Controversies in Neurology Greater Occipital Nerve Block: A Diagnostic Test? ften a neurosurgeon or orthopedic surgeon Are nerve blocks diagnostic? requests a diagnostic nerve block to deter- Neural blocks may be useful as an empirical way of Omine, prior to attempting any surgical pro- treating diverse head and neck pains, but such a cedure, whether a specific cervical nerve root is the response is also often used as the criterion for diagno- generator of the patient’s symptoms.1 sis.7,8 But such diagnoses, though clinically useful, are This statement from a contemporary textbook inexact and the procedure may be valid (if proven by reflects the commonly held view that in pain manage- properly designed trials) only as an empirical mode of ment nerve blocks are diagnostic. controlling pain. There is wide variability of headache syndromes Blondi rightly notes that “Occipital nerve blockade, … treated by greater occipital nerve (GON) blockade.1 The often results in a nonspecific regional blockade rather than putative mechanisms by which they might relate to the a specific nerve blockade and might result in a misidenti- JMS Pearce MD, FRCP is an GON are unclear. It seems a priori improbable that such fication of the occipital nerve as the source of pain.” And Emeritus Consultant Neurologist at the Hull Royal Infirmary. His diverse conditions as migraine (with its complex cere- he says: “occipital neuralgia is believed to arise from trau- interests are in Clinical Neurology bral and brainstem mechanisms), cluster headache, ma to or entrapment of the occipital nerve within the neck and the History of Medicine. occipital neuralgia, cervicogenic headache, whiplash or scalp, but the pain may also arise from the C2 spinal syndrome,2 and various tension type headaches, should root, C1–2, or C2–3 zygapophyseal joints or pathologic Correspondence to: either share a common aetiological mechanism or be change within the posterior cranial fossa.” If its source is JMS Pearce, responsive to the same treatment of a peripheral nerve. the nerve roots, how can it be rationally considered to 304 Beverley Road, be a neuralgia of the occipital nerve? Anlaby, Anatomy Despite many published studies, the diagnostic utility East Yorks, HU10 7BG, UK. The GON is composed of the medial fibres from the of employing greater occipital nerve (GON) blockade in Email: [email protected] dorsal ramus of the second cervical nerve. The ventral a variety of headaches and neck strains is unproven. ramus of C2 also contributes to the lesser occipital Many trials contain small numbers. The physician nerve and innervates deeper structures (periosteum of administering the injection in many trials is not blind- the occiput, vertebrae, etc.). In rats, a population of ed to the treatment. Follow-up assessment is commonly neurones of the dorsal horn at C2 shows convergent at about four weeks, too brief a period may have detect- input from both dura and cervical skin and muscle ter- ed significant differences in outcome. The local anaes- ritories, suggesting a functional continuum between the thetic or steroid used, and the doses vary and are com- trigeminal nucleus caudalis and upper cervical seg- monly chosen empirically. Controls are often omitted or ments involved in cranial nociception. GON stimula- poorly matched. And, interpretation is confounded by tion in rats facilitates dural stimulation, implying a cen- subjective criteria of pain relief and marked variation of tral mechanism at the second order neurone.3 C2/C3 techniques. There are therefore, several unresolved blockade is claimed to produce benefit of comparable issues concerning both rationale, claimed benefits, and order to GON blockade and both are said to be effective techniques. in the diagnosis and treatment of cervicogenic Ashkenazi and co-workers report, “The rationale of headache.4 GON blockade for the treatment of headache is based on The literature fails to incriminate specific anatomical the anatomical connections between trigeminal and upper structures as the source of cervicogenic pains. Very sim- cervical sensory fibres at the level of the trigeminal nucle- ilar diagnoses invoke structures such as nerve roots, us caudalis.”9,10 But is this alone a diagnostic foundation individual peripheral nerves, bony structures, and the or a mechanism sufficient to explain such diverse head non-specific cervicogenic pain/headache. For example: pains? “Diagnostic anesthetic blockade for the evaluation of The methodologies of some of the studies are limited cervicogenic headache can be directed to several anatomic by lack of a standardised treatment protocol or by a ret- structures such as the greater occipital nerve (dorsal ramus rospective design. In migraine, for example, improve- C2), lesser occipital nerve, atlanto-occipital joint, ment has been reported after GON blockade, but also atlantoaxial joint, C2 or C3 spinal nerve, third occipital after prophylactic drugs, and injected botulinum A nerve (dorsal ramus C3), zygapophyseal