Sural Sensory Nerve Action Potential: A Study in Healthy Indian Subjects at Tertiary Care Center of North-West India
Keywords: Healthy, Indian, sural, SNAP
Nerve conduction studies are an important tool in diagnosing polyneuropathy. Most polyneuropathies follow a length-dependent pattern, and the sensory nerves of the feet are usually affected in the early stages.,,, Sural nerve is a sensory nerve placed superficially and distally in the foot, making it easily accessible for nerve conduction studies. Sural nerve is the most frequently studied nerve in the electrophysiology for diagnosis of polyneuropathy.
Abnormal sural sensory nerve action potential (SNAP) suggests involvement at the peripheral nerve level. Incorrect technique, excessive adipose tissue, edema in limbs, absence of age-stratified reference data, and anatomical variation can lead to abnormal reporting of a normal sural SNAP. This leads to an unwanted investigation of a normal subject.
In the elderly subjects, the normal sural SNAP is lower in amplitude than that in the younger subjects.,,, A review of published literature for reference data on sural SNAP reveals very few studies in the Indian population as compared to the western population., Reference data on sural SNAP was also different in western and Indian studies. Many western studies recorded the SNAP at a distance of 14 cm from the lateral malleolus., A distance of 14 cm may not optimally apply to the Indian population because the average height of an Indian is lesser than their western counterparts, which implies a shorter limb length. The large calf girths at a distance of 14 cm, especially in obese and short-statured individuals, interfere with the recording of an optimal sural SNAP.
Indian normative data was shown in a study by Sreenivasan et al. at three sites, but this had different values as compared to western normative data studies. Thus, this study was done to obtain the reference data for the sural SNAP amplitude and latency at distances of 14, 12, and 10 cm from the active recording electrode in Indian adults for different age groups and to evaluate the effects of other parameters, such as weight, body mass index (BMI), limb length, and limb girth at stimulating site on the sural SNAP.
Two-hundred forty-four healthy subjects were included in this cross-sectional study approved by the ethics committee of our institution. Informed consent was taken before the study. The study was conducted from July 2017 to March 2019. Participants included in the study were:
All selected participants had normal neurological and general examination, without any sensory symptoms, and no past or present history of long-term treatment that can cause neuropathy.
Participants were excluded from the study if there was a history of diabetes, lumbosacral radiculopathy, abnormal serum TSH and B12 level, or trauma to the feet. All selected participants had normal neurological examination without any sensory symptoms. Age, weight, height, limb girth, and limb length (midpoint of fibular head to midpoint of lateral malleolus) of all participants were recorded. Body mass index (BMI) was calculated as weight (kg)/height (in m2).
Technique of recording the sural sensory nerve action potential
All nerve conduction tests were done by the same neurophysiologist by using the same protocol. Both sural nerves were sampled. The test was explained in detail to ensure maximum cooperation, and the participant was asked to lie comfortably in a lateral position with the leg to be assessed on top. The recording and stimulating sites were cleaned to reduce skin impedance. Filters were set between 20 Hz and 2 kHz, sweep duration was 20 ms, and sensitivity was 10 μV/divisions. Temperature was recorded at the lateral malleolus and was maintained at 30°C throughout the test. The active recording electrode was placed just behind the upper border of lateral malleolus. The recording site was marked at distances of 10, 12, and 14 cm proximal to the active electrode. Sural SNAP was recorded antidromically stimulating at three sites (14, 12, and 10 cm from the recording electrode).
The ground electrode was placed between the stimulating and recording sites. A supramaximal stimulus was used to obtain the maximum sural SNAP amplitude. Each optimal SNAP was then averaged for at least 6–8 responses to make the onset clear, and two trials were done to confirm the replicability of the response. The latency in milliseconds was measured from the onset of sweep to the onset of negative peak of SNAP waveform. SNAP amplitudes in microvolt were measured from peak to peak.
The standard statistical test was applied. The quantitative variables were expressed as Mean ± SD/Median (IQR) and compared using t test/ANOVA. P < 0.05 was considered statistically significant. SPSS version 20.0 software was used for statistical analysis.
Statistical analysis for obtaining reference value was done using mean + 2 SD as described by Robinson et al.. Percentile and quantile regression methods could not be applied due to the sample size in each group being small. Mean + 2SD were taken to define the upper limit of latency. All subjects were divided into six groups based on age (18–30, 31–40, 41–50, 51–60, 61–70, and 71–80 years).
The data of amplitude obtained was positively skewed. The amplitudes were square-root transformed to bring the data into more Gaussian distribution. The mean − 2SD of the transformed data was calculated and then reconverted into the original unit for the calculation of the lower limit of sural amplitude. ANOVA was applied to calculate the statistical difference in the sural amplitude between six age groups to assess the effect of age on amplitude.
