A Rare Presentation of Rickets Mimicking Sacroiliitis: A Case Report and Literature Review.

Hypophosphatemic rickets can cause a variety of bone and joint symptoms, one of its rare presentations is sacroiliac joint involvement, which may be mistaken for inflammatory spondylitis. Here, we report the case of a 31-year-old African American woman who presented with a two-year history of lower back pain and morning stiffness, initially suspected to be due to inflammatory spondyloarthritis. Laboratory tests revealed negative inflammatory markers, normal serum calcium, vitamin D3, and parathyroid hormone levels; however, the alkaline phosphatase levels were elevated and serum phosphorus level was low. Magnetic resonance imaging (MRI) of the lumbosacral spine revealed mild widening of the sacroiliac joint with periarticular sclerosis with no signs of osteitis or bone marrow edema. Her condition was attributed to a known diagnosis of X-linked hypophosphatemic rickets affecting her sacroiliac joints. Her symptoms gradually improved after conservative treatment with physical therapy, nonsteroidal anti-inflammatory drugs, phosphate, and vitamin D supplementations. Based on our literature review, we have come across only five rickets cases with similar presentations. Two patients had previously undiagnosed hypophosphatemic rickets at 15 and 35 years of age. One case was related to vitamin D-deficient rickets, and the final two cases were adult-onset vitamin D-resistant rickets misdiagnosed as ankylosing spondylitis. Radiological signs of sacroiliac joint involvement in these cases include narrowing of the sacroiliac joints, fusion of the sacroiliac joints, subchondral hypointense signal changes, and chondral surface irregularities. Vitamin D supplementation has significantly reduced the incidence of rickets; however, there are still cases of familial rickets that can present with a variety of symptoms, including signs and symptoms consistent with inflammatory spondylitis, which can be easily misdiagnosed or mistreated if this presentation is not recognized.