Reference values of dorsal sural sensory nerve action potential: A useful tool to diagnose peripheral neuropathy

Manoranjan Acharya; Sunil Kumar Jena

doi:10.4103/mjmsr.mjmsr_8_21

ORIGINAL ARTICLE

Year : 2021 | Volume : 12 | Issue : 1 | Page : 13-16

Reference values of dorsal sural sensory nerve action potential: A useful tool to diagnose peripheral neuropathy

Manoranjan Acharya¹, Sunil Kumar Jena²
¹ Department of Neurology, VIMSAR, Burla, Odisha, India
² Department of Physiology, VIMSAR, Burla, Odisha, India

Date of Submission	06-Mar-2021
Date of Acceptance	24-May-2021
Date of Web Publication	03-Sep-2021

Correspondence Address:
Dr. Sunil Kumar Jena
Department of Physiology, VIMSAR, Burla - 768 017, Odisha
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/mjmsr.mjmsr_8_21

Abstract

Background: Dorsal sural sensory nerve is the most distal nerve of the lower limb for which its sensory nerve action potential (SNAP) could be helpful for the diagnosis of early and subclinical peripheral neuropathy. The objective of this study was to estimate the age and sex reference data of amplitude (Amp), onset latency (OL), and conduction velocity (CV) of SNAP. Materials and Methods: A prospective cross-sectional study was conducted among 50 healthy subjects (28 male and 22 female). Participants were stratified into Group A (≤50 years) and Group B (>50 years) according to their age. Student's t-test was used to compare the data between Group A and B, between male and female and Pearson correlation was used to analyze the correlation between age and SNAP parameters. Results: OL of Group A and Group B was 2.80 ± 0.36 ms and 3.11 ± 0.55 ms, respectively (P = 0.037). CV of Group A and Group B was 45.6 ± 3.33 m/sec and 39.5 ± 1.17 m/sec, respectively (P = 0.000). Amp of Group A and Group B was 6.63 ± 0.73 μV 4.99 ± 0.47 μV, respectively (P = 0.000). OL of male and female was 2.59 ± 0.28 ms 3.34 ± 0.25 ms, respectively (P = 0.000). Pearson correlation coefficient “r” between “age– OL,” “age– CV,” and “age– Amp” was 0.135 (P = 0.351), −0.759 (P = 0.000), 0.953 (P = 0.000), respectively. Conclusion: This study provides age and sex reference values of dorsal sural SNAP in the eastern part of the Indian population.

Keywords: Age, dorsa sural nerve, reference data, sensory nerve action potential, sex

How to cite this article:
Acharya M, Jena SK. Reference values of dorsal sural sensory nerve action potential: A useful tool to diagnose peripheral neuropathy. Muller J Med Sci Res 2021;12:13-6

How to cite this URL:
Acharya M, Jena SK. Reference values of dorsal sural sensory nerve action potential: A useful tool to diagnose peripheral neuropathy. Muller J Med Sci Res [serial online] 2021 [cited 2023 Mar 20];12:13-6. Available from: https://www.mjmsr.net/text.asp?2021/12/1/13/325485

Introduction

Dorsal sural nerve, the pure sensory nerve of the foot is the continuation of sural sensory nerve distally. The course of sural nerve is in a line from middle popliteal fossa to the posterior end of lateral malleolus of foot which again continues as the dorsal sural sensory nerve along the lateral side of the foot to little toe.^[1] Nerve conduction studies of distal nerves of lower limbs play substantial role for screening as well as early diagnosis of peripheral polyneuropathies and subclinical peripheral polyneuropathies.^[2] However, this evaluation has limitation to the most distal aspect of the foot.^[3] Dorsal sural nerve is the nerve of sensory supply to the skin of lateral aspect of foot and little toe. As it is the distal sensory nerve, most probably it is affected very early in length dependent peripheral neuropathies. Sometimes, in subclinical neuropathies, the sensory nerve action potential (SNAP) values of sural and superficial peronial nerves are obtained in normal reference range. Hence, recording of SNAP of the dorsal sural nerve may be helpful in the diagnosis of early and subclinical peripheral polyneuropathies.^[4],[5],[6] Medial plantar and interdigital nerves are other distal sensory nerves of lower limbs can also be useful to record electro diagnostic study, but some technical difficulties impose their limitations. In our country, most of the people walk barefoot for which the sole is often very thick that may create problem to record the medial plantar nerve.^[7] Recording of interdigital nerves has some limitations because of using subdermal needles.^[4] It is also documented that dorsal sural sensory conduction is helpful for detecting subclinical peripheral neuropathies in impaired glucose tolerance in the pediatric age groups and adults.^[8],[9] Suggestions are there about the sensitivity of dorsal sural sensory nerve conduction to Vitamin B12 deficiency and megaloblastic anemia.^[10] Different studies reported that age and anthropometric indices affect the nerve conduction.^[11],[12] Because of its superficial location, dorsal sural nerve is easily accessible and compatible for nerve conduction study techniques.^[13] Hence, dorsal SNAP plays a crucial role in diagnosing peripheral neuropathies in the early and subclinical stages. Therefore, this study was proposed to evaluate age and sex reference values of SNAP parameters of dorsal sensory nerve in healthy participants.

