Sacroiliac Joint Involvement: An Underreported Complication of <scp>NF1</scp>

NF1-related bone dysplasia in children and young adults with neurofibromatosis type 1 (NF1) involving the sacroiliac joint has been rarely described. We report four participants who underwent whole-body magnetic resonance imaging (WB-MRI) as part of a longitudinal imaging and plexiform neurofibroma (PN) biomarker study (NCT05238909) at Ann & Robert H. Lurie Children's Hospital of Chicago in collaboration with the National Institutes of Health. The four participants were aged 12, 17, 32, and 34 at the time of WB-MRI; three out of four were female. In cases 1 through 3, there was an underlying internal PN on the side of sacroiliac dysplasia. All cases were unilateral and intraarticular, and two involved subcutaneous tissue. The PNs were too small to measure in two cases. Among the measurable lesions, one was localized, and one was diffuse with extension into the lower extremity. The localized PN showed a 20% increase in size over a one-year follow-up period. In cases 1 through 3, no sequelae such as pain or fractures were observed during the follow-up period. Case 4 was referred for surgical evaluation due to persistent pain. Our findings highlight a potentially underreported developmental abnormality of the sacroiliac joint in individuals with NF1.