DPN develops with chronic hyperglycemia and subsequent potential mechanisms induced by chronic hyperglycemia, including oxidative injury, activation of the polyol glucose metabolic pathway, deposition of advanced glycosylation end-products within nerves, and vascular insufficiency. A number of studies have reported that DPN is related to diabetes duration, hyperglycemia, current cigarette smoking, dyslipidemia, and hypertension [18,19]. In agreement with these observations, the results from a nationwide survey conducted by the Diabetic Neuropathy Study Group of the KDA in 2010 showed that older age, female sex, long duration of diabetes, presence of retinopathy, hypertension, or dyslipidemia, hyperglycemic status (i.e., being treated with insulin), and history of cerebrovascular disease or foot ulcers are independently associated with DPN [2].

Composite scoring systems that use symptoms, signs, or both have been developed to quantify general neuropathic deficits, such as the neurological symptom score of Dyck [20], the neuropathy symptom profile [21], and the neuropathy disability score (NDS) [21]. The DPN-specific symptom scoring system, the diabetic neuropathy symptom score [22], the modified NDS, which uses a sensory score (vibration perception threshold using a 128-Hz tuning fork and temperature perception, pin-prick) and reflex score (Achilles reflex) [23], neuropathy impairment score (NIS) [24], NIS of the lower limbs [25], a diabetic neuropathy examination [26], and the Toronto clinical neuropathy score are now available [27]. The Michigan neuropathy screening instrument (MNSI) for outpatients questionnaire has high specificity of 92%, and the Korean version has been validated [2,28]. The Michigan diabetic neuropathy score (MDNS) along with the MNSI score includes assessments of foot deformities and clinical sensory nerve tests to score DPN [28]. The Neuropathy Study Group of the KDA recommends the MNSI, MDNS, and modified NDS to evaluate patients with diabetes (Table 2). These scoring systems enhance diagnostic accuracy by combining the results of individual examinations. The severity of neuropathic pain and response to treatment in patients with painful DPN can be assessed using a visual analog scale or valid scales and questionnaires, such as the Korean version of the Brief Pain Inventory [3].

Clinical sensory nerve tests conducted at bedside use various devices to generate specific physical vibratory (128-Hz tuning fork) [29], pressure (10-g Semmes-Weinstein monofilament) [30], noxious (pin-prick) [31], or thermal stimuli (Tip-therm, AXON GmbH, Dusseldorf, Germany) [32], which deliver electrical signals along the sensory pathway (Table 1, Fig. 1) [33,34,35].

Quantitative sensory testing (QST) is a quantitative method usually graded using a continuous numerical scale to detect the threshold of thermal perception (cold or warm), vibration perception, current perception, pressure pain, and sudomotor function (Table 1) [28,29,32,36,37]. Vibration thresholds are particularly sensitive to detect mild or subclinical neuropathy and correlate well with other QST measures [4]. The current perception threshold (CPT) to 2,000-Hz stimulation is correlated best with vibratory thresholds, and the CPT to 5-Hz stimulation is correlated with thermal perception [37]. Sweating abnormalities may be an early manifestation of DPN. Sudoscan is a novel method to detect electrochemical skin conductance, which is proportional to the number of functional sweat glands [36].

While a sensory stimulus is an objective physical event, the response is highly subjective and depends on the examiner's experience, patient's cooperation, and confounding factors (age, sex, obesity, and smoking or alcohol consumption). This differs from an electrophysiological study of nerve conduction velocity (NCV) in which the stimulus generates evoked stimuli independent of the subjective response [38]. The QST is probably effective for documenting sensory abnormalities and changes in sensory thresholds during a longitudinal evaluation of patients with DPN. The Rochester Diabetic Neuropathy Study reported that "the QST should not be used as the sole criterion for diagnosing DPN but should be accompanied by at least one other defined abnormality before the diagnosis of DPN can be made"; therefore, the QST should be complementary to a thorough clinical assessment [39]. Although the QST has been used in clinical practice and clinical trials, it is not extensively used in clinical practice in Korea. Further studies are needed to develop the standardized test procedure, QST algorithms, and reference values from healthy test subjects.

The nerve conduction study (NCS) is a reliable and objective diagnostic method to evaluate the DPN treatment response [40]. The pathological findings of DPN are axonal loss, axonal regeneration, and demyelination in some patients [41,42]. The NCV is used to detect slowing of nerve conduction in nerve axons resulting from segmental demyelination and to measure the speed of both motor and sensory conduction, amplitude, distal latency, distance, F wave latency, and other factors [41]. The American Academy of Neurology (AAN) in conjunction with the American Association of Electrodiagnostic Medicine and the American Academy of Physical Medicine and Rehabilitation has a recommended protocol for NCS [40], which includes unilateral studies of sural, ulnar, and medial sensory nerves and peroneal, tibial, medial, and ulnar motor nerves with F waves with a minimum case detection criterion to confirm distal symmetric polyneuropathy for clinical research; "an abnormality (≥ 99th or ≤1st percentile) of any nerve conduction attribute in two separate nerves, one of which must be the sural nerve." The Nerve Conduction Criteria Study indicated subclinical distal symmetric polyneuropathy to be clinically acceptable if "≥1 among abnormal attributes in ≥2 separate nerves" for clinical practice [43]. Although a NCS is regarded as the gold standard in clinical research, it is not useful in clinical practice because it is time consuming, requires special devices and trained examiners, and has no general consensus regarding its criteria, even after multiple investigations [40]. In addition, a NCS is sensitive enough to detect abnormalities in large nerve fibers but is not sensitive enough to detect to small nerve neuropathy, which is the earliest detectable sign of DPN [44].

A nerve biopsy, typically of the sural nerve, is rarely used in clinical practice, due to its invasiveness [6]. However, a skin biopsy assessment of intraepidermal nerve fiber density (unmyelinated C fibers and small myelinated fibers) has been proposed as a valid method to detect early small nerve neuropathy, even when signs of DPN are minimal or absent and when myelinated nerve fiber morphology is still normal [45]. A nerve biopsy may detect pathological changes in small nerve fibers and correlate well with the structural pathology of axons. However, this procedure is invasive to patients and normal reference value in Korean subjects must be established.

Corneal confocal microscopy is used to assess the pathology of the corneal subbasal plexus of nerve fibers originating from the ophthalmic division of the trigeminal nerve, particularly corneal nerve fiber length, which is highly reproducible [46]. This noninvasive technique is sensitive to detect corneal nerve fiber damage during the earlier stages of DPN, and the extent of corneal nerve damage and repair correlates with peripheral nerve function. The nerve fiber regeneration response to therapeutic intervention enables documentation of the natural history of DPN in patients at follow-up [47]. This method, however, needs expensive device and expert examiner for examination.