Comparison of sensory nerve conductions in palmar cutaneous branch (PCB) and digit I nerves was applied in 50 patients suspected of having carpal tunnel syndrome (CTS) and 40 healthy persons. The abnormalities were defined as the differences in latencies and sensory nerve conduction velocities (SNCVs) of more than mean plus 2.5 SD of controls. Comparing these 2 sensory nerves, statistical differences were seen in 83.7% (36 of 43 patients) of their sensory latencies and 76.7% (33 of 43 patients) of their SNCVs. This excluded 7 patients who had unobtainable sensory nerve action potentials in digit I nerves. This method may serve as an adjunctive technique in the diagnosis of CTS. Its diagnostic sensitivity was high in comparison with other testing methods. Key words: neural conduction palmar cutaneous branch digital nerve median nerve carpal tunnel syndrome MUSCLE & NERVE 14~1173-1176 1991

COMPARISON OF SENSORY NERVE CONDUCTION IN THE PALMAR CUTANEOUS BRANCH AND FIRST DIGITAL BRANCH OF THE MEDIAN NERVE: A NEW DIAGNOSTIC METHOD FOR CARPAL TUNNEL SYNDROME CHEIN-WE1 CHANG, MD, and I-NAN LIEN, MD

C a r p a l tunnel syndrome (CTS) is the most common entrapment neuropathy seen in electrodiagnostic tests. Various methods, including nerve conduction studies and needle electromyography, are available in making the diagnosis of this syndr~me.~,~,~ In this study, we presented a new diagnostic method for CTS with comparison of sensory nerve conductions in palmar cutaneous branch (PCB) and digit I nerves. Anatomically, these 2 sensory branches of the median nerve had different passing courses and distributions. Digit 1 nerve passes through the carpal tunnel and supplies the volar thumb area. The PCB nerve passes through its own channel, radial to the carpal tunnel,' and supplies the thenar eminence. Normally,

the difference of latencies and SNCVs between these 2 nerves is small; however, in patients with CTS, the digit I nerve is commonly affected and the differences of the nerve conductions between these 2 nerves should increase. The sensitivity of this test compares well with other nerve conduction studies.

I

From the Department of Physical Medicine and Rehabilitation, National Taiwan University, College of Medicine, Taipei, Taiwan, ROC Acknowledgments: We are greatly obliged to Chi-Tsou Huang, MD, in the Department of Physical Medicine and Rehabilitation, University of Alabama at Birmingham, for his assistance and comments. Address reprint requests to Chein-Wei Chang, MD, Department of Physical Medicine and Rehabilitation, National Taiwan University Hospital No. 1 , Chang-Te Street, Taipei 10016, Taiwan, ROC. Accepted for publication December 10, 1990 CCC 0148-639)(191/01201173-04 $04.00 0 1991 John Wiley & Sons, Inc.

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Table 1. Peak latency values of SNAPS and SNCVs obtained in digit I and PCB nerves. ~~~~~~

~

~~

~~

Peak latency of SNAP (ms)

Digit I Normal controls (n = 40) Mean -c 1 SD 2.38 2 0.12 Range 1.96-2.60 Patients suspected CTS (n = 43) Mean 1 SD 3.39 k 0.64 Range 2.56-5.1 2 Normal limit Normal mean + 2.5 SD Abnormality (%)

*

~~

SNCV (m/s)

PCB

Digit I-PCB difference

Digit I

PCB

Digit I-PCB difference

2.24 2 0.18 1.92-2.56

0.10 t 0.08 0-0.28

42.21 ? 2.29 38.52-51.04

43.34 2 3.52 39.16-52.16

2.10 1.75 0-5.56

2.37 ? 0.22 1.96-3.04

1.14 2 0.68 0.08-2.48

30.48 ? 5.42 19.51-40.33

42.68 3.73 35.72-50.48

*

0.30 36/43(83.7%)

*

*

13.10 6.33 2.84-27.41 6.47 33/43(76.7%)

Abbreviations: CTS = carpal tunnel syndrome, SNAP = sensory nerve action potential, SNCV = sensory nerve conduction velocity, dfgit I = first digital nerve, PCB = palmar cutaneous branch

SUBJECTS AND METHODS

Fifty patients referred to our laboratory to rule out CTS participated in this study. There were 34 women and 16 men (age range: 25 to 64 years, mean: 42.3 years). All patients had a history of the syndrome, with intermittent paresthesia occurring spontaneously at night or after repetitive use of the affected hand. None had any underlying disease such as diabetes. Forty healthy subjects served as controls (age range: 22 to 60 years, mean: 38.6 years). SNCV studies for PCB and digit I nerves were performed using an orthodromic method. The anatomical landmarks of placing electrodes are

Latency difference (ms) PCB-Digit I

shown in Figure 1. T h e abnormalities were defined as the value beyond the mean plus 2.5 SD for the peak latency and SNCV differences, based on normal controls. T o compare with other methods for sensitivity, distal motor latency (DML), and SNCV studies over digit 11-to-wrist and palm-to-wrist segments of the median nerve by the orthodromic method were also applied. Because of the wide range of sensory nerve action potential (SNAP) amplitudes obtained in these nerve studies, the measurement of amplitude, as well as amplitude differences between these nerves, were not considered for analysis.

