Arachnoid dissection for decompression of Chiari I malformation is controversial. Whether arachnoid changes have an impact on the clinical course is not established. This paper documents the histological spectrum of arachnoid changes and evaluates correlations with preoperative, intraoperative, and postoperative data. Arachnoid samples of 162 consecutive foramen magnum decompressions from 2006 to 2016 were studied. Arachnoid thickness and degrees of fibrosis and cellularity were determined with the examiner blinded for clinical data. Based on 145 first time decompressions, a histological classification for arachnoid features was developed. The arachnoid was thicker in secondary compared with primary decompressions (176.1 ± 158.2 μm vs. 35.9 ± 43.5 μm; p = 0.0026) and in adults compared to children (37.3 ± 45.3 μm vs. 21.8 ± 7.7 μm; p = 0.0007). In primary decompressions, arachnoid thickness, degrees of fibrosis, and cellularity followed a normal distribution with all features shifted significantly to higher grades in secondary decompressions. The histological classification correlated with the preoperative severity of gait ataxia, motor weakness, and sensory deficits, whereas it had no predictive power for postoperative short- or long-term results. By comparison, the intraoperative evaluation of arachnoid changes accounting for relationships between arachnoid and surrounding tissues showed higher correlations with preoperative symptoms and had significant predictive power for postoperative short- and long-term results. Histological changes of the arachnoid correlate with preoperative symptoms. Relationships between arachnoid and surrounding tissues show even higher correlations with predictive power for short- and long-term outcomes. These findings suggest a pathophysiological role for the arachnoid in Chiari I malformation.
