In an ongoing effort to understand the pathogenesis of occipital neuralgia/headache/migraine, it is critical to describe the anatomical/tissue changes encountered during surgery. Greater occipital nerve anatomical studies mainly focus on the greater occipital nerve course through muscle/fascial planes and interaction with the occipital vessels. However, structural soft-tissue changes have not been described in detail. Anecdotally, trapezius fascia is thickened at the greater occipital nerve trigger site. This study further investigates this observation. Patients undergoing greater occipital nerve decompression surgery were enrolled prospectively in this observational study (n = 92). Tissue changes were recorded intraoperatively. The resulting data were examined. Trapezius fascia was more than 3 mm thick and appeared fibrotic in 86 patients (94 percent), whereas semispinalis muscle appeared normal in all subjects. The greater occipital nerve was macroscopically abnormal, defined as edematous, flattened, and discolored in 29 cases (32 percent). The occipital artery interacted significantly with the greater occipital nerve in 88 percent of cases. The authors conclude that the tissue structure is abnormal in patients undergoing greater occipital nerve decompression surgery. This is the first study that describes the prevalence of thickened and fibrotic appearing trapezius fascia at the occipital trigger site, a phenomenon encountered in the vast majority of patients (94 percent). This structural anomaly has a resemblance to thickened fascial tissues seen in other nerve compression syndromes, and could be related to microtrauma/overuse or actual trauma in the head and neck region.Copyright © 2020 by the American Society of Plastic Surgeons.