Preoperative Electrophysiological Findings Predicting Surgical Outcomes in Tarsal Tunnel Syndrome

Authors: Eiko SUNAMI, Satoshi IKEDA, Takao KITAMURA, Taku YONEYAMA, Arito YOZU, Qing LI, Yoichiro AOYAGI

Abstract

Tarsal tunnel syndrome is an entrapment neuropathy caused by the compression of the tibial nerve and its terminal branches in the tarsal tunnel. Electrophysiological examinations are often used to diagnose tarsal tunnel syndrome. Surgical decompression of the tibial nerve is performed in patients who are resistant to conservative treatment. However, the preoperative electrophysiological findings that predict surgical outcomes remain unknown. This study aimed to clarify the preoperative electrophysiological findings that predict the surgical outcomes of tarsal tunnel syndrome. We reviewed 28 feet of 23 patients who underwent preoperative electrophysiological examinations between November 2021 and October 2024, were diagnosed with tarsal tunnel syndrome, and subsequently underwent surgery. Electrophysiological examinations included nerve conduction study and needle electromyography. We reviewed patient characteristics and electrophysiological findings prior to surgery. Sensory plantar symptoms, such as numbness and pain, were evaluated using the Numerical Rating Scale before and after surgery. Patients were divided into the improvement and non-improvement groups based on the Numerical Rating Scale improvement rate after surgery. A comparative analysis of patient characteristics and preoperative electrophysiological findings was performed between the improvement and non-improvement groups. In a motor nerve conduction study of the tibial nerve, the amplitude of the compound motor action potential evoked by stimulation at the ankle was significantly lower in the non-improvement group than in the improvement group. In tarsal tunnel syndrome, a low compound motor action potential amplitude of the tibial nerve on preoperative motor nerve conduction study may indicate poor symptomatic improvement after surgery. Electrophysiological examinations may be useful for predicting the surgical outcomes of tarsal tunnel syndrome.

Introduction

Tarsal tunnel syndrome (TTS) is an entrapment neuropathy caused by the compression of the tibial nerve and its terminal branches, the medial plantar nerve, and the lateral plantar nerve in the tarsal tunnel. TTS is caused by space-occupying lesions such as ganglia and schwannomas, post-traumatic adhesions, compression due to dilated vessels, hypertrophy of the flexor retinaculum, and accessory muscles.^1^^)^

The symptoms of TTS include numbness, dysesthesia, and pain in the foot soles and toes. Typical clinical symptoms and Tinel's sign in the tarsal tunnel are useful for the diagnosis. Surgical decompression of the tibial nerve is performed in patients who are resistant to conservative treatment. Nerve conduction study (NCS) and needle electromyography (EMG) are often performed for differential and auxiliary diagnoses. Increased distal latency and decreased velocity and amplitude of the plantar nerve on NCS are considered useful for diagnosis.^1^^)^ However, the sensitivity and specificity of electrophysiological testing remain low,^2^^)^ and no electrophysiological diagnostic criteria have been established.

Surgical success rates vary from 44% to 96%, and several factors have been reported to predict good surgical outcomes.^1^^,^^2^^)^ Seidel et al.^3^^)^ reported that the electrophysiological evidence of tibial neuropathy was a predictor of postoperative symptom improvement. However, this was the only report to investigating the relationship between electrophysiological findings and surgical outcomes, and the details of which preoperative electrophysiological examination parameters affect surgical outcomes are unknown.

In this study, we retrospectively evaluated surgical cases of TTS to clarify the preoperative electrophysiological findings that predict surgical outcomes.

Materials and Methods

Patients and methods

We evaluated patients who visited the Nippon Medical School Hospital for sensory plantar symptoms, such as numbness and pain, underwent electrophysiological examinations from November 2021 to October 2024, were subsequently diagnosed with TTS, and underwent surgery. Electrophysiological examinations included NCS and EMG. TTS was comprehensively diagnosed based on the clinical history, symptoms, physical examination, magnetic resonance imaging (MRI) of the ankle, and electrophysiological examinations. Conservative treatment was initially administered. Conservative treatment included medications aimed at alleviating neuropathic pain, corticosteroid injections into the tarsal tunnel, and the use of an appropriate sole plate. Various medications were used for neuropathic pain, including pregabalin, mirogabalin, or duloxetine. Surgical treatment was indicated for cases in which symptoms do not improve even after 3 months of conservative treatment, cases in which medications could not be continued due to side effects, or cases in which symptoms are temporarily alleviated by corticosteroid injections into the tarsal tunnel but then recur. Cases with obvious polyneuropathy were excluded from surgical indications. In addition, cases in which NCS for tibial and medial plantar nerve showed no abnormalities were also excluded from surgical indications.

