Journal of Clinical Neuroscience 32 (2016) 83–87

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Journal of Clinical Neuroscience

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Clinical Study Modified skin incision for avoiding the lesser occipital and occipital artery during retrosigmoid craniotomy: potential applications for enhancing operative working distance and angles while minimizing the risk of postoperative neuralgias and intraoperative hemorrhage

R. Shane Tubbs a,b,c, Fabian N. Fries d, Charles Kulwin e, Martin M. Mortazavi f, Marios Loukas b, ⇑ Aaron A. Cohen-Gadol e, a Seattle Science Foundation, Seattle, WA 98122, USA b Department of Anatomical Sciences, St. George’s University, Grenada c Centre of Anatomy and Human Identification, University of Dundee, DD1 4HN, Scotland, UK d Saarland University Medical Center and Saarland University Faculty of Medicine, D66424 Homburg, Germany e Goodman Campbell Brain and Spine, Department of Neurological Surgery, Indiana University School of Medicine, 355 W. 16th Street, Suite #5100, Indianapolis, IN 46202, USA f California Brain Institute, Los Robles Hospital and Medical Center, Thousand Oaks, CA 91360, USA article info abstract

Article history: Chronic postoperative neuralgias and headache following retrosigmoid craniotomy can be uncomfortable Received 15 March 2016 for the patient. We aimed to better elucidate the regional nerve anatomy in an effort to minimize this Accepted 22 March 2016 postoperative complication. Ten adult cadaveric heads (20 sides) were dissected to observe the relation- ship between the lesser occipital nerve and a traditional linear versus modified U incision during retrosig- moid craniotomy. Additionally, the relationship between these incisions and the occipital artery were Keywords: observed. The lesser occipital nerve was found to have two types of course. Type I (60%) remained Anatomy close to the posterior border of the sternocleidomastoid muscle and some crossed anteriorly over the Lesser occipital nerve sternocleidomastoid muscle near the mastoid process. Type II nerves (40%) left the posterior border of Nerves Occiput the sternocleidomastoid muscle and swung medially (up to 4.5 cm posterior to the posterior border of Retrosigmoid craniotomy the sternocleidomastoid muscle) as they ascended over the occiput. The lesser occipital nerve was near a midpoint of a line between the external occipital protuberance and mastoid process in all specimens with the type II nerve configuration. Based on our findings, the inverted U incision would be less likely to injure the type II nerves but would necessarily cross over type I nerves, especially more cranially on the nerve at the apex of the incision. As the more traditional linear incision would most likely transect the type I nerves and more so near their trunk, the U incision may be the overall better choice in avoiding neural and occipital artery injury during retrosigmoid approaches. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The lesser occipital nerve is classically described as arising from cervical two (C2) and C3 ventral rami and then courses around the Retrosigmoid craniotomy is a standard approach used by the posterior and deep surface of the sternocleidomastoid to ascend neurosurgeon. Some patients have significant headaches following parallel with the muscle’s posterior border. Occasionally, the nerve retrosigmoid craniotomy. Such a complication has also been found may pierce the sternocleidomastoid muscle. Near the cranium, the to affect postoperative quality of life in these patients [1]. Postop- lesser occipital nerve perforates the deep fascia and passes up into erative headache following retrosigmoid craniotomy is a well- the scalp around the to supply the skin and connect with known entity and although multifactorial, may be due to occipital the great auricular and greater occipital nerves and auricular nerve transection, entrapment, or neuroma formation. branch of the facial nerve [2,3]. The nerve also supplies an auricular branch, which supplies sensory innervation to the upper and med- ial thirds of the auricle. This branch also communicates with the ⇑ Corresponding author. Tel.: +1 317 362 8760; fax: +1 317 924 8472. posterior branch of the great auricular nerve. E-mail address: [email protected] (A.A. Cohen-Gadol). http://dx.doi.org/10.1016/j.jocn.2016.03.015 0967-5868/Ó 2016 Elsevier Ltd. All rights reserved. 84 R.S. Tubbs et al. / Journal of Clinical Neuroscience 32 (2016) 83–87

