Comparing Saphenous Nerve Conduction Study Techniques at the Knee and at the Ankle and Their Relationship to Body Mass Index

Methods 

Thirty healthy adult volunteers were recruited after institutional review board approval. Subjects were excluded if they had a history of peripheral neuropathy or diabetes mellitus. After informed consent was obtained, the lower limbs were warmed with hot packs. By using a surface thermometer on the electromyography machine, leg temperatures were confirmed to be 32° to 36°C at the lateral calf. The nerves were tested in the following order in 1 leg and then the other: antidromic sural response at the calf, antidromic saphenous response at the ankle, and antidromic saphenous response at the knee.

The antidromic sural sensory response (fig 1) was obtained by using a 3-cm bar electrode with the active recording electrode posterior to the lateral malleolus. The stimulus cathode was placed 14cm proximally at the posterolateral aspect of the calf.

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Fig 1. Sural sensory response.

The saphenous sensory response was obtained by using 2 techniques. The antidromic saphenous response at the ankle (fig 2) was obtained using a 3-cm bar electrode with the active electrode placed anterior to the medial malleolus just medial to the anterior tibialis tendon. The stimulus cathode was placed 14cm proximally along the medial tibia. The antidromic saphenous response at the knee (fig 3) was obtained with a 3-cm bar with the active electrode medial to the tibia about 14cm distal to the medial popliteal fossa. The stimulus cathode was placed 14cm proximally between the sartorius and gracilis muscles, where the saphenous nerve is superficial.

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Fig 2. Saphenous sensory response at the ankle.

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Fig 3. Saphenous sensory response at the knee.

Results 

There were 30 subjects with a mean age of 35±8.4 years (range, 25–59y), of which 7 (23%) were women. Mean height was 1.8±1.5m, mean weight was 79.5±15.5kg, and mean BMI was 25.4±4.8kg/m2. No potential subjects met the exclusion criteria of a history of diabetes or peripheral neuropathy. Sural response was obtained bilaterally in all subjects tested.

The results of both saphenous nerve conduction techniques are shown in table 1. The saphenous response at the knee, in general, had a larger amplitude, shorter onset and peak latency, and faster conduction velocity. This is consistent with the fact that the more proximal nerve segments have more nerve axon fibers and have faster conduction velocities. As shown in table 2, the saphenous ankle response rate of 50% was significantly lower than the saphenous knee response rate of 77% (χ2 test=6.9, P=.03). As shown in table 3, the saphenous ankle response was affected by BMI, in which the response was less likely to be obtainable bilaterally in subjects who were overweight or obese (χ2 test=6.7, P=.03). In contrast, the saphenous knee response was not affected by BMI (χ2 test=1.2; BMI, .54kg/m2).

Table 1. Saphenous Nerve Conduction Results

	Study Parameters	Mean ± SD		Absolute Cutoff⁎	Side-to-Side Difference (mean ± SD)	Relative Cutoff⁎
		Right	Left
	Saphenous ankle
	Amplitude (μV)	4.7±3.1	4.1±1.8	1.3	1.1±2.2	57%
	Onset latency (ms)	2.8±0.4	2.8±0.3	3.6	0.0±0.4	0.7
	Peak latency (ms)	3.5±0.4	3.5±0.4	4.3	0.1±0.4	0.8
	Conduction velocity (m/s)	51.3±9.2	50.1±5.8	33.2	0.7±9.0	18.3
	Saphenous knee
	Amplitude right (μV)	5.9±3.0	6.1±2.5	2.4	0.1±2.5	74%
	Onset latency right (ms)	2.5±0.3	2.4±0.2	3.1	0.1±0.2	0.6
	Peak latency right (ms)	3.1±0.3	3.0±0.2	3.7	0.1±0.3	0.6
	Conduction velocity right (m/s)	57.8±7.5	58.4±5.7	42.0	0.9±6.7	13.9

⁎ Three percent lower limit of normal for amplitude and conduction velocity. Ninety-seven percent upper limit of normal cutoff for onset latency and peak latency.

Table 2. Bilateral Obtainable Saphenous Ankle Response Versus Saphenous Knee Response

	Nerve Conduction Result	Saphenous Ankle, n (%)	Saphenous Knee, n (%)	χ2 Test
	Responses bilaterally	15(50)	23(77)	6.9(P=.03)
	Responses not present bilaterally	15(50)	7(23)

Table 3. Bilateral Obtainable Saphenous Responses at Ankle and Knee by BMI

	Nerve Conduction Result	Saphenous Ankle Response by BMI (kg/m2)			Saphenous Knee Response by BMI (kg/m2)
		BMI ≥30	BMI 25–30	BMI <25	BMI ≥30	BMI 25–30	BMI <25
	Response not present bilaterally	4	6	5	1	3	3
	Responses bilaterally	1(20%)	2(25%)	12(71%)	4(80%)	5(63%)	14(82%)
	χ2 test	6.7(P=.03)			1.2(P=.54)

Discussion 

The first purpose of this study was to compare the ability to obtain the saphenous nerve conduction response by using 2 previously reported techniques: recording an antidromic response at the knee and recording an antidromic response at the ankle. The results show that recording from the knee obtains bilateral responses more frequently than recording from the ankle. Thus, recording from the knee can be used instead of recording from the ankle to help detect the presence of a femoral mononeuropathy or lumbar plexopathy. The saphenous knee response, because it tests nerve fibers more proximal than the saphenous ankle response, is less likely to be affected by peripheral neuropathies that tend to affect more distal fibers first. Thus, an absent knee response is more likely to be caused by a lumbar plexopathy and less likely to be caused by a peripheral neuropathy than an absent saphenous ankle response. Because the saphenous ankle response is commonly not present bilaterally, if the clinical goal is to look for a peripheral neuropathy, other distal sensory nerves (eg, sural, superficial peroneal sensory) should probably be examined instead of the saphenous ankle response. This finding cannot be compared with previous studies because we were unable to find a previous study comparing the saphenous knee response to the saphenous ankle response. Buschbacher’s study2 only used the dominant-side saphenous nerve responses to determine their prevalence of 25% nonresponse rate and did not present the prevalence of obtaining bilateral saphenous responses. The saphenous response at the knee has the disadvantage in that it is not possible to detect an injury to the saphenous nerve distal to the knee unless the response is obtained at the ankle. In addition, the response at the ankle may be more convenient to perform in conjunction with other sensory responses at the ankle.

The second purpose was to determine if BMI affects the ability to obtain a response. The results suggest that the saphenous ankle response may be difficult to obtain in subjects with BMI greater than 25kg/m2, whereas saphenous knee responses are commonly obtainable bilaterally in obese and overweight subjects.

We are also unable to directly compare this study to previous studies because Buschbacher’s study2 did not present their rate of obtaining bilateral responses at the ankle in different BMI groups. However, they did find that the BMI of subjects who had an obtainable saphenous ankle response on the dominant side differed from the BMI of subjects whose saphenous ankle response was unobtainable.

Conclusions 

Saphenous nerve conduction study (NCS) at the knee is statistically more consistent than saphenous NCS at the ankle, especially in patients with a larger BMI. However, the number of absent responses in normal subjects highlights the limits of this NCS.