SNCV difference(m/sec) PCB-Digit I

FIGURE 2. Histograms of the latency and SNCV differences between the PCB and digit I nerves in normal controls.

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Comparison of Nerve Conduction in PCB and Digit I

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v)

c

u

10

c L

0) n

f

2

5

0 0

0.2 0.4 0.6 0.8

1.0

1.2

1.4

1.6

1.8 2.0

2.2

2.4 2.6 2.8 3.0

Latency difference(ms1 PCB-Digit I

0

5

10

15

20

25

30

35

40

SNCV difference(m/sec) PCB--Digit I

FIGURE 3. Histograms of the latency and SNCV differences between the PCB and digit I nerves in patients suspected of having CTS.

RESULTS

Among the 50 patients with clinically suspected CTS, 7 patients were excluded because of their absence of SNAP in digit I nerve. The results of the peak latency of SNAP and SNCV obtained in the remaining 43 patients and 40 healthy subjects are shown in Table 1. In contrast, the values of peak latencies of SNAPS and SNCVs obtained in the digit I nerves were significantly higher in the patient group than in the normal controls (t test, P < 0.01, respectively). T h e latency and SNCV differences be-

100

tween the PCB and the digit I nerves were also higher in patients with CTS than in the normal controls ( P < 0.01). T h e results of the differences in peak latency of SNAP and SNCV obtained from the normal control and the patient groups are histographically shown in Figures 2 and 3. Based on the normal limit adopted in this study (mean + 2.5 SD), 36 of 43 subjects (83.7%)were abnormal in latency difference, whereas 33 of 43 subjects (76.7%) were abnormal in SNCV difference between the PCB and the digit I nerves. The comparison of the diagnostic sensitivities

rl

n

80

0Nor maI 60

m

u

C

Abnormal

2

P"

40

20

0

Latency diff. PCB-Digit I

SNCVdiff. PCB-Digit I

Latency Digit Il-W

SNCV Digit Il-W

Latency P-w

SNCV

DML

P-w

FIGURE 4. Sensitivities expressed as percentage of accuracy in 43 patients suspected to have CTS. SNCV = sensory nerve conduction velocity, PCB = palmar cutaneous branch, digit I = first finger, digit II = index finger, P = palm, W = wrist, and DML = distal motor latency of median nerve.

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with other testing methods is illustrated in Figure 4. DISCUSSION

The PCB of the median nerve is a small but important sensory nerve supplying the thenar eminance of the hand.* Anatomically, PCB originates at the radial side of the median nerve approximately 5.5 cm proximal to the radial sty10id.l.~It traverses the transverse carpal ligament through its own tunnel, which is 9- to 16-mm long, medially to the flexor carpi radialis tendon. Because the PCB did not pass through the carpal tunnel, it should not be affected in carpal tunnel compression. In the nerve conduction study of PCB, the orthodromic method was preferred in obtaining SNAPS, because the use of the antidromic method might create more motor artifact; however, the antidromic method will have a higher amplitude.

Adopting the peak latency of SNAP gives more reliable and reproducible results than those obtained from the take-off point of the negative deflection. Because of the small innervated area of PCB at thenar eminance, and possibly some anatomical variation in that area,7 the studies of these nerves should be done with caution and require more waves in averaging SNAP. The stimulation intensity was increased slowly by not causing a supramaximal stimulation; this prevented the spread of potential to the other nerve branches by means of volume conduction. The abnormalities were defined as the differences in the latencies and the SNCVs of more than 2.5 SD from the mean versus controls. This will provide an almost 99% confidence level. Our method of study provides an adjunctive way of diagnosing CTS. Further study is necessary to explore the false-negative or false-positive results as found in other cases of nerve entrapment syndr~me.~

REFERENCES 1. Carroll RE, Green DP: Significance of palmar cutaneous nerve at wrist. Clzn Orthoped 1972;83:24-28. 2. Hobbs RA, Magnussen PA, Tonkin MA: Palmar cutaneous branch of the median nerve.] Hand Surg 1990;15A:38-43. 3. Johnson EW: Carpal tunnel syndrome, in Johnson EW (ed): Practical Electromyography (2 ed). Baltimore, Williams and Wilkins, 1988, pp 187-205. 4. Kimura J: Electrodiagnosis in Diseases of Nerve and Muscle: Princzples and Practice (2 ed). Philadelphia, FA Davis, 1989, pp 501-504.

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5. Redmond MD, Rivner MH: False positive electrodiagnostic tests in carpal tunnel syndrome. Muscle Nerve 1988;11:511517. 6. Stevens JC: AAEM Minimonograph #26: The electrodiagnosis of carpal tunnel syndrome. Muscle Nerve 1987;10:99113.

7. Taleisnik J: Palmar cutaneous branch of median nerve and approach to carpal tunnel: anatomical study. J Bone Joint Surg 1973;55A:1212- 12 17.

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