Occipital Neuralgia

Etiology

ON is a result of GON pathophysiology in 90% of cases. Ten percent of cases are due to LON causes, and rarely is the TON thought to be involved. ON almost always results from the compression of one of these nerves at one of several anatomic points. In fact, for the various neuralgias described in the International Classification of Headache Disorders, Third Edition (ICHD-3), in the majority of cases, many of the traditional neuralgias are no longer considered to result from primary nerve pathophysiology (e.g., herpes zoster, other infections, demyelinating lesions).

Rarely, other sensory cutaneous nerves may overlap the typical distribution area of the occipital nerves. In one case report, the suboccipital nerve supplied a cutaneous branch in the normal distribution of the greater occipital nerve.[1] Such variations could contribute to unremitting neuralgia in that region.

Epidemiology

In one study investigating the incidence of facial pain in a Dutch population, ON comprised 8.3% of facial pain cases. The total incidence of ON was 3.2 per 100,000 people, with a mean age of diagnosis of 54.1 years (standard deviation of 16.2 years).[2]

Pathophysiology

An understanding of ON is incomplete without some basic knowledge of the anatomy of each of these nerves.

• GON: Arises from C2 dorsal ramus. It then passes medial to lower border of obliquus capitis inferior muscle and crosses the suboccipital triangle. It then runs rostral along the rectus capitis posterior major muscle; innervates and pierces the semispinalis capitis muscle; pierces tendon of the trapezius muscle; passes the superior nuchal line of the occipital bone; and courses toward the apex of the scalp in proximity to the occipital artery, possibly up to the coronal suture.
• LON: Arises from C2 ventral ramus; loops inferior to the spinal accessory nerve; ascends along the posterior border of the sternocleidomastoid muscle; pierces deep cervical fascia and runs across the posterior edge of the sternocleidomastoid insertion and on into the superficial fascia of the scalp; split into three branches: the auricular, mastoid, and occipital.
• TON: Arises from the dorsal ramus of C3. This divides into lateral and medial branches; then, the medial branch gives rise to superficial and deep branches, the superficial branch being the TON, then curves around the dorsolateral surfaces of C2-C3 and travels along the semispinalis capitis muscle. It turns dorsally at the C2 spinous process, piercing semispinalis capitis, splenius capitis, and trapezius muscles and exits muscles and innervates small cutaneous area right below the nuchal line. Of importance, the TON sends many branches to the GON and LON, potentially making it difficult to separate symptoms due to the TON alone.[3]

For the GON, multiple possible compression points exist that may result in neuralgia.[4] These include the C2 nerve root, the obliquus capitis inferior muscle, the semispinalis capitis muscle, instances where the nerve penetrates the trapezius muscle, and instances where the occipital artery and GON intersect.[3][4] The occipital nerve itself is quite large, measuring 2.5 to 3.5mm in diameter, which may predispose it to compressive pathology.[5] Muscle hypertrophy, tensing, or spasm of musculature in the area has been postulated to contribute to compression, as the condition is associated with stress and anxiety in many patients, and surgical sectioning of muscles in close approximation with the GON has led to pain relief.[6] In certain cases, trauma and the formation of fibrocartilage calluses or other structural changes to the bony anatomy of the skull or spine can lead to this condition.[7] Additionally, Arnold-Chiari malformation or arteriovenous malformations may contribute to nerve compression.[4] Competent treatment of the refractory disease requires knowledge of the anatomical associations of the occipital nerves; Cesmebasi et al.,[3] provide one such resource.

History and Physical

Patients presenting with a headache originating at the posterior skull base should be evaluated for ON. This condition typically presents as a paroxysmal, lancinating, or stabbing pain lasting from seconds to minutes, and therefore a continuous, aching pain likely indicates a different diagnosis. ICHD-3 criteria require that the patient also have tenderness, dysesthesia, or allodynia over the area affected. A positive Tinel’s sign may be present over the nerve's distribution, especially where the GON emerges at the base of the skull over the greater occipital notch. Pain may be unilateral or bilateral but almost always begins unilaterally and may extend into a bilateral distribution over time. Bilateral symptoms are present in one-third of cases.[7]

Evaluation

After completing a focused history and physical examination, the diagnosis is confirmed by a local anesthetic blockade of the suspected nerve. This is a required step for diagnosis per ICHD-3 standards. The patient should have pain relief with the nerve block for at least the duration of the local anesthetic. Single diagnostic blocks carry false-positive rates up to 40%, so the performance of a second block is prudent. If both blocks are positive, the provider may be more confident as to the correct diagnosis. As compression is thought to be the main etiology for ON, imaging should be considered if there is suspicion of a lesion or mass at the affected site.[3]