joint(s) or inter- toxin.11 There are claimed to be four major ‘trigger vertebral discs based on the clinical characteristics of the points’ along the course of several peripheral nerves that pain and findings of the physical examination.”5 may cause migraine headaches, which are therefore also The authors state fluoroscopic or interventional MRI- treated by injection.12 Among the peripheral nerves the guided blockade may be necessary, to assure specific greater occipital nerve has become a favourite target of localisation of the pain source; yet they fail to present needle wielders. evidence that such measures do inculpate the actual Structures said to be involved in the pathogenesis of source of pain. The opposing view (which I share) is occipital headache include the aponeurotic attachments that of Silverman: of the trapezius and semispinalis capitis muscles to the “there are no diagnostic imaging techniques of the cer- occipital bone, and entrapment of the GON within vical spine and associated structures that can determine these aponeuroses, causing symptoms of ‘occipital neu- the exact source of pain.”6 ralgia’.13 Whereas cervicogenic headache is a useful clin- In respect of diagnosis, the current evidence appraised suggests that the use of nerve blocks as the defining or pathogenetic criterion is both unsound and unreliable ACNR • VOLUME 8 NUMBER 3 • JULY/AUGUST 2008 I 15 Controversies in Neurology trapezius muscle was penetrated by the Table 1: Variation of anatomical sites for greater occipital nerve. GON in 45% of cases, the semispinalis Study Vertical location of GON (cm) Lateral location of GON (cm) muscle of the head was penetrated in 90% 1. Mosser et al. 3cm below EOP 1.5cm from midline of cases, and the inferior oblique muscle of head in 7.5% of cases. Macroscopic find- 2. Loukas et al. 2cm below EOP 2cm from EOP ings of possible compression were made in 3. Natsis et al. The site where the semispinalis capitis is pierced by the GON 11 cases (27.5%), “indicating that nerve 4. Bovim et al. no anatomical landmark given for injection compression per se may be of minor impor- 5. Becser et al. Along intermastoid line 0.5 to 2.8cm from midline tance since it seems to exist in the absence of 6. Tubbs et al. 2cm above intermastoid line 4cm lateral to EOP headache.” 5. Becser, Bovim and Sjaastad reported EOP = external occipital protuberance. topography shown by dissection and care- ful measurements of 10 embalmed cadav- ers.18 A great variability in nerve topogra- ical description,14 it is not a diagnosis that trace the normal course of the greater phy was seen interindividually and intrain- accurately inculpates the pathogenic struc- occipital nerve from the semispinalis mus- dividually. The greater occipital nerve tures involved, nor the mechanism of pain. cle penetration to the superior nuchal line. ascended between 5mm and 28mm from The International Headache Society (IHS) Standardised measurements were per- the midline along the intermastoid line. proposed diagnostic criteria15 for cervicogenic formed on 14 specimens to determine the The minor occipital nerve was found headache (11.2.1) are: location of the emergence of the nerve between 32mm and 90mm from the mid- 1. Pain referred from a source in the neck and using the midline and occipital protuber- line along the same landmark. In most perceived in one or more regions of the ance as landmarks. The location of emer- cases, both the GON and the minor occip- head and/or face. gence was determined to be at a point cen- ital nerve pierced the aponeurosis after 2. Clinical, laboratory and/or imaging evi- tered approximately 3cm below the occipi- branching. Thirteen GONs and eight dence of a disorder or lesion within the tal protuberance and 1.5cm lateral to the minor occipital nerves also were embed- cervical spine or soft tissues of the neck, midline.12 ded in this tissue. Twelve of the 20 GONs known to be, or generally accepted as, a 2. Loukas et al.13 examined the course and formed a rich network around the occipi- valid cause of headache. distribution of GON and its relation to the tal artery. Importantly they commented: 3. Evidence that the pain can be attributed to aponeuroses of the trapezius and semi- “anatomic structures with an imminent risk the neck disorder or lesion, based on either spinalis capitis in 100 formalin-fixed adult of causing entrapment were not observed. … clinical signs that implicate a source of cadavers. The greater occipital nerve was results suggest that optimal locations for pain in the neck or abolition of headache located at a mean distance of 3.8cm (range blockade techniques should be reconsidered.” following diagnostic nerve block. 1.5-7.5cm) lateral to a vertical line through 6. Tubbs and colleagues noted the surprising 4. Pain resolving within three months after the external occipital protuberance and the lack of surgical landmarks in the literature successful treatment of causative disorder spinous processes of the cervical vertebrae.