3 Surgical Anatomy of the Frontal and Occipital Trigger Sites
The frontal trigger site includes the supraorbital and supratrochlear nerves (STN). Both nerves are derived from the frontal nerve, which is the largest branch of the ophthalmic division of the trigeminal nerve (V1).
Eighty-three percent of individuals have a notch, 27% have a foramen, and 10% have both.
The supraorbital notch contains a fascial band 86% of the time.
When a supraorbital foramen is present, it is located on average 31 mm lateral to the midline, 1 although this location is inconsistent and can be asymmetric even between the two sides of a single individual.
After passing through the supraorbital notch or foramen, the SON divides into a deep branch and a superficial branch. The deep branch is located approximately 0.56 mm lateral to a vertical line through the medial limbus.
The deep branch of the supraorbital nerve (SON) is susceptible to compression inferolaterally.
As they travel cranially, one or both branches of the SON course directly through the corrugator supercilii muscle (CSM) in 78% of individuals.
Supraorbital artery travels medial to the SON.
The STN usually divides into two branches in the retro-orbicularis fat.
Similar to the SON, one or both branches of the STN may travel through the CSM, which comprises a potential compression or trigger point of the supratrochlear.
In 84% of individuals, both branches of the STN enter the CSM 18.8 mm lateral to the midline, and exit it 19.6 mm lateral to the midline and 15 mm cranial to the superior orbital rim. 2
The occipital trigger site comprises three sets of nerves, constituting three distinct trigger sites, one medially along the posterior midline (the third occipital), one about 1.5 cm lateral to the midline (greater occipital), and one laterally close to the hairline (the lesser occipital).
The greater occipital artery can be compressed or irritated at six sites.
The greater occipital nerve (GON) travels around the lower border of the obliquus capitis inferior muscle, where fascial bands between this muscle and the nerve constitute the first potential compression point of the GON.
The entrance of the GON into the deep aspect (undersurface) of the semispinalis capitis muscle constitutes the second potential compression point, located 17.5 mm lateral and 59.7 mm caudal to the occipital protuberance.
The emergence of the GON from the superficial aspect of the semispinalis capitis muscle constitutes the third potential compression point, located 15.5 mm lateral and 34.5 mm caudal to the external occipital protuberance (EOP).
The GON then continues cranially deep to the trapezius muscle, before entering the deep surface, the fourth potential compression point. 3
The GON then emerges from the tendinous insertion of the trapezius into the nuchal line, at a point 37.1 mm lateral and 4.4 mm caudal to the EOP, and this constitutes the fifth potential compression point. 3
The lesser occipital nerve emerges around the posterior border of the sternocleidomastoid muscle in 86.7% of individuals, 6 or through the muscle in the remaining 13.3% of individuals, and this constitutes the first potential compression point.
The lesser occipital compression point is located 61.3 to 68.9 mm lateral to the midline, and 53.2 mm below the level of the external auditory canals.
The lesser occipital nerve intersects with the occipital artery in 55% of individuals, 7 and this constitutes the second potential compression point, located 51 mm lateral to the midline and 20 mm caudal to the level of the external auditory canal.
The anatomy of the frontal and occipital trigger sites has been studied extensively as it pertains to possible contributions to migraine headaches. Each of these trigger sites includes several potential trigger points that highlight the wide variety of structures that may interact with sensory nerves to produce headache symptoms, including muscle, bone, fascia, and vessel. In this chapter, the anatomy of the supraorbital, supratrochlear, greater occipital, third occipital, and lesser occipital nerves (LON) is detailed, including all known compression and irritation points.
3.2 Frontal Trigger Site
The frontal trigger site includes the supraorbital nerve (SON) and supratrochlear nerve (STN). Both nerves are derived from the frontal nerve, which is the largest branch of the ophthalmic division of the trigeminal nerve (V1). It also includes muscle, particularly the corrugator supercilii, the procerus, and depressor supercilii, as well as bone, fascia, and vessel.
3.2.1 Supraorbital Nerve
The SON may exit the orbit through a notch in the supraorbital rim or through a foramen just cranial to the rim, and this notch or foramen constitutes the first potential compression point of the SON. Fallucco et al found that individuals are most likely to have a supraorbital notch than a foramen (83% of individuals have a notch, 27% have a foramen, and 10% have both). 2 In a separate anatomical study, Webster et al found bilateral supraorbital notches in 50% of individuals, bilateral supraorbital foramina in 25% of individuals, and a notch on one side with a foramen on the contralateral side in 25% of individuals. 8 When a supraorbital notch is present, it is located on average 25 mm lateral to the midline. 9 The notch contains a fascial band 86% of the time. Fallucco et al divided those fascial bands into three types 2 : type I bands (51.2%) consist of a single fascial band at the caudal edge of the notch. Type II bands (30.2%) consist of bony spicules and a fascial band at the caudal edge of the notch. Type III bands (18.6%) consist of a septum within the notch that creates multiple passageways for the neurovascular bundle (►Fig. 3.1). When a supraorbital foramen is present, it is located on average 31 mm lateral to the midline, 1 although this location is inconsistent and can be asymmetric even between the two sides of a single individual. The location of the foramen also varies by gender (more lateral in males) 10 and by race (more lateral in Blacks). 11
After passing through the supraorbital notch or foramen, the SON divides into a deep branch and a superficial branch. The deep branch is located approximately 0.56 mm lateral to a vertical line through the medial limbus. 12 Both branches may interact with the corrugator supercilii muscle (CSM) at multiple points, and precise knowledge of the location and topography of the CSM is essential to achieve adequate chemodenervation or surgical resection of this muscle. In the transverse plane, the CSM extends from approximately 3 mm lateral to the vertical midline to a distance approximately 85% of the distance to the lateral orbital rim. The apex of the muscle is located approximately 33 mm above the horizontal plane drawn between the nasion and lateral orbital rim at the level of the lateral limbus (►Fig. 3.2). 13
Along its inferior free border, the CSM is surrounded by a galeal fat pad, 14 and nerve compression there is unlikely due to this padding. However, inferolaterally, the CSM interdigitates with the orbicularis oculi muscle, and the deep branch of the SON is susceptible to compression there due to repetitive muscle contraction. 13 An additional potential compression point of the deep branch of the SON occurs when it changes direction from its firmly adherent location on the periosteum to a more superficial plane, leading to a potential kink in the nerve. 13
As they travel cranially, one or both branches of the SON course directly through the CSM in 78% of individuals. 13 , 15 The branching pattern of the SON in relation to the CSM has been studied by Janis et al, who identified four patterns 15 : a type I pattern (40%), where branches off the deep branch of the SON (SON-D) travels through the CSM, a type II pattern (34%), where branches off both the superficial (SON-S) and deep branches of the SON travel through the CSM, a type III pattern (4%), where branches off the SON-S travel through the CSM, and a type IV pattern (22%) where there are no branches from either the SON-D or SON-S that travel through the CSM (►Fig. 3.3).
More cranially, the horizontal fibers of the CSM interdigitate with the vertical fibers of the frontalis muscle, and those opposing muscle forces may apply torque to one or both branches of the SON, leading to compression. 15
After passing through or deep to the CSM, the deep branch of the SON travels cranially in the layer between the galea aponeurotica and the periosteum to provide sensory innervation to the frontoparietal scalp. The superficial branch of the SON divides into multiple smaller branches that pierce the frontalis muscle before reaching the subcutaneous tissue and skin of the lateral forehead. 15 , 16 The supraorbital artery travels with the SON and often it is located medial to it.