Recently, the number of diabetic patients has increased very rapidly and is accompanied
by an increasing development of diabetic neuropathies (DNs). The incidence and prevalence
of DNs associated with duration of diabetes affects up to 50% of diabetic patients
after 25 years of disease. Although DNs are known as the most common complications
of diabetes, they was not adequately and properly diagnosed, and their severity is
not estimated reliably in clinical practice by current methods. DNs have a wide variability
in prevalence, from a few percentage points to over 50%. This could be explained by
a lack of consensus about defining criteria; lack of sensitive, objective and quantitative
diagnostic tools; and lack of homogeneity in research subjects of DNs. In addition,
we do not have accurate and reproducible clinical end-point assessment modalities
by each researcher. The importance of accurate and early detection of DNs is emphasized
by the prediction of all-cause and disease-specific mortality in patients with diabetes
accompanied with intensive glycemic control.
After the 1988 San Antonio conference on DNs, several diagnostic criteria for DNs
were proposed, and recently, Tesfaye et al.1 proposed separate criteria for DNs. They
proposed that DNs are symmetrical, length-dependent polyneuropathies attributable
to metabolic and microvessel alterations as a result of chronic hyperglycemia exposure
(diabetes) and cardiovascular risk covariates. Abnormalities in nerve conduction studies
(NCS), which are frequently detected in subclinical DN conditions, are known to be
the first objective quantitative indication of this condition. Furthermore, for research
purposes, the authors suggested that confirmed and subclinical DNs must be evaluated
by NCS.
If we suspect DNs in diabetic patients, to diagnose and characterize the condition
we must exclude other causes of sensory motor neuropathies. Precise patient history
and neurological examinations must be carried out to obtain much more information
about the general characteristics. For reported symptoms and signs, and other clinical
neurophysiological tests, such as quantitative sensory tests (QSTs), abnormal results
were required to characterize the symptoms, signs and overall severity of the DNs.
Because assessing methodologies and techniques, evaluation time and reference values,
and validated QSTs are not sufficient for clinicians, the usual clinical evaluation
and neurophysiological tests for DNs diagnosis and staging results have many limitations.
Furthermore, the usual clinical evaluation and neurophysiological tests cannot define
the nature of the pathophysiological changes and the clinical features that specify
the distribution of nerve involvement or the time-course and stage. To accurately
and reliably evaluate the kind, severity and distribution of sensation loss, we require
more standardized, validated and referenced methods. Also, the clinician's proficiency
has been added for adequate assessment of DNs by other researchers2.
As we know, DNs involve motor, sensory and autonomic nerves, and specify their symptoms
and signs by damaged nerve fiber size and type. Therefore, variable parameters of
damaged nerves and their manifestations are important for the diagnosis of DNs, in
addition to a patient's neuropathic symptoms and signs. Because NCS investigate only
large myelinated fibers, occasionally there is a discrepancy between the morphology
and physiology of such small fiber neuropathy. Nerve fiber evaluation tests reflect
three conditions: normal, axonal injury and myelin loss of peripheral nerves. Normal
means that axons and myelinated fibers are intact, axonal injury shows that damaged
axons disconnect fibers from sensory nerves or motor nerves. Finally, myelin loss
occurs at multiple sites along the nerve, and results in the slowing of conduction
velocities. Conduction velocity can be mildly slowed by metabolic causes, such as
hyperglycemia, does not structurally affect myelin and is reversed by correction of
underlying metabolic abnormalities. Parameters of NCS, such as peroneal conduction
velocity and sural sensory nerve action potential (SNAP) amplitude expressed as percentiles
and adjusted for variables of age and anthropomorphic variables, are especially sensitive
indicators of DNs3. Various factors affect the rate of nerve conduction. Most important
in NCS are the temperature of the tested nerve, normal variations among nerves and
nerve segments, and patient age. NCS is slowed at lower temperatures in a linear manner,
and the effects of temperature are more apparent with sensory than with motor nerves.
With lower nerve temperatures, SNAPs are longer in duration, resulting in less phase
cancellation and larger SNAP amplitudes. The lower nerve temperature slows conduction,
by approximately 2 m/s/°C. To prevent misdiagnosis, the limb skin temperature of the
patient should be maintained at over 31°C3.
In the clinically relevant late stage of DN complications, although the nerve parameter
of NCS can use surrogate markers and widely accepted objective methods for the diagnosis
of DNs, it cannot adequately evaluate the sensation loss area and severity. Therefore,
the attributes of NCS are weak measures of DNs severity, and provide only limited
information about the kind and distribution of sensory loss. Furthermore, NCS is a
complex, time-consuming procedure, and requires specialized equipment and experts.