Two hundred and forty-four patients between ages 18 years to 80 years were included in the study. Mean age of included subjects was 43.28 years. Maximum leg girth was at 14 cm. There was a significant difference in the leg girth at all three sites (P < 0.001). The baseline data and anthropometric parameters of the patients are shown in [Table 1]. Age wise distribution of patients is shown in [Figure 1].
Sural SNAP onset latency at 14 cm was maximum (2.69 ms) in the 51–60-years age group. Reference data for sural SNAP latency was calculated for each age group at distances of 10, 12, and 14 cm and are shown in [Table 2]. Sural SNAP latency at each stimulating site was compared in different age groups. There was no significant difference between different age groups (P = 0.19).
Sural SNAP amplitude at all three sites was maximum in the 18–30-years age group. The mean amplitude was 17.65, 18.76, and 19.56 microvolt (uv) at a distance of 14, 12, and 10 cm, respectively. Data for sural SNAP peak to peak amplitude and base to peak amplitude was calculated for each age group at a distance of 10, 12, and 14 cm and are shown in [Table 3]A and [Table 3]B.
The data of amplitude obtained was positively skewed and required optimal transformation. The mean − 2SD of the transformed data was calculated and then reconverted into the original unit for the calculation of the lower limit of sural amplitude as shown in [Table 4]. Maximum amplitude was in the 18–30-years age group, and the amplitude was minimum in the 71–80-years age group (4.34 and 2.79, respectively). The difference in the amplitude recorded in the different age groups was found to be statistically different (P < 0.001).
Transformed data for amplitude were analyzed. The difference in the amplitude recorded at each site was found to be statistically different (P < 0.001) as shown in [Table 5]. Linear regression of the transformed data of sural nerve amplitude showed age as the covariate with maximum effect (r2 = 0.35), as shown in [Figure 3].
The sensory nerve action potential is an important parameter in the evaluation of a patient with suspected peripheral neuropathy.,,,, Abnormality of SNAP is primarily determined by comparing amplitude with available reference data. The low amplitude or absent SNAP is a basic abnormality in length-dependent peripheral neuropathy.
A literature search reveals only a few studies for the reference data on sural SNAP in a healthy Indian population. Sural SNAP is also affected by age of the patient. Unavailability of age-matched reference data could lead to a false interpretation of nerve conduction. This can result in false diagnosis of peripheral neuropathy and can lead to multiple unwarranted investigations.
In our study, we stimulated sural nerve at three sites 14, 12, and 10 cm above the active recording electrode in the calf. ANOVA showed a significant difference in the leg girth at all three sites (P < 0.001) [Figure 2]. Stimulation near the recording electrode at 10 cm produces large artifacts and required rotation to optimize the SNAP. Stimulation at 14 cm was technically difficult in short and obese individuals; it requires an increase in stimulation duration. Stimulation at 12 cm from the recording electrode was found to be the most convenient. Multiple international studies have used a distance of 14 or 12 cm from the recording electrodes.,,
Previous studies have shown different results about sural SNAP above the age of 60 years with abnormal results of the lower limit of amplitude. Some studies showed values less than zero. We found no subject above age 60 years with a nonrecordable surface sural SNAP. Our results were similar to the study by Horowitch et al. that showed that although amplitude decreases with age, sural response was obtained in all ages (5–90 years).
The amplitude of SNAP decreases with age. Similar results were found in our study. A similar correlation with age has been established in previous studies.,, Other factors such as weight, height, BMI, leg girth, and limb length did not add significant variation to SNAP amplitude.
The reference value of sural SNAP amplitude in our study is comparable to the results obtained by Horowitz & Esper et al., In this study, we used the Mean − 2SD method after transforming the skewed data as suggested by Robinson et al. A similar method for transformation was used by Aarthika et al. Some studies used the percentile method to obtain sural SNAP amplitude. The number of subjects in each group was inadequate for the reliable application of the percentile method. Most of the subjects in our study were from western India; thus, the study population was from a limited geographical area.
This is the first study with a large sample size (244 subjects) to provide age-stratified reference data for SNAP in the Indian population by using three sites of stimulation at distances of 14, 12, and 10 cm from the recording electrode. This study shows that sural SNAP amplitude varies with the age of the subject and distance from stimulation. Thus, a single cut-off value for sural SNAP without age and site of stimulation consideration may lead to wrong conclusions. Further studies with a large sample size in each age group are required for age-specific reference value of SNAP amplitudes.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]