Measure of NCS and CTS diagnosis at our hospital
At our hospital, patients suspected to have CTS were initially assessed by neurologists for clinical histories; their physical conditions were examined. Physical examinations included assessing the responses to the Tinel and Phalen tests and the thenar eminence, specifically, observing thenar muscle wasting according to the American Academy of Orthopaedic Surgeons guidelines [7]. We performed an NCS for all patients diagnosed with CTS based on clinical symptoms and provocation test results [8, 9]. During the NCS procedure at our hospital (Fig. 1), CTS was diagnosed if the CMAP terminal latency (TL) indicated a delay of 4.0 ms or more during a motor NCS (surface electrode on the abductor pollicis brevis [APB] and stimulation of the median nerve at the wrist and elbow), the sensory nerve conduction velocity was less than 50 m/s during a sensory NCS (surface electrode on the second finger and stimulation of the median nerve at the wrist and elbow), and the TL of the median nerve was 0.4 ms or longer compared with that of the ulnar nerve during an assessment of the CMAP (surface electrode on the second lumbrical and the first palmar interosseous and stimulation of the median nerve and ulnar nerve) and SNAP (surface electrode on the fourth finger and stimulation of the median nerve and ulnar nerve). Furthermore, by stimulating the palmar region of the median nerve during assessment of the CMAP (surface electrode on the APB) and SNAP (surface electrode on the third finger), measuring the amplitudes of the CMAP and SNAP, and comparing the amplitudes of the wrist and palm, we can evaluate the motor and sensory nerve demyelination status (conduction block) and the number of remaining axons [10]. The same laboratory technician (E.S) performed the NCS and severity classification using the following Bland’s classification [11]; grade 0 represents no neurophysiology abnormality, grade 1 is a very mild abnormality (detected only in two sensitive tests), grade 2 is mild CTS (sensory conduction velocity from index finger to wrist < 40 m/s with motor terminal latency from wrist to ABP < 4.5 ms), grade 3 is moderately severe CTS (motor terminal latency > 4.5 ms and < 6.5 ms with preserved index finger SNAP), grade 4 is severe CTS (motor terminal latency > 4.5 ms and < 6.5 ms with absent SNAP), grade 5 is very severe CTS (motor terminal latency > 6.5 ms), and grade 6 is extremely severe CTS (surface motor potential from APB < 0.2 mV).
Surgical criteria and surgical method for CTS at our hospital
We performed surgery for patients with impaired motor nerves classified as grade 3 or higher according to Bland’s classification and who do not show improvement with conservative treatment. In particular, for patients with cases classified as grade 5 or 6 and who showed severe impairment in the motor nerves that control the APB, we recommended surgery as the continuation of conservative treatment could cause APB atrophy.
The same surgeon (T.T) performed the surgery for all patients. All patients underwent surgery under a magnifying glass. After the upper arm was fastened with a tourniquet, surgery was initiated under local skin anesthesia with 1% epinephrine-containing lidocaine. A median longitudinal incision of approximately 35 mm that did not exceed the distal carpal line and a transverse incision of approximately 10 mm on the ulnar side of the proximal side were created. The palmar longus aponeurosis was split bilaterally to reveal the flexor ligament underneath. After checking the thickness of the flexor ligament and the presence or absence of calcification, the flexor ligament was carefully cut from the distal side to the proximal side without damaging the median nerve. In the proximal region, the flexor ligament may be thick and calcified; therefore, careful attention is necessary to ensure that the proximal operative field is fully deployed and that the proximal side of the flexor ligament remains uncut. The standard site for the distal incision is the perineural adipose tissue. After confirming sufficient decompression of the median nerve, compression from the tourniquet was released, the surrounding bleeding was stopped, and the wound was closed with a 4–0 nylon thread mattress suture [12].
Materials and measurements
This study was approved by the local ethics committee of Atsugi City Hospital (approval number R1–06). Overall, 93 patients (129 hands, 74 right and 55 left hands) with CTS underwent carpal tunnel surgery at our hospital between April 2014 and March 2019. The mean age was 67.1 ± 12.4 years; 26 patients were males, and 67 were females. The 60 hands (37 right hands and 23 left hands) of 46 patients with a preoperative Bland’s classification of grade 5 or 6 were assessed for the changes in Bland’s classification grade before surgery and 6 months after surgery. The mean age of these 46 patients was 66.2 ± 12.9 years, and 13 were males, while 33 were females. Among patients with a preoperative Bland classification of grade 5 or 6, those who showed improvement to postoperative grades 1–4 were included in the improvement group, whereas those who did not show improvement and had postoperative grade 5 or 6 were included in the non-improvement group. Amplitudes of the CMAP and SNAP of the palms in preoperative NCS were compared in the two groups. The Wilcoxon rank-sum test was used for statistical analyses. Numerical values are shown as mean ± standard deviation. A p-value < 0.05 was considered statistically significant.
Furthermore, we assessed for the degree of numbness of the fingers, primarily in the thumb and index, middle, and ring fingers; the presence or absence of APB atrophy; and the presence or absence of opposition movements of the thumb before surgery. The patients were asked to complete a question postoperatively to assess changes in clinical symptoms [13]. Six months postoperatively, patients were asked to compare their symptoms with those before surgery, rate them, and select any of the following five options: cured, much better, better, unchanged, and worse. Based on these results, the correlation between changes in Bland’s classification and the degree of clinical symptom improvement before and after surgery was examined. Informed consent for the procedure was obtained from all patients, as was permission to use their anonymized data.