Medial plantar nerve block for prolonged pain relief in a rare presentation of chronic post-herpetic neuralgia of the great toe

Dear Editor,

Post-herpetic neuralgia (PHN) is the most common chronic complication of varicella-zoster virus reactivation and is characterised by persistent neuropathic pain, most frequently involving thoracic and cranial dermatomes after rash resolution.[1] Involvement of distal peripheral nerve branches of the foot is exceptionally rare.[2,3] We report successful management of long-standing PHN confined to the distal plantar aspect of the foot, refractory to conventional oral therapy.

A 74-year-old male with no known comorbidities and normal blood sugar levels presented with persistent left foot pain for 6 months. He had developed vesicular eruptions over the plantar aspect seven months earlier, consistent with herpes zoster (HZ), which resolved within 1 month. However, dull aching pain with intermittent tingling persisted and progressively worsened. Pain was localised predominantly to the plantar surface of the great toe and medial sole. The baseline pain score was 7/10, assessed on 0–10 Numeric Rating Scale (NRS), which was aggravated by standing, walking, and minimal contact. The pain was refractory to pregabalin 75 mg twice daily and topical 2% lignocaine gel with only 5–10% relief. The patient demonstrated significantly impaired SF-36 (Short Form Health Survey) functional scores (5/100). Examination revealed healed lesions over the plantar great toe with marked allodynia and hyperalgesia. There was no swelling or erythema, and vascular and motor examinations were normal. The clinical picture was consistent with PHN involving the medial plantar nerve (MPN), a terminal branch of the tibial nerve.

As the patient had travelled from afar and desired rapid, sustained relief, an ultrasound-guided MPN block was performed for diagnostic confirmation and therapy. Under aseptic precautions and monitoring, a high-frequency linear probe identified the MPN between the abductor hallucis and flexor digitorum brevis muscles [Figure 1]. Using an in-plane approach, a 22-gauge needle was advanced to a perineural position. After negative aspiration, 2 mL of 1% lignocaine with 4 mg dexamethasone was injected, and circumferential spread was confirmed sonographically. Pain decreased to NRS score 2/10 at discharge with marked improvement in allodynia. No complications were observed. Weekly follow-up for 6 weeks demonstrated sustained analgesia (NRS score 1–2/10) without adverse effects with improvement of SF-36 scores to 75/100 with regards to physical functioning.

Figure 1.

Ultrasound-guided medial plantar nerve block at the level of the tarsal tunnel. The left panel shows probe positioning over the medial malleolus (MM) at the ankle. The right panel demonstrates a transverse short-axis sonographic view beneath the transverse tarsal ligament (flexor retinaculum). The medial plantar nerve (MPN), lateral plantar nerve (LPN), and medial calcaneal nerve (MCN) are identified adjacent to the posterior tibial artery and vein (AV). The white arrow indicates the needle trajectory towards the medial plantar nerve

However, pain recurred from the 7^th week with a progressive increase in NRS score to 7/10. Hence, long-duration pulsed radiofrequency (PRF) of the MPN was performed for 10 minutes at the 8^th week. At 2-week follow-up post-procedure, the patient reported significant pain relief and continues to be under regular follow-up.

PHN results from virus-induced injury to peripheral sensory neurons, causing peripheral and central sensitisation. In anatomically restricted presentations, targeted peripheral nerve intervention may interrupt nociceptive input and reduce peripheral sensitisation.[1] Muché et al.[2] reported post-surgical acute HZ affecting the plantar surface managed with serial posterior tibial nerve blocks (3 mL 0.5% bupivacaine), resulting in pain reduction at 1 month. Another report described PHN in a 55-year-old female with persistent burning pain over the dorsum and sole after relief of leg pain with serial sciatic nerve blocks. A pentablock with 0.25% bupivacaine (10 mL) and clonidine (75 μg) provided relief for only 48–72 hours and required repeat procedures.[3] To our knowledge, PHN confined predominantly to the MPN distribution has not been previously described in the literature.

While steroids have demonstrated benefits as adjuvants in peripheral nerve blocks for non-PHN conditions, their role in PHN is less well documented. A review has shown improved outcomes with steroid addition in chronic postoperative inguinal pain.[4] Another report described 1-month relief in intractable upper-limb PHN following ultrasound-guided musculocutaneous nerve block with triamcinolone and lignocaine.[5] However, reports of steroid use for prolonged benefits in focal plantar PHN are lacking. Persistent inflammation has been demonstrated in PHN patients persisting years after HZ.[6] Nevertheless, dexamethasone in our patient may have contributed to sustained analgesia through anti-inflammatory and neuromodulatory effects, attenuating neurogenic inflammation and ectopic neural discharge.

In conclusion, addition of dexamethasone to local anaesthetic in ultrasound-guided MPN block provided prompt and prolonged relief in this rare case of focal distal plantar PHN. This technique can be helpful in resource-limited settings as a low-cost alternative to PRF. Larger studies are needed to establish reproducibility and long-term efficacy.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient consented to the images and other clinical information to be reported in the journal. The patient understands that his name and initials will not be published and due efforts will be made to conceal his identity, but anonymity cannot be guaranteed.

Disclosure of use of artificial intelligence (AI)-assistive or generative tools

The authors declare that no artificial intelligence (AI)-assistive or generative tools were used in the preparation, writing, or editing of this manuscript.

Author contributions

SR: Conceptualisation, patient management, literature review, manuscript drafting, and final approval of the manuscript. HG: Clinical supervision, manuscript review, and final approval. JP: Manuscript conception, critical revision of the manuscript, and final approval.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.