Materials and Methods

This study included 50 apparently healthy participants (28 male and 22 female) of adult age group. It was conducted in the department of neurology in collaboration with the department of physiology in a health institute in Eastern India. This prospective, cross-sectional study was approved by the Ethics Committee of our institution. The present study was completed between July 2017 and June 2019. Participants were well explained about the study protocol and its output. Informed written consent was taken from each subject beforehand. Healthy persons accompany the patients to neurology department, relatives of patients, and volunteers from locality were selected as study participants for this study. Selected study participants had normal general and neurological examination, no sensory abnormality, sensory system intact and no past history of long-term treatment which can cause neuropathy. During selection of participants, we excluded the participants if there was a habit of sitting cross legged on the floor for long duration, history of diabetes, lumbosacral radiculopathy, and trauma to the feet. The participants were classified into two groups by their age. Subjects' age ≤50 years were included in Group A and >50 years were included in Group B.

Technique of recording the sural dorsal sensory nerve action potential

Participants were well explained about the technique of recording the nerve conduction in detail for their maximum cooperation. During recording, the subject was asked to lie comfortably in the lateral position with the leg to be assessed on top. The stimulating and recording sites were cleaned by rectified spirit to reduce the impedance of the skin. Medicaide Neurostim machine was used for the test. Filters were set between 20 Hz and 2 kHz, sensitivity was 10 μV/divisions, and sweep duration was 20 ms. Temperature of the laboratory was maintained at 30°C during the test. Trained technicians under the guidance of neurophysiologists conducted the test. Inactive electrode was placed in the web space of digits 4 and 5, whereas the active electrode was placed 3 cm proximal to inactive electrode. The stimulating site was in a line from the posterior end of lateral malleolus to just below the tip of the lateral malleolus. The ground electrode was placed between the recording and stimulating sites.^[7] A supramaximal stimulus was applied to obtain the maximum dorsal sural SNAP amplitude. The latency in milliseconds was calculated from the onset of sweep to the onset of negative peak of SNAP wave form. The amplitude of SNAP in microvolt was obtained from peak to peak. Conduction velocity (CV) was calculated from distance and time.

Statistical analysis

Data were analyzed using IBM SPSS 20 version (Statistical Package for Social Sciences) IBM Corporation, Armonk, New York. Right side dorsal sural SNAP parameters of 50 participants were processed for the statistical analysis. Dorsal sural SNAP parameters onset latency (OL), CV, and amplitude (Amp) were compared between Group A and B as well as between male and female by Student's t-test. Correlation between age and different parameters of SNAP was tested by Pearson correlation. P < 0.05 was considered to be statistically significant.

Results

The results of our study were presented in tabular and graphical form. [Table 1] depicts the comparison of OL, CV, and Amp of SNAP between Group A (age ≤50 years) and B (age >50 years). OL of Group A was 2.80 ± 0.36 ms and Group B was 3.11 ± 0.55 ms with P = 0.037. CV of Group A was 45.6 ± 3.33 m/sec and Group B was 39.5 ± 1.17 m/sec with P = 0.000. Amp of Group A was 6.63 ± 0.73 μV and Group B was 4.99 ± 0.47 μV with P = 0.000.

