Key Insights On Nerve Testing

Author(s): 
By Alexander Reyzelman, DPM, Joseph Fiorito, Cody Hoover and Michael Brewer

In the podiatric profession, we are frequently faced with chronic painful musculoskeletal processes that get labeled as arthritis, chronic plantar fasciitis, neuroma, etc. Perhaps it would behoove us to start thinking of an underlying neurological pathology that may be responsible for foot or ankle pain.

In the senior author’s opinion, there is more neurological pathology causing foot and ankle pain than is being diagnosed. This may explain the cases in which patients have chronic musculoskeletal conditions and undergo surgery, but their pain never improves. Accordingly, let us take a closer look at peripheral nerve pathologies that are common in the lower extremity and their physical examination findings along with appropriate diagnostic tools that may be useful.

Polyneuropathies, both axonal and demyelinating, are peripheral neuropathies that are generalized and symmetrical. They are often described as a “stocking and glove” distribution that preferentially affects lower extremities before upper extremities. The symptoms of polyneuropathy can be divided into positive and negative ones. Positive symptoms include pain, paresthesias and fasciculations. Negative symptoms of polyneuropathy may include numbness, weakness, imbalance and gait instability.

The loss of peripheral nerve function almost always involves axonal loss, which may occur uniformly in all types of axons or may predominantly affect one fiber type or size. There are five major function classes of axons that may be assessed by neurophysiologic techniques. These classes include: large myelinated motor axons (skeletal muscle control and reflexes); large myelinated sensory axons (vibration and proprioception); small myelinated sensory axons (cold and warmth sensation); unmyelinated sensory axons (pain); and autonomic axons (cardiac rate, blood pressure, sweating).

In order to effectively diagnose neuropathy, one must ask subjective questions of patients as to whether they are experiencing positive or negative symptoms, and pursue an appropriate detailed neurologic examination of all the potentially affected nerve fibers. In regard to the neurologic exam, this would involve reliable testing procedures that are standardized, reproducible, simple, noninvasive and not time consuming.

When You Suspect Large Fiber Neuropathy
As we mentioned above, the symptoms of large fiber neuropathy consist of tingling numbness and poor balance. Testing for large fiber neuropathy should include deep tendon reflex, vibration perception threshold (VPT) and proprioception testing.

Deep tendon reflexes (DTR) can be helpful in identifying motor nerve involvement. One can test DTRs with a reflex hammer. The test provides information associated with the integrity of the central and peripheral nervous system. Generally, decreased reflexes indicate a peripheral problem and lively or exaggerated reflexes indicate a central nervous system defect. One may quantify DTRs with a grading system.

The reflexes one may test in lower extremity examinations should include patellar reflex or knee jerk reflex (L3), Achilles reflex (S1), plantar reflex or Babinski reflex (S1).

Elicit the plantar reflex (PR) by stroking the lateral or outer border of the sole of the foot with the thumbnail or a blunt point like the end of the handle of the reflex hammer or the tip of a key. Direct the stimulus from the heel forward toward the little toe. Upon reaching the foot pad, direct the stimulus transversely across the metatarsal pad from the little toe to the base of the great toe. The stimulus should stop short of the base of the toes because extending the stroke to the base of the toes produces unpredictable movements.

The plantar response may signal:
• a normal flexor plantar response (plantarflexion of the foot and adduction of toes); or
• a pathologic or abnormal extensor plantar response (Babinski’s sign), which involves dorsiflexion or extension of the great toe with or without fanning or abduction of the other toes.

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