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ACMT Toxicology Visual Pearl: Danger in the Shallows

In addition to local pain and dermal injury, stings from this marine animal can result in what systemic symptoms?

  1. Acute liver injury
  2. Hypotension, arrhythmia, and cardiac arrest
  3. Nausea, vomiting, and respiratory arrest
  4. Numbness, tingling, and muscle paralysis

[Image courtesy of Guido Gautsch, Wikimedia Commons]

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By |2025-08-29T09:45:18-07:00Sep 9, 2025|ACMT Visual Pearls, Expert Peer Reviewed (Clinical), Tox & Medications|
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ACMT Toxicology Visual Pearl: Turning Blue

What commercially available product can cause blue-grey discoloration of the skin and conjunctiva with long term use?

  1. Benzocaine, 20% oral gel
  2. Colloidal silver, 30 ppm liquid solution
  3. Ferrous sulfate, 325 mg tablet
  4. Methylene blue, 1% oral solution

[Image from Herbert L. Fred, MD and Hendrik A. van Dijk via Wikimedia Commons]

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By |2025-04-02T16:28:10-07:00Apr 16, 2025|ACMT Visual Pearls, Expert Peer Reviewed (Clinical), Tox & Medications|
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Greater Occipital Nerve Block in the Emergency Department

Chief complaints of non-traumatic headaches represent approximately 2% of emergency department (ED) visits in the United States [1]. Headaches are classified as primary (standalone condition) or secondary (a symptom of another medical condition), with primary headaches being the more common type [2].

In the ED, it is critical to rule out life-threatening causes of headaches such as subarachnoid hemorrhages, stroke, or meningitis. Headaches such as migraines, occipital neuralgia, cervicogenic headaches, and cluster headaches while more benign may still be debilitating for patients (Figure 1).

GON headache types

Figure 1. Headaches that may benefit from greater occipital nerve blockade [illustration by Dr. Felipe Ocampo]

Treatment of such headaches includes supportive care, medications, and procedures, including blockade of the greater occipital nerve (GON) [3, 4]. Peripheral nerve blocks such as GON blockade provide pain relief and its effects may outlast the duration of the local anesthetic. The GON block is an efficient, low-cost, and safe intervention for treatment of such headaches in the ED [5, 6]. In this article, we review the GON block, its relevant anatomy, indications, and procedural technique.

Identifying the Greater Occipital Nerve

Anatomy

Bilaterally, the GON originates from the second cervical spinal nerve (C2) and innervates the posterior scalp [7].

After arising from the C2 spinal nerve, the fibers of the GON ascend through the fascial plane between the obliquus capitis inferior and semispinalis capitis muscles. The fibers then pierce the semispinalis capitis and travel deep to the trapezius muscle until exiting the aponeurosis inferior to the superior nuchal line where it lies subcutaneously, medial to the occipital artery (Figure 2). The nerve measures approximately 2.5 to 3.5 mm in diameter [8].

Anatomy of greater occipital nerve for block

Figure 2: Anatomy around the greater occipital nerve (occipital aspect of the skull) [illustration by Dr. Felipe Ocampo]

GON Block: Necessary Equipment

The basic supplies one should collect for the GON block procedure:

  • Chlorhexidine gluconate or isopropyl alcohol applicator
  • 5 mL syringe
  • 18 gauge needle for drawing solution
  • 2 to 4 mL of either: 0.25 – 0.5% bupivacaine or 1 – 2% lidocaine
    • Consider a 1:1 mix in the same syringe for both short and longer lasting relief,
    • Example: 1.5 mL 1% lidocaine + 1.5 mL 0.25% bupivacaine
  • 1.5 inch 25- or 27-gauge needle for injecting solution

Note about inclusion of corticosteroids: The data is weak except in the case of cluster headaches where it is moderate [9].

equipment for greater occipital nerve block

Figure 3. Basic equipment necessary to perform a GON block [illustration by Dr. Felipe Ocampo]