Also, although abnormal parameters in NCS can predict the outcome of DNs, there is
only limited data for the prediction of incipient DNs at a stage that precedes its
complications. To evaluate the stage and severity of DNs, objective and/or quantitative
measures, such as NCS and QSTs, are required. As the severity of DNs is a combination
of neuropathic symptoms and signs, abnormal neurophysiological test results, and other
neuropathic dysfunctions and impairments, the sum scores of various measures of neurological
signs and symptoms, neurophysiological test scores, or scores of function of quality
of life are require, and provide the grade of severity1.
Recently, some researchers reported that NCS could be used for the early detection
and prediction of DNs. Hyllienmark et al.4 carried out a study to examine whether
subclinical nerve dysfunction as reflected by electrodiagnostic testing predicts the
development of clinical neuropathy in 59 type 1 diabetic patients who were aged 15.5 ± 3.22 years
and duration of diabetes was 6.8 ± 3.3 years. At baseline, patients' nerve conduction
velocities and amplitudes were modestly reduced without clinical neuropathy evidence
compared with healthy controls. At follow up, approximately 13 years later, nine patients
(15%) showed clinical neuropathy, and they showed more significant reductions in all
tested nerve velocities and amplitudes, and showed a negative correlation between
peroneal motor conduction velocity, sural sensory conduction velocity, sural SNAP,
and peroneal compound muscle action potential and age. Also, patients' glycated hemoglobin
was 6.9 ± 1.03% at baseline to 7.4 ± 0.94% at follow up, and was correlated with NCS
results and neuropathy impairment assessment. From these results, they concluded that
subclinical nerve dysfunction, as defined by NCS data, predicted clinical neuropathy
many years later, and that the strongest predictor for the presence of clinical neuropathy
after an average of 20 years with type 1 diabetes was poor metabolic control during
the first years of the disease. Therefore, they emphasized that the role of early
NCS and good metabolic control during the early years of type 1 diabetes was important
to detect and predict DNs development. Although the role of early detection and prediction
of NCS in DNs is useful, a limitation of that study was that the sample was composed
of type 1 diabetes patients only. Therefore, further research into the early detection
and predictive roles of NCSs as markers of nerve damage in type 2 diabetespatients
is required. Weisman et al.5 carried out a different study in 406 participants (61
with type 1 diabetes and 345 with type 2 diabetes) to determine the measurement of
single and simple combinations of NCS parameters for identification and future prediction
of DNs. At baseline, 246 (60%) patients were prevalent cases, and after 4 years of
follow up, 25 (23%) of the 109 prevalent controls that followed became incident DNs
cases. From that study, they reported that threshold values for peroneal conduction
velocity and sural SNAP best identified prevalent cases, and baseline tibial F-wave
latency, peroneal conduction velocity and the sum of three lower limb nerve conduction
velocities (sural, peroneal and tibial) best predicted 4-year incidence. Also, they
concluded that individual NCS parameters or their simple combinations were valuable
for identification and future prediction of DNs. However, that study required further
study for the implication of the amplitude potential and conduction velocity threshold
value that differ from the normal distributions of NCS parameters in detection and
prediction of DNs.
In conclusion, because of the lack of unified diagnostic criteria for DN, we have
many problems carrying out various clinical trials, such as therapeutic efficacy and
epidemic research with unified research protocols. Therefore, DNs epidemic study results
are variable by current diagnostic tools. To solve these problems, we required only
one reasonable consensus definition for DNs diagnosis and prediction. Furthermore,
each of the current diagnostic tools and procedures has their advantages and weakness.
Both NCS and QSTs have high reproducibility and are complementary to each other. Neurological
examinations, and neuropathic signs and symptoms are important in early detection
and severity evaluation (Figure1). However, these assessments require good performance
and proficiency of the physicians. For early detection and prediction of DNs, we must
carry out several neurological examinations and NCSs in the lower extremities and
both feet. It is well known that NCS changes in DNs patients occur earlier than clinical
symptoms and signs without small fiber neuropathy, therefore NCS results are important
in the early detection of DNs. Although current NCS performance has many limitations,
proper nerve and machine selection for reproducible and convenient measurement is
more important and valuable for early detection and prediction of DNs.
Figure 1
Abnormalities of diagnostic tool by diabetic peripheral neuropathy type. CPT, current
perception threshold; DM, diabetes mellitus; DN, diabetic neuropathies; IENFD, intra-epidermal
nerve fiber density; NCS, nerve conduction study; QST, quantitative sensory test;
SFN, small fiber neuropathy; VPT, vibration perception threshold.