Table 1: Comparison of sensory nerve action potential between Groups A and B

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[Table 2] depicts the comparison of OL, CV, and Amp of SNAP between male and female. OL of male was 2.59 ± 0.28 ms and female was 3.34 ± 0.25 ms with P = 0.000. CV of male was 44.0 ± 4.13 m/sec and female was 42.3 ± 3.77 m/sec with P = 0.143. Amp of male was 6.10 ± 1.00 μV and female was 5.89 ± 1.07 μV with P = 0.481.

Table 2: Comparison of sensory nerve action potential between male and female

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[Table 3] depicts the correlation between age and parameters (OL, CV, and Amp) of SNAP. Pearson correlation coefficient “r” between “age– OL,” “age– CV,” and “age– Amp” was 0.135 (P = 0.351), −0.759 (P = 0.000), −0.953 (P = 0.000), respectively.

Table 3: Correlation between age and parameters of sensory nerve action potential

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Discussion

Reference SNAP values of healthy participants are useful to assess the functional status of peripheral nerves. SNAP values beyond normal reference range are considered to be abnormal peripheral nerve functioning. Clinicians rarely considered age and sex as covariate of SNAP. These reference data will be helpful for the diagnosis and prognosis of peripheral neuropathies. SNAP is an important predictor in the electrodiagnostic study of patients with peripheral neuropathies.^{[14],[15],[16],[17]} Previous studies documented that dorsal sural sensory nerve conduction study could be possible with ease because of its superficial position. It is the most distal nerve of the lower limb for which it could be assessed for the early diagnosis of peripheral neuropathies and subclinical neuropathies. It was possible to record SNAP of dorsal sural sensory nerve in all age groups.^[3],[7] The location of this nerve is the distal part of the lower limb and very minimal chance of entrapment increases its possibilities to assess the functional status of the peripheral nervous system.^[18] The present study suggested that other variables such as height, weight, body mass index (BMI), limb length, and distance between the electrodes did not contribute to any variation in the parameters of SNAP of dorsal sural sensory nerve but some other studies reported that these variables affect nerve conduction.^[7]

In our study, we found that the OL in Group B was significantly more than Group A, CV and Amp in Group B was significantly less than Group A. Among the three parameters of SNAP, only OL was significantly more in female than male. Rest two parameters, i.e. CV and Amp did not show any significant variation between male and female. There was no significant correlation between age and OL, but significant negative correlation found between “age– CV” [Figure 1] and “age– Amp” [Figure 2].

Figure 1: Correlation between age and amplitude

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Figure 2: Correlation between age and conduction velocity

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In a study, SNAP of the dorsal sural sensory nerve was done extensively taking 294 healthy subjects. They compared the amplitude and CV of SNAP of dorsa sural sensory nerve.^[13] Their result was similar to our result, but in our study, we found one additional finding, i.e., OL was prolonged in Group B than Group A. In other studies, researchers reported that amplitude of participants aged more than 70 years was more than the participants of aged 50–59 years.^{[19],[20],[21]} This finding contradicts to our study in which we found older age group subjects amplitude was less than younger age group and negative correlation of amplitude of SNAP with age. Some studies suggested that age is a covariate that affects the parameters of nerve conduction.^{[11],[12],[22]} With increasing age there is neuronal remodeling, less number of nerve fibers reducing axonal diameter, and changes in property of membrane may also contribute to these changes.^[12] In one study, researchers found that 26% of participants' dorsa sural sensory SNAP was not possible to record. Hence, they concluded that dorsa sural sensory SNAP has no value for the diagnosis of peripheral neuropathies. However, our study contradicts their statement as we could able to record SNAP in all selected subjects for which this study recommends its usefulness for the diagnosis of peripheral neuropathies.^[17] In our study, only OL showed significant variation between male and female which is a new finding. We did not find any significant variation in CV and Amp between male and female for which further study is required.

Limitations of study

If we could stratify the subjects according to their BMI, it could have been a better prediction. A lot of people in our country sit cross legged for which there may be chance of focal neuropathy. Hence, further study may be done excluding this covariate.

Conclusion

This study has obtained age and sex reference values of OL, amplitude, and CV of dorsal sural nerve. Age and sex may be considered a covariate and these data may be applied in the clinical diagnosis of polyneuropathies.

Acknowledgment

We are thankful to the laboratory technicians in the neurology department of VIMSAR.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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