The release of compressed or irritated afferent sensory nerves involves, next to the resection of muscles, bony structures, fascia, and blood vessels [6, 20, 24]. The supraorbital rim is an important potential compression site, in particular when the resection of muscle for nerve decompression yields unsatisfactory results. The exit point of the SoN, irrespective of whether it is a notch or a foramen, is approximately at the midpupillary line [20, 29]. A more reliable topographical landmark may be a vertical line along the medial iris (Fig. 4.3).

There is a multitude of studies available on the proportion of notches versus foramen at the supraorbital rim. The prevalence of the SoN going through a bony foramen ranges from 26 to 50%, whereas a notch is present in 50–86% of reported specimens [13, 14]. A double passageway is found in up to 7% [29]. In 6%, yet another variant is one large notch through which both the SoN and StN passed [26]. Lastly, the SoN can go across the bony edge without any opening on the supraorbital margin. This type is called “pithecoid” meaning ape-like. In addition, all the above-described options can be different from side to side. Previous studies demonstrated that fascial bands covered the supraorbital notches in 83% of cases with simple bands being the most common followed by a partial bony covering [20] (Fig. 4.2). The StN passes the orbital rim through a notch in the majority of cases [13, 14, 26]. In 72% of these cases, the notch is covered by a band. In 8%, the nerve pierced through this band instead of running behind it. The prevalence of a true bony foramen as the passageway for the StN seems less clear. It is reported to range from 18 to 40% [26, 29].

All of the anatomical studies are carried out in individuals without any known history of migraine. Statistical knowledge about the morphological characteristics of the supraorbital rim in the general population allows only limited conclusions of its association with migraine. In contrast to mere neuralgia, migraine is very unlikely to be caused by a constricted nerve. In the presence of existing migraine though, it can constitute a primary compression site proximal to the entrance of the nerves into the corrugator muscle (Figs. 4.3 and 4.4) [23, 27]. It would be interesting to compare the findings at the orbital rim in migraine versus non-migraine patients, for example, through ultrasound or neuroimaging [20].

Patients who achieve only partial or no improvement of their migraine headaches after the paralysis of the corrugator through botulinum toxin, despite clearly pointing out the eyebrow and forehead area to be relevant pain localization, should be investigated for nerve compression at the supraorbital rim [4, 12, 27]. A possible stepwise method would be chemodenervation followed by injection of local anesthesia at the nerve passage at the rim with a delay of several weeks. In case of a significant change of symptoms following the second step, the anatomical characteristics could be elucidated.

4.2 Supraorbital Nerve

Once the SoN passes the orbital rim, it divides into a medial, superficial part, and a lateral, deep part (Figs. 4.5 and 4.6). Whereas the superficial part of the nerve only supplies sensation to the forehead, the deep part terminates higher up at the coronal suture (Figs. 4.7 and 4.8) [8, 12]. The deep branch has a more consistent course and runs between the galea aponeurotica and the periosteum toward the temporal fusion line laterally and provides sensation to the frontoparietal scalp (Fig. 4.5). The deep branch of the SoN is to be located an average of 0.56 mm from a vertical line drawn tangentially to the medial limbus of the iris (Fig. 4.3). The superficial branch of the SoN is more variable in location. It pierces the frontalis muscle in a fanlike pattern with numerous branches and provides sensory innervation to the forehead skin and anterior scalp (Figs. 4.4 and 4.5) [13].

This section is about the more proximal part of the nerve as it exits its notch or foramen at the supraorbital rim in relation to the CSM [20]. The SoN is usually thought to run behind the CSM [26]. Branches of the deep part of the SoN are shown to run on the underside of the muscle in 40%. In another 34%, such branches come off the superficial as well the deep parts of the nerve (Figs. 4.6 and 4.9). It is important to note that these nerve branches do not actually penetrate the muscle but are only closely attached to it. Whereas in 74% these nerves are branching off the deep division of the SoN, in 22% there is no specific relation of the nerves and the muscle (Figs. 4.10 and 4.11). Overall, 60% reported that parts of the nerve penetrated the muscle (Figs. 4.5, 4.8, 4.10, and 4.11). It is unclear what the clinical significance of these findings is. The absence of closely attached nerve fibers to the muscle could be a possible reason for the unsuccessful paralysis or decompression of the CSM.

4.2.1 Points of Compression and External Landmarks

The first compression point of the SoN consists of either the supraorbital notch or foramen (Figs. 4.1 and 4.12) [20]. When a supraorbital notch is present as the SoN exits from the superior orbital rim, there is frequently a fascial band that completes the circular shape of the notch and can compress the SoN against the frontal bone (Figs. 4.6 and 4.13) (supraorbital notch in 83%, a foramen in 27%, and both a notch and a foramen in 10% of specimens) [12, 13]. They found that 86% of supraorbital notches had a fascial band divided into three classications. Type 1 bands, which occurred in 51.2% of specimens, are described as “simple” and consisted of a single fascial band. Type 2 bands, occurring in 30.2% of specimens, consisted of bony spicules with a fascial band completing the bridge overlying the supraorbital notch. Type 3 bands, occurring in 18.6% of specimens, contained a septum that allowed for more than a single passageway for the neurovascular bundle through the supraorbital notch.