Wrist – Digit II, Wrist – Digit V

and Mario Di Napoli¹

(1)

Neurological Service, S. Camillo de’ Lellis General Hospital, Rieti, Italy

Original Settings

Sensitivity was 20 μV/division, sweep speed was 2 ms/division, duration of pulse was 0.1–0.4 ms, and the machine used was a Tektronix electromyograph. Low-frequency filter, high-frequency filter, and rate of pulse were not specified.

Position

This study was performed in the supine position.

Recording

Following the antidromic method [1], signals were recorded using surface electrodes from the median and ulnar nerves separately (Fig. 1). For the median nerve (R1), the active recording electrode (A) was placed around the proximal interphalangeal joint of digit II; the reference electrode (R) was placed over the distal interphalangeal joint of digit II. For the ulnar nerve (R2), the active recording electrode (A) was placed around the proximal interphalangeal joint of digit V; the reference electrode (R) was placed over the distal interphalangeal joint of digit V. The proximal interphalangeal joint rather than the base of the finger was selected as the site for the proximal ring electrode to reduce or eliminate the volume-conducted motor response. Adjacent digits were kept from inadvertently contacting each other. Ground (G) was placed on the dorsum of the hand, between the recording and the stimulating electrodes (the figure shows the ground electrode placed on the palm). The author used the same technique described previously [2].

Fig. 1

Antidromic sensory nerve action potentials (SNAPs) recorded to digit II (upper trace) and to digit V (lower trace), stimulation of the median and ulnar nerves at the wrist

Stimulation

Stimulations were applied over the median and ulnar nerves at the wrist. Median and ulnar nerves were stimulated separately (S1, S2), both at 14 cm from the active recording electrode (A) placed around the proximal interphalangeal joint of digit II (R1) and digit V (R2). The cathodes were placed proximally. The author used only supramaximal stimulation with a duration of 0.1–0.4 ms.

Measurements

Peak to peak amplitude (μV) of the sensory nerve action potential (SNAP) was measured from the negative peak to the positive peak of the SNAP. In each hand of each volunteer, the amplitude of the median SNAP was also expressed as a percentage of the amplitude of the ulnar SNAP. Skin and room temperature data were not given. Median to ulnar amplitude ratio (Amp Rat) was determined by dividing the median digital sensory amplitude (stimulating at the wrist and recording to digit II) by the ulnar digital sensory amplitude (stimulating at the wrist and recording to digit V) as follows:

$\mathrm{Amplitude}\;\mathrm{ratio}\;\left(\%\right)=\frac{\mathrm{Digital}\kern0.24em \mathrm{sensory}\kern0.24em \mathrm{amplitude}\kern0.24em \mathrm{t}\mathrm{o}\kern0.24em \mathrm{t}\mathrm{he}\kern0.24em \mathrm{digit}\kern0.24em \mathrm{I}\mathrm{I},\kern0.24em \left(\mu \mathrm{V}\right)}{\mathrm{Digital}\kern0.24em \mathrm{sensory}\kern0.24em \mathrm{amplitude}\kern0.24em \mathrm{t}\mathrm{o}\kern0.24em \mathrm{t}\mathrm{he}\kern0.24em \mathrm{digit}\kern0.24em \mathrm{V},\kern0.24em \left(\mu \mathrm{V}\right)}$

The author studied 100 normal hands from 50 healthy volunteers (Table 1) and 100 hands from 60 patients (Table 2) with the diagnosis of CTS (in 22 patients, the diagnosis was bilateral).

Table 1

Normal values [1]

Median nerve – peak to peak Amplitude (μV) wrist–digit II	Mean ± SD	Range
All subjects	56.9 ± 21.5	20–128

Ulnar nerve – peak to peak Amplitude (μV) wrist–digit V	Mean ± SD	Range
All subjects	47.5 ± 21.5	10–116

Median–ulnar nerves Amplitude ratio (peak to peak)	Mean ± SD	Range	Limit of normal (−2 SD)
All subjects	1.254 ± 0.326	0.67–2.0	≥0.60

Median–ulnar nerves Amplitude ratio (%)	<50	50–59	60–69	70–79	80–89	90–99	>100
All subjects (100 hands)	–	–	1	2	4	4	89

Table 2

Pathological values [1]

Median nerve – peak to peak Amplitude (μV) wrist–digit II	Mean ± SD	Range
Group I – CTS with normal DSL	27.0 ± 15.5	2.4–70
Group II – peak DSL between 3.9 and 4.9 ms	22.7 ± 12.3	8–60
Group III – peak DSL ≥5.0 ms	15.2 ± 7.9	4–30
Group IV – absent median DSL	–	–

Ulnar nerve – peak to peak Amplitude (μV) wrist–digit V	Mean ± SD	Range
Group I – CTS with normal DSL	32.9 ± 13.5	12–60
Group II – peak DSL between 3.9 and 4.9 ms	28.1 ± 15.4	10–76
Group III – peak DSL ≥5.0 ms	26.9 ± 12.2	10–50
Group IV – absent median DSL	19.4 ± 6.9	10–30