Morphological study of the ansa cervicalis and the phrenic nerve.

Morphological features of ansa cervicalis and phrenic nerve were studied in 106 cadavers. Ansa cervicalis was located medial to the internal jugular vein in 63% (medial type) and lateral to the vein in 33.7% (lateral type). Ansa cervicalis was derived from a combination of C1-C4 spinal segments, with C1-C3 being the most frequent pattern (87.5%). In >60% the ansa was bilaterally symmetrical. The distribution of medial and lateral types was equal on left and right sides of the body. The segmental composition of the inferior root was higher in the medial type and also on the left side of the body. In the lateral type the branches that formed the inferior root frequently (75%) formed a common trunk before joining the superior root, but in 74.8% of the medial type they joined the superior root independently. The phrenic nerve was derived from C4 and C5 in 52%. The C4 segment was present in the phrenic nerve in all cases except one. Additional phrenic components that pass anterior to the subclavian vein were defined as accessory phrenic nerves and found in 28.7%, while those passing posterior to the same vein were defined as secondary phrenic nerves (19.8%). Most of the accessory phrenic nerves contained a C5 segment and the nerve to subclavius was the commonest source. Various relationships between the ansa cervicalis and the phrenic nerve are investigated and, based on these findings, two separate classifications for the two nerves are suggested.