The present study was a retrospective investigation of patients with DM greater than 20 years of age who underwent NCS at the electrodiagnostic clinic of Incheon St. Mary's Hospital from June 2011 to November 2014. This study was approved by the Institutional Review Board.

We excluded the patients who had previously undergone surgery for CTS; who had a previous history of trauma and had been diagnosed with fractures of the wrist; who had been diagnosed with cervical radiculopathy, brachial plexopathy, or a focal peripheral neuropathy other than CTS involving the upper extremities; who had medical diseases that could evoke peripheral polyneuropathies other than DM, such as chronic renal insufficiency, chronic alcoholism, toxin exposure, or hypothyroidism. We included a total of 63 dominant hands of diabetic patients, and all patients completed the Boston Carpal Tunnel Questionnaire (BCTQ) and underwent NCS and ultrasonographic evaluation of the median nerve CSA at the carpal tunnel on the same day.

On the basis of the results of these tests, the patients were classified into the following three groups: group 1 (n=16), in which NCS results did not reveal CTS; group 2 (n=19), in which NCS results revealed CTS but the group scored 0 points on the BCTQ; and group 3 (n=28), in which NCS results revealed CTS and the group scored >1 point on the BCTQ.

Prior to the electrodiagnostic test, a physiatrist administered the BCTQ [13]. This is a self-report scale that assesses the degree of symptom severity and upper extremity function on the basis of the patient's responses according to symptom and functional scale scores. Asymptomatic CTS was defined as a score of 0 points, and symptomatic CTS was defined as a score of >1 point following summation of symptom and functional scale scores. The validity and reliability of the BCTQ for the detection of symptom severity and functional status of CTS patients have been confirmed in many studies [14,15,16,17,18]. We used the Korean version of the BCTQ, which has also been shown to have good validity and reliability [19].

All subjects underwent NCS using an electromyography device (VIASYS Healthcare, Surrey, UK) and surface electrodes (VIASYS Healthcare). The skin temperature of the patients was measured prior to the test, and the temperature of each hand was maintained above 32℃. Motor conduction studies were performed on the median, ulnar, peroneal, and tibial nerves. F-waves were recorded from the median and tibial nerves. Sensory nerve action potentials (SNAPs) were obtained from the median, ulnar, and sural nerves. The median motor response was recorded from the abductor pollicis brevis muscle using supramaximal stimulation 7 cm from the active electrode; the onset latency and baseline-to-peak amplitude were then measured. The median sensory response was measured antidromically in the third digit, and stimulation was applied at two points: 7 and 14 cm proximal to the active electrode. The ulnar motor response was recorded from the abductor digiti minimi muscle using stimulation 7 cm from the active electrode. The ulnar sensory response was measured antidromically in the fifth digit, and stimulation was applied to the wrist 14 cm proximal to the active electrode. A sensory comparative study of the ring finger was performed when the peak latency of the median SNAP at the wrist was delayed. The electrode was placed over the fourth digit, and stimulations were applied to the median and ulnar sides of the wrist 14 cm proximal to the active electrode for each position. Bilateral superficial peroneal, sural, deep peroneal, and tibial nerves were also assessed for making the diagnosis of diabetic polyneuropathy (DPN). Superficial peroneal and sural nerves were antidromically recorded 14 cm distal to the stimulated sites.

CTS was diagnosed according to the American Association of Electrodiagnostic Medicine, American Academy of Neurology, and American Academy of Physical Medicine and Rehabilitation, practice parameters for electrodiagnostic studies in CTS: summary statement [20]. CTS was diagnosed in patients if they met the following criteria: (1) the peak latency of the median SNAP at the wrist (14 cm proximal to the active electrode) was delayed >3.6 ms; (2) the median-to-ulnar peak latency difference at the ring finger (14 cm proximal to the active electrode) was >0.5 ms; and (3) the wrist-to-palm latency difference was greater than the palm-to-digit latency difference in the median sensory nerve. The diagnosis of DPN was based on the Diabetes Control and Complications Trial [21]. All reference values were based on Dumitru et al.'s description [22].

An experienced Korean physician who was board certified in musculoskeletal ultrasound medicine performed the ultrasound examination using an HD11 XE device (Philips, Bothell, WA, USA) and a 7- to 12-MHz line-array transducer. The physician was blinded to the BCTQ scores and electrodiagnostic results. Ultrasonography was performed while the subject was lying supine with the wrist resting in a neutral position with the palm facing upwards. CSA of the median nerve was measured at the level of the pisiform bone using a direct tracing method within the echogenic boundary of the nerve. If the patient had a bifid median nerve, the summated CSA value of the nerves was used. CSA of the median nerve in each hand was estimated twice, and the mean value was recorded.

All statistical analyses were performed using SPSS ver. 18.0 for Windows (SPSS Inc., Chicago, IL, USA). Normality of the sample was tested by applying the Shapiro-Wilk test. The age and duration of DM were assessed using the Kruskal-Wallis test. HbA1c and the results of NCS were assessed using one-way analysis of variance (ANOVA). Comparisons of the CSA of the median nerve at the carpal tunnel among the three groups were analyzed by Mann-Whitney U test after Kruskal-Wallis test. The rate of DPN was analyzed using the chi-square test, and a p-value <0.05 was considered to indicate statistical significance.