GON Block: Procedural Technique

Procedural Technique #1: Palpation Approach

Traditionally, the GON block can be done by palpating the external occipital protuberance (OP) and the mastoid process (MP). Anesthetic is injected approximately one-third the distance from the OP to the MP (Figure 4) [6]. With this approach, the nerve is targeted more distally from its origin, where it is found more superficially at a median depth of 8 mm [10]. Here, the occipital artery can typically be palpated lateral to the location of the greater occipital nerve but anatomy can vary [11].

surface anatomy injection site for greater occipital nerve block

Figure 4: Injection site for the greater occipital nerve block along the occipital surface of the scalp [illustration by Dr. Felipe Ocampo]

  1. Position patients with their head flexed forward with either technique:
    • Sitting on the edge of the bed with their palms (or a procedure stand with a pillow) supporting their face
    • Prone with a pillow under the chest
  2. Find landmarks by palpating the external OP and MP
  3. Your target will be approximately one-third the distance from the OP to MP
  4. Ensure there is no palpable pulse at your target
  5. After sterilizing the area, insert the needle approaching from an inferior angle.
    • If you hit periosteum/skull, withdraw the needle slightly
    • Aspirate to ensure that you are not in the occipital artery or another vessel
    • Inject the anesthetic solution (typically 1-3 mL per side)
    • Consider slightly withdrawing and advancing while injecting to bathe multiple planes in anesthetic
  6. Assess for numbness along the posterior scalp (within 5 minutes for lidocaine and 10-15 minutes for bupivacaine)

Note: A modified version of this technique is to ask patients to pinpoint the area where pain is maximal or originates and if in general distribution of the GON between the OP and MP, this can be targeted assuming no palpable pulse and/or blood on withdrawal of syringe.

Procedural Technique #2: Ultrasound-Guided Approach

For the ultrasound-guided approach, the GON is typically targeted more proximally from its origin, at the level of the C2 vertebra. Here, the GON lies within the fascia above the obliquus capitis inferior (OCI) at a median depth of 1.8 cm [10].

  1. Positioning: same as landmark approach above.
  2. Prep the skin and transducer before insertion of the needle.
  3. Apply a generous amount of ultrasound gel to the lower posterior scalp
  4. A high-frequency linear probe is used with the indicator pointing towards the scanner’s left in the axial view starting at the midline external occipital protuberance (OP). With this view, one can see the semispinalis capitis (SSC) on either side. Note that this view may not be attainable in a patient with longer hair.
  5. Then, the probe can be translated downwards until the bifid C2 spinous process is visualized – here the trapezius, SSC, and obliquus capitis inferior (OCI) are seen on either side.
  6. By rotating the probe obliquely and slightly laterally towards the ear of the affected side, one can visualize the GON lying in the fascia above the OCI. Once this view is achieved, lateral to medial in-plane technique can be used to position the needle (25-27 gauge needle) adjacent to the GON, injecting 1-3 mL of solution to achieve spread around the nerve.
    • Pro Tip: The GON is often not visualized on ultrasound, but as long as your other landmarks are clear (between the OCI and SSC), injecting the solution into the plane is sufficient as long as you visualize your needle tip and confirm you are not intravascular.
  7. As with the palpation method, assess for numbness along the posterior scalp after 5-15 minutes, depending on anesthetic used.

ultrasound GON block

Figure 5: Illustrated sonographic views around the greater occipital nerve (yellow); SSC – semispinalis capitis, OP – occipital protuberance, TM – trapezius muscle, OCI – obliquus capitis inferior, C2 – C2 vertebra [illustration by Dr. Felipe Ocampo]

Greater occipital nerve block GON ultrasound anatomy

Figure 6: Sonographic anatomy of the greater occipital nerve (GON); TM- trapezius muscle, SSC – semispinalis capitis, OCI – obliquus capitis inferior, C2 – C2 vertebra [image from Dr. Felipe Ocampo]

GON Block: Comparing Techniques

Choosing a GON block technique may be up to clinician and/or patient preference or equipment availability.

