Sonography of the normal greater occipital nerve and obliquus capitis inferior muscle.

BACKGROUND To use sonography to measure the cross-sectional area (CSA) of the greater occipital nerve (GON) and the adjacent obliquus capitis inferior muscle (OCI) in normal subjects. METHODS Data from 30 asymptomatic subjects between the ages of 22 and 35 were collected. CSA and circumference of the GON and CSA of OCI were measured using sonography. Interexaminer reliability analysis was performed using the intraclass correlation coefficient. RESULTS The CSA of the GON and OCI were 2 mm2 +/- 1 mm2 and 1.86 cm2 +/- 0.51 cm2, respectively. The average circumference of the GON was 4.8 mm +/- 1.3 mm. The interexaminer reliability of the measurements was excellent with intraclass correlation coefficient coefficients of 0.91, 0.84, and 0.73 for the GON CSA, GON circumference, and OCI CSA, respectively. CONCLUSION We report the normal values of the CSA of the GON and OCI. Knowledge of these normal values may facilitate the diagnosis of GON entrapment and provide outcome measures in therapeutic interventions.