Bilateral variation of the suboccipital region musculature

Background: This case report documents bilateral anatomic muscular variation observed in a detailed dissection of the suboccipital region of an 81-year-old male cadaver. The suboccipital muscle group consists of four paired muscles located inferior to the occipital bone. The muscles in this group include the rectus capitis posterior major muscle, rectus capitis posterior minor muscle, obliquus capitis superior muscle, and obliquus capitis inferior muscle. They are innervated by the suboccipital nerve and lie deep to the trapezius muscle and the semispinalis capitis muscle. In this case report, the rectus capitis posterior major muscles were doubled bilaterally. Accessory muscles were also noted bilaterally, immediately superficial to the suboccipital muscles. The two sets of anatomical variants described have little to no previous documentation. Doubling, or division, of the rectus capitis posterior major muscle has been reported, but mentions of a bilateral doubling of the muscle are infrequent. Accessory muscles have been discovered in this area before, but none with the same attachments or morphology as the ones noted here. Case Information: Two accessory muscles lying deep to the semispinalis capitis muscle on each side were observed, as well as a bilateral doubling of the rectus capitis posterior major muscle. The accessory muscles ran from fascial attachments to the nuchal ligament at the level of the second and third cervical vertebrae to insert on the occipital bone below the superior nuchal line. In addition, the left greater occipital nerve was split in two by the medial band of the left accessory muscle. The nerve traveled in two separate parts around the muscle belly, and converged immediately cranial to the accessory muscle to continue its course to innervate the skin of the posterior scalp. Conclusions: The suboccipital region contains neurologic and vascular structures that have been implicated in the etiology of cervicogenic pain, chronic headaches, and occipital neuralgia. Variations in the musculature, like those observed in this case report, have the potential to create structural interactions causing pain or other symptoms. Anatomic variation should be considered in the diagnosis and treatment of pain and other conditions of the suboccipital region.