Compression or stretching of the lesser occipital nerve may The 7 cm linear incision was localized 0.5 cm posterior and par- result in cervicogenic headaches or occipitoparietal migraines, thus allel to the mastoid groove and one third above and two thirds the lack of a definitive nerve blockade site makes it difficult to treat below the transverse sinus. The curvilinear incision was deter- these headaches [1]. mined as follows: A line connecting the inion to the root of zygoma The senior author (A. C-G.) has been using the reverse ‘‘U” inci- was marked. The junction of this line with extension of the line sion (Fig. 1) for retrosigmoid craniotomy as described by Dandy [4] marking the mastoid groove was noted as the summit of the inci- for the past 3 years and has noted a decreased rate of postoperative sion. A wide-based curvilinear ‘‘U” incision dependent on this sum- headaches and more specifically occipital pain among his patients. mit was marked (Fig. 1). Relationships between the linear and Therefore, we performed the current cadaveric study to better elu- modified incision and the occipital artery were also observed. All cidate the regional anatomy and to compare this incision with the measurements were made with digital calipers (Mitutoyo, Japan). more typical linear incision in regard to regional neurovascular No specimen was found to have prior surgery or gross injury to bundles. The advantages of the ‘‘U” incision for improving the the areas dissected. Statistical analysis between sides and sex operative working distance and angles will also be reviewed. was performed using Statistica for Windows (Dell; Tulsa, OK USA) with significance set at p < 0.05.

2. Materials and methods 3. Results

For the present study, 10 adult formalin-fixed cadaveric heads No pathology was identified in the region of dissection for any (20 sides) were used. These specimens were derived from five male specimen. A lesser occipital nerve was identified on all sides. The and five female adult cadavers with an average age at death of nerve was not duplicated or triplicated on any side. The lesser 63 years (range 55 to 79 years). In the prone position, the subcuta- occipital nerve was on average 6.8 cm lateral to the external occip- neous tissues of the posterior head and upper cervical spine, from ital protuberance. However, the nerve was found to have two types posterior to the pinna to approximately 2 cm lateral to the midline of course. Type I nerves (12 sides; 60%) (Fig. 2) remained close (no were dissected. The relationship between the lesser occipital nerve greater than 1 cm) to the posterior border of the sternocleidomas- in the region where a standard retrosigmoid craniotomy would be toid muscle along its entirety, and continued onto the occiput more performed was observed. Specifically, the relationship with the les- or less in line with this posterior border even after the muscle ser occipital nerve to a traditional linear and modified U incision attached to the skull but with a small deviation toward the poste- was recorded. rior pinna. Some type I nerves crossed anteriorly over the stern- ocleidomastoid muscle near the mastoid process.

Fig. 1. Schematic drawing of the positioning of a craniotomy for left-sided Fig. 2. Left-side cadaveric dissection of the lesser occipital nerve. Note that this retrosigmoid approaches. The dotted white line connects the root of the zygoma represents a type I nerve that hugs the posterior border of the sternocleidomastoid and the inion and marks the level of the transverse sinus. The vertical blue line is muscle along most of its course and in this case courses laterally and crosses the the mastoid groove. The junction of these two lines is the summit of the incision muscle. The locations of the linear and curvilinear incisions are marked. Note that and approximates the junction of the transverse and sigmoid sinuses. [With the linear incision transects the nerve while the curvilinear incision cross over the permission from The Neurosurgical Atlas by Aaron Cohen-Gadol, MD] [30]. nerve, especially more cranially on the nerve at the apex of the incision. R.S. Tubbs et al. / Journal of Clinical Neuroscience 32 (2016) 83–87 85

