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 Most
metabolic and toxic neuropathies typically affect sensory
nerves before motor nerves and present with a dying back distribution in the
tips of the toes first and later the fingers. Mapping the distribution of
sensory impairment resulting from polyneuropathy and noting the type of
axonal dysfunction permits the differential diagnosis of diffuse
demyelinating vs distal axonal polyneuropathies. CPT measures from the big
toe (A) are generally the first to be affected, ranging from the earliest
hyperesthetic stage to the advanced hypoesthetic condition. Normal sensation
at proximal test sites (B, C) confirms the clinical diagnosis of distal
dying back polyneuropathy. In contrast, demyelinating polyneuropathies
typically affect both proximal and distal CPT measures associated with
myelinated fiber function (2000 Hz and 250 Hz CPTs) but not measures
associated with unmyelinated fiber function (5 Hz CPTs).
CPT testing at the big toe incorporates both the superficial
and the deep peroneal nerves (4th and 5th lumbar dermatomes respectively,
site A above). This neurological overlap prevents focal nerve lesions or
mono-radiculopathies from affecting the CPT measures at this site.
Conversely, symmetrical dying back polyneuropathies affecting both the
superficial and deep peroneal nerves are easily detectable at this location.
The figure to the right is the standardized
sNCT CPT Test Sites.
See cited
publications characterizing polyneuropathy by Neurometer evaluation.
Polyneuropathy Utilization Guidelines
Distal Symmetrical Polyneuropathy
Sensory Nerve Conduction Threshold (sNCT™) evaluations are conducted at
symptomatic and asymptomatic sites to document abnormal distributions of
sensory nerve function and assist in the diagnosis. Diagnoses commonly
include the following categories: radiculopathy, compressive/focal lesions
and polyneuropathy. Standardized test sites and protocols provide a
documented basis consistent with a clinical diagnosis.
sNCT evaluations from the tips of the ring fingers may
be indicated when abnormal measures are obtained from the tests on the great
toes to determine the presence of polyneuropathy in the upper extremity. If
sNCT findings from these sites are normal, then no further testing is
required. If sNCT findings are uniformly anesthetic at a site, then more
proximal testing on a site where sNCT measures are obtainable is
appropriate. When proximal testing is conducted on the same nerve additional
billing is inappropriate.
(Note: Proximal testing determines the extent of the neuropathy, aids in
performing a differential diagnosis and enables the physician to quantify
changes in the patient's condition in a follow-up evaluation.)
Asymmetric
Polyneuropathy
Asymmetric polyneuropathy is diagnosed through bilateral, proximal and
distal sNCT evaluations. Sites evaluated include the peroneal nerve (distal
limb nerve), lateral antebrachial cutaneous (mid-limb nerve) and C2
dermatome (proximal segment nerve).
(Note: Asymmetric polyneuropathies are primarily immune mediated and
associated with conditions such as Chronic Inflammatory Demyelinating
Polyneuropathy (CIDP).)
(Cutaneous nerve illustration
by Frank Netter, MD)
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