We reviewed the patient characteristics and preoperative electrophysiological findings collected from 0-12 months before surgery. The patient characteristics included age, sex, symptom duration, history of lumbar spine surgery, presence of diabetes, symptoms, physical examination, and MRI of the ankle. Sensory plantar symptoms were evaluated using the Numerical Rating Scale (NRS).^4^^)^ The distribution of sensory plantar symptoms was also evaluated. Physical examination included evaluation of Tinel's sign of the tibial nerve, sensory disturbances, and muscle strength. All patients underwent preoperative MRI of the ankle to assess space-occupying lesions and nerve compression.

One month postoperatively, sensory plantar symptoms, assessed using the NRS, were evaluated as the surgical outcome. Patients whose NRS scores after surgery improved by ≥50% compared to preoperative scores were defined as the “improvement group,” while those whose scores improved less than that were defined as the “non-improvement group.” A comparative analysis of patient characteristics and preoperative NCS and EMG parameters was performed between the 2 groups.

Electrophysiological examination

A motor nerve conduction study (MNCS) of the tibial nerve was performed bilaterally. The tibial nerve was stimulated at the ankle (proximal to the flexor retinaculum and posterior to the medial malleolus) and popliteal fossa. Surface recording electrodes were placed at the motor point of the abductor hallucis muscle (Figure 1). Distal motor latency was evaluated. The compound muscle action potential (CMAP) amplitude was measured from baseline to the negative peak. Motor conduction velocity was measured from the popliteal fossa to the ankle.

A sensory nerve conduction study (SNCS) of the medial plantar nerve was performed bilaterally. The tibial nerve was stimulated at the ankle (proximal to the flexor retinaculum and posterior to the medial malleolus). The antidromic sensory potential of the medial plantar nerve was obtained by attaching a ring electrode to the big toe. The sensory nerve action potential (SNAP) was recorded by averaging the responses. The SNAP amplitude was measured from baseline to the negative peak. Sensory conduction velocity was calculated by measuring the distance from the stimulation point to the ring electrode.

Standard needle EMG of the abductor hallucis muscle was performed in all patients. EMG of the gastrocnemius muscle was also evaluated in 25 feet (20 patients). The EMG included a search for denervation activity at rest (fibrillations and positive sharp waves), evaluation of the motor unit action potential (MUP), and interference pattern at maximum effort. MUP was considered neurogenic if the polyphasia and/or MUP peak to peak amplitude was ≥2.5 mV and/or MUP duration was ≥15 ms. When neurogenic MUP and/or decreased interference were recorded, it was considered a chronic neurogenic change.

Surgical procedure

All patients underwent surgery under local anesthesia involving injection of 1% lidocaine into the superficial skin over the affected area. Surgery was performed on the affected side down to the lateral position using a microscope. A 30 mm bow-like skin incision was made in the tarsal tunnel. The flexor retinaculum was opened, and the posterior tibial artery was confirmed. For nerve decompression, we relocated the posterior tibial artery away from the posterior tibial nerve to the medial malleolus side and sutured it with 6-0 polydioxanone suture at the superficial to the flexor retinaculum. For arterial transposition, some small vessels were removed from the artery.

Statistical analysis

Descriptive statistics, including patient characteristics and parameters of the preoperative electrophysiological examination, were conducted for all cases and each group. The descriptive analysis of numerical variables was shown as numbers or as an arithmetic mean ± standard deviation (SD). The Mann-Whitney U test and Fisher's exact test were applied to analyze the differences in patient characteristics and preoperative electrophysiological findings between the improvement and non-improvement groups. Statistical significance was set at P < 0.05. The correlation between improvement rate of NRS and parameters of electrophysiological examination were analyzed using Pearson correlation test. Statistical significance was set at P < 0.05. All statistical analyses were performed using the IBM SPSS statistics version 29.0 (IBM Corp., Armonk, NY, USA).

Ethics approval

This study adhered to the tenets of the Declaration of Helsinki, and was approved by the Ethics Committee of the Nippon Medical School (approval M-2025-280). Because this was a retrospective observational study, informed consent was obtained using an opt-out approach. Details of the study were disclosed on the institutional website, and patients were allowed to refuse participation.

Results

Between November 2021 and October 2024, electrophysiological examination was performed on 59 patients with suspected TTS at Nippon Medical School Hospital (Figure 2). Of the 59 patients, 54 patients were diagnosed with TTS, and the remaining 5 patients were diagnosed with diseases other than TTS (polyneuropathy, lumbar radiculopathy, and others). Of the 54 patients with TTS, 19 patients were followed up with conservative treatment, and 35 patients underwent surgical treatment. Overall, 35 patients who underwent surgery were initially considered for our analysis. However, 12 patients were excluded due to incomplete and missing electrophysiological data. Ultimately, 28 feet from 23 patients were included in this study. Of the 28 feet (23 patients) included 23 (19 patients) were classified in the improvement group and 5 (4 patients) were classified in the non-improvement group.

