A 28-year-old woman presented with a two-year history of left-sided neck pain and paresthesia radiating superiorly and laterally toward the occipital and postauricular regions, which had exacerbated over the previous few days. She had been diagnosed with occipital neuralgia 10 years earlier following blunt trauma. Multiple blind greater occipital nerve (GON) and lesser occipital nerve (LON) blocks, including a local anesthetic injection without imaging guidance performed at another clinic three months earlier, had been attempted; however, none provided symptomatic relief.

On physical examination, focal pain and tenderness were elicited over the occipital region, while the remainder of the neurological examination was unremarkable. Ultrasound (US) evaluation revealed an oval-shaped lymph node without power Doppler activity and a small accessory sternocleidomastoid (SCM) muscle compartment within the posterior cervical triangle (Figure 1, Video 1). Sonographic tracking demonstrated that the accessory structure was in close contact with (impinging on) the LON (Video 1). Sonopalpation reproduced the patient’s characteristic symptoms along the LON trajectory. The morphological US findings, together with positive sonopalpation, suggested LON entrapment secondary to an SCM anatomical variation.

Figure 1.

Ultrasound image demonstrates a lymph node (arrowheads) and an aberrant muscular structure (arrow) adjacent to it. scm = sternocleidomastoid muscle.

For both definitive diagnosis and treatment, a US-guided injection was performed using an in-plane approach with a 27-gauge needle. The injectate (6.43 mg betamethasone dipropionate, 2.63 mg betamethasone sodium phosphate and 2 mL prilocaine) was administered to hydrodissect the LON from the accessory muscle (Video 2). The patient reported complete pain relief within 15 minutes, which persisted at the three-month follow-up. She remains under ongoing follow-up.

Occipital neuralgia is clinically defined as pain and paresthesia involving the posterior head and neck. ^{Reference Djavaherian and Guthmiller1} Cervicogenic headache, myofascial pain, C2 radiculopathy and auriculotemporal neuralgia are common entities that should be considered in the differential diagnosis. While the majority of cases are attributed to the GON, entrapment of the LON – a branch of the superficial cervical plexus arising from the ventral rami of the C2–C3 nerve roots – accounts for approximately 10% of cases. ^{Reference Djavaherian and Guthmiller1} The posterior border of the SCM is a recognized entrapment site. ^{Reference Yu and Wang2} Other reported causes of LON compression include osteochondroma, herpes zoster, cervical osteoarthritis, lipoma, vascular anomalies and lymphadenopathy. ^{Reference Han, Kim, Rhie and Moon3–Reference Platzgummer, Moritz and Gruber5} Herein, we report a case of LON entrapment attributable to an unusually distinct etiologic factor.

The SCM has two heads (sternal and clavicular) that are typically divided into five compartments; however, anatomical variations or remnant compartmentalization may occur. ^{Reference Kaymak, Gürçay, Ata, Kara and Özçakar6} In such cases, individualized morphological assessment is essential to guide targeted interventions. In our patient, real-time US was pivotal in identifying the SCM variation – an anomalous accessory muscular slip – and enabling precise hydrodissection of the LON. ^{Reference Özçakar, Abacıoğlu and Leblebicioğlu7} This high-volume perineural injection technique may provide improved autonomic modulation by minimizing involvement of the vasa nervorum and nervi nervorum, thereby achieving more favorable clinical outcomes compared with traditional perineural injections. ^{Reference Colorado, McNeill and Norbury8} Beyond its therapeutic benefits, it may also reduce the risk of systemic toxicity associated with the high doses of local anesthetic typically used in conventional blocks. ^{Reference Lam, Reeves and Cheng9}

The optimal injectate remains controversial in the literature, and an individualized approach is recommended. ^{Reference Colorado, McNeill and Norbury8} Although corticosteroid injections are generally not advised at short intervals, we preferred corticosteroid administration in our patient since more than three months had elapsed since the previous injection. ^{Reference Colorado, McNeill and Norbury8} In refractory cases, surgical decompression of the nerve from the accessory compartment may also be a viable option.

In closing, this case highlights how a comprehensive US examination can expand the management of occipital neuralgia beyond traditional diagnostic nerve blocks. Notably, a closer inspection with US may reveal an otherwise subtle anatomical variation that should necessarily be targeted during intervention.