Type II nerves (8 sides; 40%) (Fig. 3) left the posterior border of ified U incision was less likely to transect the artery, especially the sternocleidomastoid muscle and swung medially (up to 4.5 cm when the medial limb of the incision was shortened. No statistical posterior to the posterior border of the sternocleidomastoid mus- differences were seen comparing male or female specimens. cle) over the nuchal region as they ascended over the occiput. The lesser occipital nerve was near a midpoint of a line between 4. Discussion the external occipital protuberance and mastoid process in all specimens with a type II nerve configuration. However, type I There are relatively few anatomic landmarks that have been nerves were found at a point about one third of the distance along cited in the literature for identifying the lesser occipital nerve a line drawn from the mastoid process to the external occipital [5,6]. Most studies have focused on landmarks for the more medi- protuberance. The linear incision placed the type I nerves at greater ally positioned [7]. Dash et al. [8] found that risk of injury while type II nerves were less likely to be injured via blockage of the lesser occipital nerve was best performed in an area such an incision (Fig. 1). approximately 3 cm in diameter centered at a point 6.5 cm from The inverted U incision was less likely to injure the type II the midline and 5.3 cm inferior to a line connecting the external nerves but would necessarily cross over type I nerves, especially auditory canals. Another study found that the main lesser occipital more cranially on the nerve at the apex of the incision. On two nerve trunk was found on average 7 cm lateral to the external (10%) right sides, the lesser occipital nerve pierced the sternoclei- occipital protuberance [9]. Our study found that the lesser occipital domastoid muscle prior to the muscle’s attachment onto the skull. nerve has two types and that type I nerves were closer to the On one (5%) left side, the nerve pierced the splenius capitis muscle region of the mastoid process in their distribution and were briefly before continuing to the skin overlying the skull. Each of therefore, more likely to be damaged with the traditionally used these nerves that pierced muscles were type I nerves. Although linear skin incision for retrosigmoid craniotomies. The modified type II lesser occipital nerves tended to be more frequent on left U incision, especially when a shorter medial arm was used, was sides, this did not reach statistical significance. less likely to transect the main trunk of the lesser occipital nerve Additionally, the traditional linear incision leads to accumula- and moreover, avoided the main stem of the occipital artery tion of the scalp and underlying muscle under the retractor that (Fig. 5). increases the working distance of the surgeon toward the cerebel- Patients undergoing craniotomy are at risk for chronic head- lopontine angle (CP). The curvilinear incision mobilizes the scalp aches [10–20]. Schankin et al. [21] investigated patients with post- flap inferiorly and avoids this problem (Fig. 4). operative headaches following craniotomy and classified the types Lastly, traditional linear incisions in this area were more likely of headaches based on criteria defined by the International Head- to transect the main trunk of the occipital artery (Fig. 5). The mod- ache Society [22]. These authors identified five different types of headaches and occipital neuralgia was the second most common type making up 16.6% of postoperative headaches sufferers [21]. These postoperative headaches, therefore, might be amenable to surgical decompression of the nerves overlying the occiput, which might be entrapped following craniotomy [23,24]. Moreover, Ducic et al. [1] identified patients with medically recalcitrant postopera- tive headache where the headache resolved following excision of the injured cutaneous nerve overlying the occiput. These authors concluded that all chronic cases of postoperative headache should be screened for potential occipital nerve injury. In their series, 7 patients with postoperative headache had evidence of occipital nerve injury (neuroma/scar tissue entrapment) and those who underwent occipital nerve excision were found to have an 80% or more decrease in headache. Unrecognized occipital nerve injuries have been found to be a leading cause of chronic occipital neuralgia in patients who have undergone vestibular schwannoma resection. Treatment of this chronic ipsilateral pain by neuroma excision with implantation of the nerve stump to muscle has been described [25]. However, some examples of midline crossing of the occipital nerves have been reported so that unilateral injury could result in bilateral pain [5]. Additionally, multiple proximal interconnections between the upper cervical nerves can often result in mixed clinical presenta- tions where the treatment options are not clear [26,27]. Interest- ingly, Schessel et al. [28] modified their incision for cranioplasty in an attempt to avoid occipital nerve injury and found that this had no effect on the incidence and severity of pain. We have used the ‘‘U” incision with a wider base for large posterior fossa tumors, including vestibular schwannomas and have noted rare instances of patients suffering from chronic headaches.

4.1. The effect of incision on operative distance and working angles

Fig. 3. Left-sided cadaveric dissection of the lesser occipital nerve. Note that this We initially abandoned the linear incision for these retrosig- represents a type II nerve that after emerging from the posterior border of the sternocleidomastoid muscle, the nerve swings away from the muscle and closer to moid approaches in favor of the ‘‘U” incision due to the difficulties the midline. we encountered while using the linear incision. These difficulties 86 R.S. Tubbs et al. / Journal of Clinical Neuroscience 32 (2016) 83–87

Fig. 4. (a) The linear incision leads to accumulation of the scalp and underlying muscle under the retractor that increases the working distance of the surgeon toward the cerebellopontine angle, (b) the curvilinear incision mobilizes the scalp flap inferiorly and avoids this problem (with permission from Clarian/IU Health).

Moreover, this increase in the working distance led to limitation of working angles within the CP cisterns. These limitations were especially problematic among obese patients. Even though our original intent was to use the ‘‘U” incision for improving our work- ing distance and angles, we have noticed a decreased rate of post- operative headaches among our patients. However, this conclusion should be confirmed through a randomized trial.

5. Conclusions

The main trunk of the lesser occipital nerve is near the tradi- tional incisions used for retrosigmoid craniotomy. Based on our findings, the inverted U incision, would be less likely to injure the type II nerves but would necessarily cross over type I nerves, especially more cranially on the nerve at the apex of the incision. As the more traditional linear incision would also most likely tran- sect the type I nerves, the U incision would be the overall better choice in avoiding superficial neural injury of the main trunk of the lesser occipital nerve and occipital artery during retrosigmoid approaches. These findings support our recent experience with the U incision and patients who seem to experience apparently less postoperative discomfort and improved wound healing.

Conflicts of Interest/Disclosures

The authors declare that they have no financial or other con- flicts of interest in relation to this research and its publication.

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