Patients characteristics

Table 1 shows a comparison of the patient characteristics between the improvement and non-improvement groups. All patients had sensory plantar symptoms, such as numbness and pain, before surgery. Symptoms were bilateral in 91.3% of patients. In all patients, the NRS score 1 month after surgery was lower than that before surgery, indicating symptom improvement. Ankle MRI revealed a ganglion in only one patient in the improvement group, but no space-occupying lesions were observed in the other patients. There were no significant differences in the preoperative NRS scores between the improvement and non-improvement groups. There were no significant differences in the patient characteristics between the improvement and non-improvement groups.

Electrophysiological examinations

Table 2 presents a comparison of the electrophysiological examinations between the improvement and non-improvement groups. In the MNCS of the tibial nerve, the amplitude of CMAP evoked by stimulation at the ankle was 8.2 ± 2.1 mV (mean ± SD) in the improvement group and 6.0 ± 1.3 mV (mean ± SD) in the non-improvement group, which was significantly lower in the non-improvement group (p = 0.027). In the MNCS of the tibial nerve, there was significant positive correlation between the improvement rate of NRS and the amplitude of CMAP evoked by stimulation at the ankle (Pearson correlation, r = 0.384, p = 0.043) (Supplemental Figure 1). No significant correlation between the improvement rate of NRS and the amplitude of CMAP evoked by stimulation at the popliteal fossa was noted. The conduction velocity between the popliteal fossa and the ankle was significantly faster in the non-improvement group (p = 0.013).

In the SNCS of the medial plantar nerve, SNAP was evoked in 7 of the total 28 feet (25%). SNAP of the medial planter nerve was evoked in 26.1% in the improvement group and 20% in the non-improvement group. For 7 feet in which SNAP were evoked, the SNAP amplitude was 2.1 ± 1.9 μV (mean ± SD), and the sensory conduction velocity was 39.4 ± 4.2 m/s (mean ± SD).

In the EMG of the abductor hallucis muscle, there were no significant differences between the 2 groups in the parameters of MUP, presence or absence of denervation activity, and presence or absence of chronic neurogenic changes. Chronic neurogenic changes were observed in 91.3% and 100% of the patients in the improvement and non-improvement-groups, respectively.

EMG of the gastrocnemius muscle was evaluated in 25 feet (20 patients). Table 3 shows a comparison of the EMG of the gastrocnemius muscle between the improvement and non-improvement groups. The improvement group included 20 feet (16 patients), while the non-improvement group included 5 feet (4 patients). There were no significant differences between the 2 groups in the parameters of the MUP, presence or absence of denervation activity, or the presence or absence of chronic neurogenic changes. Chronic neurogenic changes were observed in 55% and 100% of the patients in the improvement and non-improvement groups, respectively.

Discussion

In the present study, the surgical outcomes of TTS 1 month after surgery were divided into improvement group and non-improvement group, resulting in 82% of the feet were classified as improvement group. Preoperative electrophysiological findings and patient characteristics were compared between the 2 groups. The results showed that in the tibial nerve MNCS, the amplitude of the CMAP evoked by stimulation at the ankle was significantly lower in the non-improvement group than in the improvement group. In the tibial nerve MNCS, there was significant positive correlation between the improvement rate of NRS and the amplitude of CMAP evoked by stimulation at the ankle. In TTS, NCS is commonly used as an adjunct diagnosis because of the possibility of false-negative and false-positive results.^1^^)^ However, our results indicate that the preoperative NCS may be a useful predictor of surgical outcomes.

In the present study, the stimulation site of the tibial nerve at the ankle was proximal to the flexor retinaculum and posterior to the medial malleolus. The low CMAP amplitude in the non-improvement group suggested axonal degeneration of the nerve within the tarsal tunnel or more distally. Furthermore, chronic neurogenic changes were found in 100% of the EMG of the abductor hallucis muscles in the non-improvement group, supporting axonal degeneration of the tibial and medial plantar nerves. Decreased CMAP amplitudes of the medial plantar nerve in TTS have been reported in previous studies.^5-7^^)^ In TTS, the nerves have been reported to show demyelination and axonal degeneration,^6^^)^ and it has been suggested that severe demyelination may lead to secondary axonal degeneration.^8^^)^ In this study, stimulation was not performed across the tarsal tunnel and distal latency was not measured at a constant distance; therefore, demyelination could not be evaluated. However, the low CMAP amplitude of the tibial nerve in the non-improvement group may have been due to severe nerve damage and axonal degeneration, leading to poor surgical efficacy. In contrast, the amplitude of CMAP evoked by stimulation at the popliteal fossa did not differ between the 2 groups. This may be because the conduction distance by stimulation at the popliteal fossa is longer than that at the ankle, making it difficult to distinguish between the 2 groups. Further studies involving larger numbers of patients are required to validate these results.

