The Lesser Occipital Nerve in Fetuses

The lesser occipital nerve (LON) is an ascending superficial branch of the cervical plexus that has a variable origin either from the ventral ramus of the second cervical nerve or second and third cervical nerves and is purely sensory. Forty fetuses (right side: 40/80; left: 40/80) with gestational ages between 15 to 28 weeks were microdissected to document the anatomy of the LON. Results: a) Incidence and Morphometry: LON present in 100% specimens, with average length on the right and left sides of 23.59 +/- 2.32 mm and 23.45 +/- 2.27 mm, respectively; b) Course: In its ascent towards the occipital region, the LON was located on the splenius capitus muscle in 85% of specimens and in 15% of the specimens, it ascended vertically on the sternocleidomastoid muscle towards the ear, innervating its superior third; c) Branching pattern: LON displayed (i) single: 70%; (ii) duplicate: 26% and (iii) triplicate: 4% patterns; d) Variation in the course of LON was observed in 6% of the specimens. Knowledge of the anatomy and variations of the LON may assist in the understanding of cervicogenic headaches and may be of assistance to anesthetists performing regional anesthesia for surgical procedures in the neck.