Anatomical variations of the occipital nerves: implications for the treatment of chronic headaches.

BACKGROUND The anatomy of the greater and lesser occipital nerves has come under increased scrutiny with the increased appreciation of their role in the causation of chronic occipital headaches. Classic anatomical descriptions of their paths have differed from some recent published reports. METHODS Measurements of the courses of the greater and lesser occipital nerves were conducted bilaterally in 125 individuals, consisting of 112 live intraoperative measurements and 13 cadaver specimens. In addition to nerve width and trajectory in the occiput, measurements of the distance of the nerves from the occipital protuberance were performed. RESULTS The greater occipital nerve had a diameter of 3.8 +/- 1.6 mm, and emerged from the semispinalis capitis muscle 14.9 +/- 4.5 mm lateral to the midline and 30.2 +/- 5.1 mm inferior to the occipital protuberance. The nerve almost always (98.5 percent) pierces the body of the semispinalis capitis muscle, and in 6.1 percent of individuals it is split by fibers of this muscle or in the trapezial tunnel. The nerve then travels in a superolateral course. In 43.9 percent of patients, the nerves were asymmetric on the two sides. The lesser occipital nerve had a diameter of 1.2 +/- 1.6 mm and was often located along the posterior border of the sternocleidomastoid muscle. CONCLUSIONS The course of these two nerves differs in several critical aspects from that described in classic anatomical reports. These findings have direct implications for application of nerve blocks and surgical decompression of these nerves.