Classification and clinical anatomy of the first spinal nerve: surgical implications.

Object: Data regarding the first cervical nerve are scanty and conflicting, however this nerve may need to be identified for neurosurgical procedures such as rhizotomy for torticollis and suboccipital pain syndromes. The authors performed the present study to elucidate further the detailed anatomy of the first cervical nerve and review its clinical relevance.

Methods: Forty adult cadavers (80 sides) were used in this study. Dissection was performed at the craniocervical junction with special attention to the formation and presence of the components of the C-1 spinal nerve. Additionally, connections between the C-1 nerve and the spinal accessory nerves were recorded.

Results: The authors classified the C-1 nerves into Types Ia, Ib, and II, in 34, 9, and 37 sides, respectively. Type Ia was composed of ventral and dorsal roots with a dorsal root ganglion, Type Ib was composed of ventral and dorsal roots and no dorsal root ganglion, and Type II was composed of only ventral roots. All types contained both dorsal and ventral rami. Mackenzie's nerve was identified on 2 left sides (2.5%). On 48 sides (60%), the C-1 nerve received a mean of 2.5 dorsal rootlets. In the remaining specimens, C-1 did not receive any dorsal rootlets. On the sides found to receive C-1 dorsal rootlets, 14 (30%) were found to have a distinct dorsal root ganglion present, and in 21 (44%) the spinal accessory nerve joined with the dorsal rootlets. The first cervical vertebra in these cases did not possess a dorsal root ganglion. A dorsal ramus of the C-1 spinal nerve was identified on all sides. Communication between the dorsal rami of C-1 and C-2 near their posterior elements was found on 12 sides (15%).

Conclusions: A detailed knowledge of C-1 nerve anatomy may be of use to the surgeon operating in the vicinity. Specifically, this knowledge may be helpful in procedures involving C-1 rhizotomy, including peripheral denervation procedures for cervical dystonia and occipital neuralgia.