EDX technique |
DSL (msec) |
DML (msec) |
Amplitude (µV) |
NCV (m/s) |
Distal motor latency (DML) of the median nerve
|
- |
2.2-4.2 |
5000-25000 |
50-60 |
Median DML of APB muscle compared to ulnar DML of ADM muscle
|
- |
1.0 |
|
|
Distal sensory latency (DSL) of median nerve, digit 2
|
2.9-3.6 |
- |
10-100 |
48-65 |
Median DSL of digit 2 compared to ulnar DSL of digit 5
|
< 0.5 |
- |
- |
- |
Median DSL of digit 4 compared to ulnar DSL of digit 4
|
< 0.6 |
- |
- |
- |
(8 cm distance);
(14 cm distance); APB: Abductor pollicis bravis; ADM: Abductor digiti minimi
The CTS has been reported to be more frequent in diabetic patients with DPN than in general population17, 18. The CTS and DPN often coexist and can be difficult to distinguish by EDX findings in clinical practice18-20, because CTS and DPN may produce similar abnormalities in median NCS. The use of EDX criteria in these patients results in a high rate of false-positive diagnosis. On the other hand, uncertainty of the EDX criteria of CTS in diabetic patients with DPN, have not been established14-20.
Usefulness of Additional Electrodiagnostic Techniques for Median neuropathy at the wrist (Carpal Tunnel Syndrome) in Patients with Diabetic Polyneuropathy
AAEM recommended guidelines for the study of CTS, which include looking at the median nerve sensory conduction across short segments. By comparing a proximal sensory segment across the carpal ligament with a more distal segment, it is possible to differentiate between CTS and DPN16-21. The CTS patients will have more slowing nerve conduction velocity (NCV) across the carpal ligament, while the DPN involves more distal nerves commonly in the upper extremity, especially the ulnar and median nerves will be involved. While in the CTS, the median nerve will be significantly slowed.
The objectives of this article are finding out and review the literatures, in the year 2005-201116-27, which studied the additional EDX studies distoproximal latency ratio (DPLR)16, 20, 22-26, wrist-palm median sensory conduction velocity (W-P SCV)18, 23-25, 2nd lumbrical-interosseous median-ulnar distal latency difference (2nd LIMULD)17, 20, 26, median-radial distal sensory latency difference (M-RSLD)25, 27, have been proposed to determine CTS with DPN, but there is no consensus of these tests is most reliable16,17, 21.
The Distoproximal latency ratio (DPLR) and Wristpalm median sensory conduction velocity (WP SCV)
The median nerve was obtained antidromically, it was stimulated 3 cm proximal to the distal wrist crease, and 3 cm distal to the distal wrist crease (mid-palm), recording by ring electrodes from the index finger22-25. Median palm digit and wrist palm latencies compared by using the distoproximal latency ratio (DPLR) (Fig. 1). It is calculated as follows: DPLR = palm digit latency/[wrist digit latency - palm digit latency]16, 20, 22-26. Wrist-palm median sensory conduction velocity (WP SCV) 17, 23-25 was calculated using distal latency difference of the median nerve at the time of wrist and palm stimulation (Fig 2).

Figure 1 The positions of stimulating and recording electrodes for measurement of Distoproximal latency ratio (DPLR) (applied from references 22-25).
The DPLR for the cut-off value of 1.0, and median-ulnar distal sensory latency difference (M-USLD) to digit 4 for the cut-off value of 0.35, showed the highest sensitivity (90% for both DPLR and M-USLD) and specificity (81% for DPLR, 85% for M-USLD) in the diagnosis of CTS in DPN patients (n CTS+DPN group = 62, n CTS group = 140, n total = 349 hands)25.

Figure 2 The positions of stimulating and recording electrodes for measurement of Wrist-palm median sensory conduction velocity (WP SCV)23, 25
Wristpalm median sensory conduction velocity (WP SCV)23, 25, with the cut-off value of 41.4 m/sec, showed high sensitivity (85%) and specificity (77%). It could be a useful technique for EDX of CTS in DPN patients (n CTS+DPN group = 62, n CTS group = 140, n total = 349 hands)25.
The 2nd lumbrical-interosseous median-ulnar distal latency difference (2nd LIMULD)
The 2nd lumbrical-interosseous median-ulnar distal latency difference (2nd LIMULD) is comparison study of median distal motor latency (2nd lumbrical) to the ulnar distal motor latency (2nd interosseous). The 2nd LIMULD with a distance of 3 cm proximal to distal wrist crease and 8 cm from cup-shaped active electrode at 2nd lumbrical and interosseous muscles (Fig. 3). The 2nd LIMULD can identify the CTS in diabetic DPN patients (sensitivity of 88.4%) better than median-radial distal sensory latency difference (M-RSLD) with a distance of 9 cm (sensitivity of 73%), and M-USLD with a distance of 12-13 cm (sensitivity of 54%). The difference values are more than 0.4 msec for 2nd LIMULD, 0.5 msec for both M-RSLD and M-USLD (n CTS+DPN group = 43, n CTS group = 45, n total = 180 hands)26.

Figure 3 The positions of stimulation and recording electrodes of the comparison study of latency difference between a) median distal motor latency (2nd lumbrical muscle) to the b) ulnar distal motor latency (2nd interosseous muscle)26
The median-radial distal sensory latency difference (M-RSLD)
The M-RSLD to digit 1 (distance of 9 cm, cut-off value of 0.55 msec) showed high sensitivity (82%) and specificity (80%) for CTS patients with DPN (n CTS+DPN group = 62, n CTS group = 140, n total = 349 hands)25. And the other study, combined sensory index (CSI) showed the M-RSLD (distance of 10 cm, cut-off value of 0.5 msec; or thumb-diff) (Fig 4) had high sensitivity (100%) but low specificity (52.2%). The M-USLD (distance of 14 cm, cut-off value of 0.4 msec; or ring-diff) and orthodromic median-ulnar distal sensory latency difference (distance of 8 cm, cut-off value of 0.3 msec; or palm-diff) also had high sensitivity (100%) with moderate to high specificity (82%) in the CTS patients with DPN (n = 52 hands)27.

Figure 4 The positions of stimulation and recording electrodes of the median-radial distal sensory latency difference (M-RSLD) study25, 27
Conclusion
Segmental median NCSs like the DPLR or WP SCV and sensory comparative tests, such as M-RSLD and M-USLD, in combination with standard NCS techniques should result in more accurate diagnosis of sensory involved CTS in DPN patients. Among the motor NCS, the 2nd LIMULD also is additional usefulness technique in motor involved CTS in DPN patients.
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