceps C7, so each reflex localises a specific segment.
- Knee and ankle jerks: the knee jerk tests L3 and L4 through the femoral nerve, while the ankle jerk tests S1 through the tibial nerve.
- Grading: 0 is absent, 1+ diminished, 2+ normal, 3+ brisk and 4+ brisk with clonus, always compared between the two sides.
- Babinski sign: extension of the great toe with fanning of the other toes, a pyramidal sign indicating an upper motor neuron lesion after infancy.
- Infant plantar response: an extensor response is physiological up to about one year and may persist somewhat longer, so age must be considered.
- Clonus: rhythmic involuntary beats after sustained stretch, most often at the ankle, indicating a brisk upper motor neuron lesion.
- Spinal shock: an acute cord lesion abolishes reflexes early, and hyperreflexia and clonus appear only as recovery proceeds over days to weeks.
- Jendrassik manoeuvre: clenching the teeth or hooking the fingers reinforces faint reflexes, which helps in peripheral neuropathy.
A bedside method for grading reflexes
Reflex examination is useful only when it is performed the same way each time. A consistent sequence allows the examiner to compare sides and detect asymmetry early.
- Position and relax the patient: Support the limb in a relaxed position, and use the Jendrassik manoeuvre if the reflex is faint.
- Strike the tendon briskly: Use a quick, controlled blow over the tendon, comparing both sides at the same angle and force.
- Grade each reflex: Record absence, diminished, normal, brisk or clonus on the zero to four scale, and note the root level tested.
- Test the plantar response: Stroke the lateral sole from the heel to the base of the little toe and then across the forefoot, watching the great toe.
- Look for associated signs: Check tone, power, Hoffmann's sign in the upper limb and ankle clonus to confirm whether an upper motor neuron pattern is genuine.
- Interpret the pattern: Combine reflexes, tone and plantar response to choose between upper motor neuron, lower motor neuron and peripheral patterns, then localise the lesion.
Where the exam tries to catch you
Reflex questions usually turn on a single sign that is easy to misread under time pressure.
- Trap: Calling an absent ankle jerk normal because the patient is elderly Correct: ankle reflexes fade with age, but absence still needs a search for peripheral neuropathy, and reinforcement can unmask a faint response.
- Trap: Reading a flexor plantar response as abnormal Correct: a flexor, downgoing plantar response is normal in adults, and only extension after infancy is pathological.
- Trap: Expecting hyperreflexia immediately after acute cord injury Correct: spinal shock causes early areflexia and flaccidity, and exaggerated reflexes emerge only as recovery proceeds.
- Trap: Swapping the knee and ankle root values Correct: the knee jerk is L3 and L4 and the ankle jerk is S1, so a lost ankle jerk points to the sacral segment or tibial nerve.
Frequently asked questions
Which spinal segments does the knee jerk test?
The knee jerk tests the L3 and L4 segments through the femoral nerve. Absence of the knee jerk suggests a lesion of these roots, the femoral nerve or the quadriceps, or a generalised peripheral neuropathy. Comparing both sides is essential for a valid interpretation.
What does an extensor plantar response signify in adults?
An extensor plantar response, also called a positive Babinski sign, signifies an upper motor neuron lesion of the corticospinal tract. It is pathological in adults and older children, and it is usually accompanied by other upper motor neuron signs such as increased tone and brisk reflexes.
How does spinal shock change reflexes after acute cord injury?
Spinal shock abolishes reflexes below the level of an acute cord injury, causing flaccid areflexia. Reflexes return over days to weeks and then become exaggerated, with clonus and an extensor plantar response. Early examination findings therefore do not represent the final pattern.
What is the Jendrassik manoeuvre used for?
The Jendrassik manoeuvre is a reinforcement technique in which the patient clenches the teeth or hooks the fingers while the reflex is tested. It helps reveal faint or absent reflexes, particularly in peripheral neuropathy. Reinforced responses are documented as present, with the manoeuvre noted.
Why is the plantar response normally extensor in infants?
In infants the corticospinal tract is not yet fully myelinated, so an extensor plantar response is physiological up to about one year of age. The flexor response becomes the normal adult pattern as myelination matures. Persistence beyond this age warrants neurological review.