Fiber Spectra Distribution of a Typical Peripheral Sensory Nerve

Sensory neuropathy is often selective for an individual sub-population of nerve fibers while sparing others.  This results in various types of “painless” and "painful" nerve damage or neuro-pathy.  This table illustrates the neuro-selectivity of standard sensory nerve tests and the Neurometer evaluation.  In one automated painless procedure, the Neurometer tests all three sub-populations of sensory nerve fibers .  The thermal, vibratory and monofilament tests are receptor mediated.  These thresholds are effected by burns from skin freeze lesions that result in receptor mediated inflammation.   In contrast neuroselective CPT measures are not effected by burns from skin freeze lesions (see Lötsch J, Angst MS. The mu-opioid agonist remifentanil attenuates hyperalgesia evoked by blunt and punctuated stimuli with different potency: a pharmacological evaluation of the freeze lesion in humans. Pain Vol. 102:151-161, 2003). 

Direct Nerve Fiber Function Test:  The sNCT CPT evaluation directly excites the nerve fibers. CPT measures are not mediated by local nerve cell membrane receptors at the site of stimulation.  The CPT measures are dependent upon the electrical excitability of the nerve fibers at the site of stimulation.  The CPT being a functional electrodiagnostic evaluation has the unique ability to detect hyperesthesia or abnormally low sensory thresholds.  Hyperesthesia is common in early inflammatory neuropathic conditions ranging from HIV infection, carpal tunnel syndrome to the earliest stages of diabetic neuropathy.  In the advanced stages of neuropathy nerve function deteriorates and predominantly hyposesthetic measures are detected.

Fiber Spectra Profile of a Typical Sensory Nerve

Peripheral nerves are composed of three distinct sub-populations of nerve fibers that are characterized by their diameter.  The above figure above illustrates the number of fibers vs. diameter that comprise the typical sensory nerve.  These various sub-populations of nerve fibers transmit different sensations. The smallest (unmyelinated) fibers transmit dull pain and temperature, the middle diameter (small myelinated) fibers transmit fast pain, temperature and pressure sensation, and the largest (myelinated) fibers transmit touch and pressure sensation.  These three sub populations of nerve fibers work together to transmit normal sensation.  For example, when one touches a glass, first the glass is felt (at (50 m/s) and only afterward the temperature (at 1 m/s).  The right table below compares the neuroselectivity of the Neurometer CPT evaluation versus the Skin Punch Biopsy and the Nerve Conduction Velocity and the Somatosensory Evoked Potential evaluations. Additional diagnostic test comparisons are available at this link.

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