All Restricted Spines are not Spondyloarthritis: Fibrodysplasia Ossificans Progressiva (FOP) in Monozygotic Twins presenting to Rheumatology Clinic.

Fibrodysplasia ossificans progressiva (FOP) is a rare and progressive debilitating disease that is often misdiagnosed. We present FOP in monozygotic twins in their teen years, presenting to an adult rheumatology outpatient clinic with restricted neck and spine and a referral to rule out ankylosing spondylitis. The classical feature of recurrent episodes of painful lumps on their body, along with classical deformity of their big toes and radiography, clinched the clinical diagnosis. This was further confirmed by a genetic analysis. We review here the pathogenesis and literature on newer treatment options for FOP. The first FDA-approved drug, palovarotene, was approved in 2023. It showed a reduction in heterotopic ossifications. This highlights the need for awareness of this condition among both adult and paediatric rheumatologists so that harmful biopsies and surgeries can be avoided, and patients can start on newer therapies early in the disease. It can be considered a rare mimic of ankylosing spondylitis; however, the characteristic features can very well identify the disorder clinically.