March 27, 2026, 3:49 pm | Read time: 6 minutes
A painful swelling that initially seems like a harmless inflammation can mean much more in the case of FOP than a typical bodily reaction. The rare genetic disorder often begins inconspicuously but gradually alters muscles, connective tissue, and the entire daily life of those affected–with consequences that many families initially struggle to understand.
Symptoms: How FOP Manifests
The disease often appears very early. A typical warning sign is shortened or deformed big toes, which are often noticeable at birth. Later, painful swellings usually occur suddenly and may initially seem like inflammations, bruises, or other minor complaints.
These swellings often subside after a few days or weeks. However, the affected tissue can then transform into bone. Such episodes are often referred to as “flare-ups.” They are frequently accompanied by pain, burning, warmth, redness, and limited mobility.
As the disease progresses, stiffness and movement restrictions increase. Initially, the neck, shoulders, and back are often affected, later also arms, legs, hips, knees, or the jaw. In severe cases, the chest can also become restricted in its mobility, making breathing difficult.1
What Kind of Disease Is This?
FOP is a congenital, genetically determined, and non-contagious disease. It is among the rarest diseases worldwide. Estimates suggest that about one in two million people is affected. Globally, only a few hundred cases are known, with likely only a few dozen in Germany.
The unique aspect of FOP is that the body forms bone in areas where bones should not normally develop–such as in muscles, tendons, ligaments, or connective tissue. Experts refer to this as heterotopic ossification (bone formation).2
Although the first symptoms often appear in childhood, FOP is not a typical childhood disease. The condition is present from birth and accompanies those affected throughout their lives.
Also of interest: This disease hinders development and speech
Causes: Why FOP Develops
The cause of FOP lies in a mutation of the ACVR1 gene. This gene is involved in regulating bone formation in the body. Due to the mutation, this mechanism is disrupted. The body then responds to injuries or inflammations abnormally, forming bone in incorrect locations.
FOP is inherited in an autosomal dominant manner. This means that a single altered gene copy is sufficient to trigger the disease. In most cases, however, the mutation occurs spontaneously, even though both parents are healthy.3
Family Planning–What Affected Individuals Should Know
In principle, people with FOP can have children if the disease does not affect the reproductive system. However, many affected individuals choose not to have children. There are two main reasons for this: increasing physical limitations and the heritability of the disease.4
Since FOP can be genetically passed on, genetic counseling is particularly important for those wishing to have children. It can help better assess individual risks and make informed decisions.
How the Disease Develops
FOP does not progress in clearly defined stages but in episodes. The disease usually begins in early childhood. Initially, recurring swellings and inflammatory reactions occur. Later, these lead to permanent bone formations.
The bone formation often begins in the neck, shoulders, and back. From there, it gradually spreads. With each new episode, mobility can be further lost. Everyday activities like dressing, writing, eating, or personal care become increasingly difficult.
As the disease progresses, the jaw and respiratory muscles can also be affected. If the chest can no longer move sufficiently, the risk of breathing problems and infections increases. Many affected individuals rely on aids or support in daily life by young adulthood.
Challenges in Daily Life
Living with FOP involves significant burdens. A particular challenge is that even minor injuries can trigger new bone formations. Even slight bumps, falls, overexertion, or medical procedures can be problematic.5
Therefore, affected individuals and their families often need to organize daily life very consciously. This includes reducing injury risks, adapting the home, and avoiding stress as much as possible. Travel, school, work, and leisure activities also often require more planning.
Additionally, there are pains, recurring inflammations, and the psychological burden of living with a progressive and little-known disease. Many affected individuals experience increasing dependence on support, which can also be emotionally taxing.
Degree of Disability and Care Level
FOP often leads to severe disability in the long term. However, a degree of disability is not automatically granted; it depends on how much daily life is concretely restricted. Affected individuals must demonstrate this in an application to the social welfare office.
The care level is different. It becomes relevant when assistance with self-care is needed. The care level must be applied for separately with the care insurance fund. Because the applications can be complex, support from counseling centers or social associations can be helpful.
What Is the Difference Between Arthritis and Arthrosis?
How to Prevent Your Dental Implant from Becoming Inflamed
Why FOP Is Often Diagnosed Late
The diagnosis of FOP is challenging because the disease is so rare and the initial symptoms are often mistaken for other conditions. Especially the early swellings are not always correctly interpreted. A crucial clue is the typical malformations of the big toes.
Two features are particularly decisive for the diagnosis: the noticeable big toes and the bone formations in soft tissue. Imaging techniques can make these changes visible. A genetic test can confirm the diagnosis by detecting the mutation in the ACVR1 gene.6
An early diagnosis is particularly important because incorrect treatments–such as unnecessary interventions in the tissue–can worsen the course of the disease.
Treatment of the Disease
There is currently no cure for FOP. Treatment therefore aims to alleviate symptoms, mitigate episodes, and avoid new bone formations as much as possible.
During acute episodes, pain relievers, anti-inflammatory medications, and sometimes corticosteroids are used. It is also important to consistently avoid injuries. Unnecessary surgeries, biopsies, or intramuscular injections should be avoided if possible, as they can trigger new episodes.
Gentle, adapted physiotherapy can help maintain existing mobility for as long as possible. Aids such as wheelchairs, grabbers, dressing aids, adapted seating solutions, or voice-controlled devices can significantly ease daily life. International research is being conducted on new therapies. However, a specifically effective medication is not currently approved in Europe.