Studies comparing the 2 approaches seem to favor ultrasound-guided GON blockade at the proximal (C2) site due to:

  • Improved analgesia [10]
  • A theoretical lower risk of damaging the occipital artery because (a) it is further from the GON in the proximal ultrasound-guided approach and (b) the ability to visualize structures [11]
  • Easier skin disinfection as injection site tends to be below the hairline [10, 11]
  • Anatomical variants in GON location making the landmark-based/palpation approach less accurate [14]

GON Block: Contraindications and Adverse Effects

Absolute Contraindications

  • Patient refusal
  • Anesthetic allergy
  • Open skull defect
  • Infection at procedural site

Relative Contraindications

  • Coagulopathy
  • Arnold-Chiari Malformations
  • Inability to lie still

Adverse Effects

  • Hematoma
  • Local infection
  • Lesion to nerve
  • Allergy to local anesthetics
  • Local anesthetic systemic toxicity (LAST)
  • Intradural infiltration
  • Vasovagal syncope
  • Alopecia around injection site

Common Indications for GON Block

1. Occipital Neuralgia

Occipital Neuralgia (ON) is a painful condition characterized by paroxysmal, lancinating, stabbing headaches along the posterior scalp that can last seconds to minutes [2]. The etiology of ON is thought to be due to irritation or compression of the occipital nerves, most commonly the GON. Symptoms are usually unilateral but can be bilateral in a third of cases [15, 16].

On exam, there may be dysesthesia (abnormal perceptions of touch) or allodynia (pain to typically non-painful stimuli) of the posterior scalp. There may also be a positive Tinel’s sign along the course of the GON.

A GON block can be both diagnostic and therapeutic by providing immediate relief and can provide up to 2 months of symptom relief [12].

2. Migraine

A migraine is a complex disorder that can be divided into two major subtypes: without aura (75%) and with aura. The etiology of migraines is unclear but some theories describe involvement of the peripheral and central nervous system, the former of which may involve sensory input from nerves including the GON [17].

Migraine characteristics:

  • Migraine without aura involves recurrent episodes of headache lasting 4-72 hours with at least two of the following characteristics: unilateral, pulsatile, moderate to severe in intensity, aggravated by or causing avoidance of physical activity. A migraine headache occurs with at least one of the following symptoms: nausea and/or vomiting, photophobia, phonophobia.
  • Migraine with aura is characterized by transient neurological symptoms that precede or accompany the headache described above. Auras involve at least one unilateral disturbance of: visual, sensory, speech and language, motor, brainstem.
  • A diagnosis is made with at least 5 attacks meeting the above criteria.
  • Chronic migraines are diagnosed when episodes occur 15 or more days per month for 3 months or longer [2].

GON blockade may be helpful for the immediate treatment of migraine episodes or as an alternative treatment in refractory episodes [12]. It has been shown to be as effective as a regimen of intravenous NSAIDs and metoclopramide in patients with acute migraine headaches in the ED [18]. In a case series of patients in the ED with migraine headaches, there were improved symptoms on 2-week call backs of patients who had received a GON block [19]. For chronic migraines, GON block may be as effective as current prophylactic treatments [12].

3. Cervicogenic Headache

Cervicogenic headaches are thought to be due to referred pain from disorders or lesions of the cervical spine or soft tissues of the neck. These headaches tend to be unilateral, moderate to severe in intensity, and may localize to the occipital, frontotemporal, and periorbital regions [2, 20]. Etiology is thought to be due to nociceptive signals from injury of structures in the neck innervated by C1-3 converging into the trigeminal cervical complex [12]. There may be clinical or imaging evidence of cervical lesions.

On exam, there may be decreased cervical range of motion and worsening of the headache with provoking maneuvers.