Treatment / Management

There are multiple treatment options for ON. The most conservative treatments, such as immobilization of the neck by the cervical collar, physiotherapy, and cryotherapy, have not been shown to perform better than placebo.[7] Non-steroidal anti-inflammatory drugs, tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors, and anticonvulsants may help to alleviate symptoms. Following diagnostic nerve blocks, therapeutic blocks may be attempted. Typically, a steroid is added to the local anesthetic with variable results. Botulinum Toxin A injection has emerged as a treatment with a conceptually lower side effect profile than many other techniques described here, with most recent trials demonstrating 50% or more improvement.[4][7](B3)

It remains a common practice to utilize a landmark-only approach when performing greater and lesser occipital nerve blocks. For blockade of both nerves, medication is infiltrated along the nuchal ridge. While easy to perform and relatively safe if done correctly, this technique may not be particularly accurate and, as a result, could theoretically increase the risk of a false-positive result. To improve accuracy, ultrasound-guided techniques were developed. The original ultrasound-guided technique for injection of the GON (utilized routinely by this article's editor for diagnostic injection and cryoablation) was described by Greher et al. in 2010; it targets the nerve as it courses superficial to the obliquus capitis inferior muscle at the C1-C2 level.[8]

There are several advanced interventional procedures in clinical use.

• Pulsed or thermal radiofrequency ablation (RFA) may be considered for longer-lasting relief after a local anesthetic blockade confirms the diagnosis. Thermal RFA aimed at destroying the nerve architecture can render long-term analgesia but also comes with the potential risks of hypesthesia, dysesthesia, anesthesia dolorosa, and painful neuroma formation. Chemical neurolysis with alcohol or phenol carries the same risks as thermal RFA.[8][5] There is no such risk with pulsed RF; however, some question its efficacy compared to other procedures.
• Neuromodulation of the occipital nerve(s) involves the placement of nerve stimulator leads in a horizontal or oblique orientation at the base of the skull across where the greater occipital nerve emerges. Patients should be trialed with temporary leads first, and greater than 50% pain relief for several days is considered a successful trial, after which permanent implantation may be considered. Risks include surgical site infection and lead or generator displacement or fracture after the operation.[4][5]
• Ultrasound-guided percutaneous cryoablation of the GON is a commonly performed procedure by many clinicians. At the correct temperature, there should be stunning but not permanent damage to the nerve, but at temperatures below negative 70 degrees Celsius, nerve injury is possible. Most recently, in the literature, a 2018 article by Kastler et al. described 7 patients who underwent cryoneurolysis in a non-blinded fashion to good effect, but the follow-up was limited to 3 months.[9] This editor has seen between 3 months to 1.5 years of benefit (typically around 6 months) with each treatment. 
• Surgical decompression is often considered to be the last resort. In one study of 11 patients, only two patients did not experience significant pain relief postoperatively. The mean pain episodes per month decreased from 17.1 to 4.1, with mean pain intensity scores also decreasing from 7.18 to 1.73.[6] Resection of part of the obliquus capitis inferior muscle has shown success in patients who experience an exacerbation of their pain with flexion of the cervical spine.[4] Another popular surgical technique is C2 gangliotomy, even though patients are left with several days of intermittent nausea and dizziness.[7] As with any large nerve resection, there is a theoretical risk of developing a deafferentation syndrome, though arguably, the risk is lower if the resection is pre-ganglionic.
(B3)

Differential Diagnosis

The differential diagnosis for ON includes any disorder which similarly presents with a headache or facial pain. Because connections are possible between the occipital nerves and cranial nerves VIII, IX, and X, patients can sometimes present with confusing symptoms, such as vision impairment, dizziness, or sinus congestion.[4] The conditions most easily mistaken with ON for other headache and facial pain disorders include migraine, cluster headache, tension headache, and hemicrania continua. Mechanical neck pain from an upper disc, facet, or musculoligamentous sources may refer to the occiput but is not classically lancinating or otherwise neuropathic and should not be confused with ON. A crucial step in differentiating ON from other disorders is relief with an occipital nerve block. 

Prognosis

Short-term relief is common with basic interventional treatments. Rarely, diagnostic injections of local anesthetic with or without steroids can yield up to several months of analgesia.[4] Advanced treatments mentioned above may result in improvements ranging from weeks to years.

Complications

Complications from procedures have been reported in the literature. In a case series reviewing over a hundred thermal RFA procedures, one patient developed intraventricular hemorrhage and died after the case, which the authors attribute to hypertension during the procedure.[7] The same paper reported a patient who developed Brown-Sequard syndrome from post-traumatic cervical syringomyelia, also after RFA.