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Neurometer® Polyneuropathy Diagnosis: Anatomic & Neuroselective

Mapping Polyneuropathy: Axonal vs. Demyelinating

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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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rev 10/16/09