GON blockade may support diagnosis of cervicogenic headaches by rapidly relieving symptoms and may provide prolonged remission of headaches [12]. One study showed reduced analgesic medication consumption for months in patients with refractory cervicogenic headaches who had received GON blockade [21].

4. Cluster Headache

Cluster headaches are excruciating unilateral headaches that typically localize to the orbit with attacks lasting 15 minutes to 3 hours in duration [2]. This condition is rare, affecting approximately 0.1% of the population, most commonly males in their thirties. The etiology of cluster headaches is unclear but theories describe an association with the trigeminovascular system, parasympathetic nerve fibers, and the hypothalamus [22].

Cluster headache characteristics:

  • Severe, unilateral, orbital or supraorbital and/or temporal in location
  • Attacks occur with at least one ipsilateral autonomic symptom of:
    • Conjunctival injection or lacrimation
    • Nasal congestion or rhinorrhea
    • Eyelid edema or forehead or facial sweating
    • Miosis or ptosis
    • Unilateral photophobia or phonophobia
  • May involve agitation or restlessness
  • Occur with a frequency of once every other day to 8 times per day

GON blocks have been shown to be effective in both the acute and prophylactic treatment of cluster headaches [13, 23, 24]. The data for using corticosteroids in GON blocks is strongest for cluster headaches [12].

Conclusions

In the emergency department, it is critical to evaluate for life-threatening headaches. However, when indicated for patients presenting with specific headaches, the GON block may be an effective and efficient tool in providing analgesia.