Patients with TTS commonly complain of sensory symptoms and rarely experience muscle weakness. However, atrophy of the intrinsic foot muscles may be observed in advanced cases. In the present study, atrophy of the abductor hallucis muscles was frequently observed. Sodani et al.^6^^)^ reported that MNCS is a useful screening tool in clinically suspected idiopathic TTS. Similarly, our study indicates that MNCS of the tibial and plantar nerves may sensitively reflect nerve damage in TTS.

In our study, the motor conduction velocity of the tibial nerve was significantly faster in the non-improvement group than in the improvement group. However, because the conduction velocity is between the ankle and the popliteal fossa, the results may not be directly related to the pathophysiology of TTS. The SNAP of the medial plantar nerve had a low elicitation rate in both the improvement and non-improvement groups, and the SNAP amplitude was low. This is consistent with prior reports of a high rate of abnormalities in sensory action potential of plantar nerve in TTS.^9^^,^^10^^)^

In the present study, chronic neurogenic changes in the abductor hallucis muscle were frequently observed in both improvement and non-improvement groups. Mondelli et al.^5^^)^ reported that neurogenic changes were observed in 45% of the sole muscles in TTS. The frequency was higher in this study, which may support the diagnosis of TTS.

We also evaluated the EMG parameters of the gastrocnemius muscles in 25 feet. No significant difference was observed in the frequency of chronic neurogenic changes in the gastrocnemius muscle between the improvement and non-improvement groups. However, chronic neurogenic changes were observed in 100% of the gastrocnemius muscles in the non-improvement group. The presence of neurogenic changes in the gastrocnemius muscle implies that the tibial nerve proximal to the tarsal tunnel is involved. As patients with obvious polyneuropathy are excluded from surgical indications, lumbosacral radiculopathy is suspected to be a proximal lesion. It has been reported that concomitant TTS was found 4.8% patients with lumbosacral radiculopathy.^11^^)^ Future studies with a larger number of patients are needed to clarify whether EMG findings in the leg muscles proximal to the tarsal tunnel affect surgical outcomes.

This study involved patients who underwent surgery for TTS who were resistant to conservative treatment. Previous literature has shown that surgical success rates for TTS vary from 44-96%, and some symptoms may persist.^1^^)^ In previous studies, surgical treatment achieves the best results in young patients, those with a positive Tinel sign before surgery, a short history of symptoms, a clear etiology, and no previous ankle pathology.^2^^)^ In our study, there were no significant patient characteristics factors that predicted surgical outcomes. Our study suggested preoperative electrophysiological findings may be useful as an adjunct when considering surgical indications. Usually, conservative treatment is undertaken before surgical treatment and gives good results in TTS.^2^^)^ However, there has been little research into which patients fail conservative treatment and require surgical treatment. Although it has been reported that abnormally slow nerve conduction across the tibial nerve is predictive of failed conservative treatment,^12^^)^ other factors remain unclear. A prospective study is required to examine changes in symptoms and electrophysiological findings before and after treatment in patients undergoing surgical or conservative treatment. This will help clarify the electrophysiological parameters for surgical indications in TTS.

Limitations

This study has several limitations. Firstly, we evaluated only the medial plantar nerve and not the lateral plantar nerve. Second, the SNAP of the medial plantar nerve was recorded antidromically using surface electrodes. Obtaining the SNAP of the plantar nerve using surface electrodes is technically difficult and commonly requires averaging frequent stimuli. Therefore, several evaluation methods have been studied, including near-nerve sensory nerve conduction,^13^^)^ and the measurement of mixed nerve action potentials.^3^^,^^10^^)^ The low elicitation rate of SNAP in this study may have been due to the evaluation method used. Third, the surgical outcome was evaluated only 1 month after surgery, and the NRS score improved in all cases. In prior studies, the postoperative follow-up period for TTS was longer.^14-16^^)^ Since symptoms may recur after surgery for TTS, long-term postoperative evaluation is required. Finally, the number of patients in this study was small, particularly in the non-improvement group. Prospective studies with a larger number of cases are needed to clarify the preoperative NCS and EMG parameters that predict surgical outcomes. This will also clarify electrophysiological findings that are useful for the diagnosis of TTS.

Conclusion

In TTS, a low CMAP amplitude of the tibial nerve on preoperative MNCS may indicate poor symptomatic improvement after surgery. Electrophysiological examinations may be useful for predicting the surgical outcomes of TTS.

Conflicts of Interest Disclosure

All authors have no conflict of interest.

Supplementary Material