References

  1. Goldstein JN, Camargo CA Jr, Pelletier AJ, Edlow JA. Headache in United States emergency departments: demographics, work-up and frequency of pathological diagnoses. Cephalalgia. 2006;26(6):684-690. doi:10.1111/j.1468-2982.2006.01093.x PMID 16686907
  2. Headache Classification Committee of the International Headache Society (IHS). The International Classification of Headache Disorders, 3rd edition (beta version). Cephalalgia. 2013;33(9):629-808. doi:10.1177/0333102413485658. PMID 23771276
  3. Austin M, Hinson MR. Occipital Nerve Block. [Updated 2023 Apr 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  4. Afridi SK, Shields KG, Bhola R, Goadsby PJ. Greater occipital nerve injection in primary headache syndromes–prolonged effects from a single injection. Pain. 2006;122(1-2):126-129. doi:10.1016/j.pain.2006.01.016. PMID 16527404
  5. Guner D, Bilgin S. Efficacy of Adding a Distal Level Block to a C2 Level Greater Occipital Nerve Block under Ultrasound Guidance in Chronic Migraine. Ann Indian Acad Neurol. 2023;26(4):513-519. doi:10.4103/aian.aian_169_23. PMID 37970254
  6. Levin M. Nerve blocks in the treatment of headache. Neurotherapeutics. 2010;7(2):197-203. doi:10.1016/j.nurt.2010.03.001. PMID 20430319
  7. Yu M, Wang SM. Anatomy, Head and Neck, Occipital Nerves. [Updated 2022 Oct 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  8. Güvençer M, Akyer P, Sayhan S, Tetik S. The importance of the greater occipital nerve in the occipital and the suboccipital region for nerve blockade and surgical approaches–an anatomic study on cadavers. Clin Neurol Neurosurg. 2011;113(4):289-294. doi:10.1016/j.clineuro.2010.11.021. PMID 21208741
  9. Benzon HT, Elmofty D, Shankar H, et al. Use of corticosteroids for adult chronic pain interventions: sympathetic and peripheral nerve blocks, trigger point injections – guidelines from the American Society of Regional Anesthesia and Pain Medicine, the American Academy of Pain Medicine, the American Society of Interventional Pain Physicians, the International Pain and Spine Intervention Society, and the North American Spine Society. Reg Anesth Pain Med. Published online August 7, 2024. doi:10.1136/rapm-2024-105593. PMID 39019502
  10. Greher M, Moriggl B, Curatolo M, Kirchmair L, Eichenberger U. Sonographic visualization and ultrasound-guided blockade of the greater occipital nerve: a comparison of two selective techniques confirmed by anatomical dissection. Br J Anaesth. 2010;104(5):637-642. doi:10.1093/bja/aeq052. PMID 20299347
  11. Gürsoy G, Tuna HA. Comparison of two methods of greater occipital nerve block in patients with chronic migraine: ultrasound-guided and landmark-based techniques. BMC Neurol. 2024;24(1):311. Published 2024 Sep 4. doi:10.1186/s12883-024-03816-8. PMID 39232647
  12. Santos Lasaosa S, Cuadrado Pérez ML, Guerrero Peral AL, et al. Consensus recommendations for anaesthetic peripheral nerve block. Guía consenso sobre técnicas de infiltración anestésica de nervios pericraneales. Neurologia. 2017;32(5):316-330. doi:10.1016/j.nrl.2016.04.017. PMID 27342391
  13. Blumenfeld A, Ashkenazi A, Napchan U, et al. Expert consensus recommendations for the performance of peripheral nerve blocks for headaches–a narrative review. Headache. 2013;53(3):437-446. doi:10.1111/head.12053. PMID 23406160
  14. Shim JH, Ko SY, Bang MR, et al. Ultrasound-guided greater occipital nerve block for patients with occipital headache and short term follow up. Korean J Anesthesiol. 2011;61(1):50-54. doi:10.4097/kjae.2011.61.1.50. PMID 21860751
  15. Djavaherian DM, Guthmiller KB. Occipital Neuralgia. [Updated 2023 Mar 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  16. Choi I, Jeon SR. Neuralgias of the Head: Occipital Neuralgia. J Korean Med Sci. 2016 Apr;31(4):479-488. https://doi.org/10.3346/jkms.2016.31.4.479. PMID 27051229
  17. Pescador Ruschel MA, De Jesus O. Migraine Headache. [Updated 2024 Jul 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  18. Korucu O, Dagar S, Çorbacioglu ŞK, Emektar E, Cevik Y. The effectiveness of greater occipital nerve blockade in treating acute migraine-related headaches in emergency departments. Acta Neurol Scand. 2018;138(3):212-218. doi:10.1111/ane.12952. PMID 29744871
  19. Yanuck J, Shah S, Jen M, Dayal R. Occipital Nerve Blocks in the Emergency Department for Initial Medication-Refractory Acute Occipital Migraines. Clin Pract Cases Emerg Med. 2019;3(1):6-10. Published 2019 Jan 22. doi:10.5811/cpcem.2019.1.39910. PMID 30775654
  20. Al Khalili Y, Ly N, Murphy PB. Cervicogenic Headache. [Updated 2022 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  21. Gabrhelík T, Michálek P, Adamus M. Pulsed radiofrequency therapy versus greater occipital nerve block in the management of refractory cervicogenic headache – a pilot study. Prague Med Rep. 2011;112(4):279-287. PMID 22142523
  22. Kandel SA, Mandiga P. Cluster Headache. [Updated 2023 Jul 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
  23. Gönen M, Balgetir F, Aytaç E, Taşcı İ, Demir CF, Müngen B. Suboccipital steroid injection alone as a preventive treatment for cluster headache. J Clin Neurosci. 2019;68:140-145. doi:10.1016/j.jocn.2019.07.009. PMID 31326284
  24. Gordon A, Roe T, Villar-Martínez MD, Moreno-Ajona D, Goadsby PJ, Hoffmann J. Effectiveness and safety profile of greater occipital nerve blockade in cluster headache: a systematic review. J Neurol Neurosurg Psychiatry. 2023;95(1):73-85. Published 2023 Dec 14. doi:10.1136/jnnp-2023-331066. PMID 36948579
By |2025-03-26T14:01:20-07:00Mar 28, 2025|Expert Peer Reviewed (Clinical), Neurology|
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