Nail patella syndrome (NPS) is a rare genetic condition that affects the nails, kneecaps, elbows, and pelvis. Sometimes called Fong disease, it can also involve the kidneys, eyes, and nervous system. Most people with the condition live a normal lifespan, though about 4 in 10 develop kidney problems that require ongoing monitoring.
The Four Hallmark Features
NPS is defined by a classic set of four physical findings: abnormal fingernails, missing or underdeveloped kneecaps, elbow deformities, and bony growths on the pelvis called iliac horns. Not every person with the syndrome has all four, and severity varies widely even within the same family.
Nail changes are usually the most visible sign. Fingernails may be small, ridged, split, discolored, or missing entirely. The thumbnails tend to be the most affected, with involvement becoming less noticeable as you move toward the pinky finger. Toenails are less commonly involved.
The kneecaps can be undersized, irregularly shaped, or completely absent. This often leads to knee instability, frequent dislocations, and difficulty with activities like climbing stairs or running. Many people with NPS develop osteoarthritis in the knees earlier than expected.
Elbow problems typically show up as limited range of motion, particularly difficulty fully straightening or rotating the forearm. You might not be able to turn your palms fully upward, for instance. Like the knee issues, elbow deformities can lead to early arthritis.
Iliac Horns: A Unique Skeletal Marker
Iliac horns are small, cone-shaped bony projections that grow from the back of the pelvis. They are considered pathognomonic for NPS, meaning they don’t occur in any other condition. Despite that diagnostic significance, they only appear in 70% to 80% of people with the syndrome. These projections sit at the attachment point of the gluteus medius muscles and can sometimes be felt through the skin, though they rarely cause pain or affect how you walk. They show up clearly on a pelvic X-ray or CT scan and can even be detected on fetal ultrasound as early as the third trimester.
What Causes It
NPS is caused by a mutation in a gene called LMX1B, which plays a key role in the development of limbs, kidneys, and eyes during embryonic growth. The condition follows an autosomal dominant inheritance pattern, meaning you only need one copy of the altered gene to be affected. If one parent has NPS, each child has a 50% chance of inheriting it. The mutation has complete penetrance, so virtually everyone who carries it shows some signs of the condition, though the specific features and their severity can differ dramatically from person to person.
Some cases arise from a new (spontaneous) mutation in someone with no family history.
Kidney Involvement
Roughly 40% of people with NPS develop some degree of kidney disease. The earliest warning signs are usually small amounts of blood in the urine, protein in the urine, high blood pressure, or unexplained swelling in the legs or around the eyes. These changes can appear in childhood or adulthood.
For most people, kidney involvement follows a relatively slow course. But in a smaller subset, it progresses to full kidney failure requiring dialysis or a transplant. Kidney disease is the main factor that can shorten life expectancy in NPS, which is why regular urine tests and blood pressure checks are an important part of long-term care.
Glaucoma and Eye Health
Glaucoma is the most common eye problem linked to NPS, found in roughly 30% of affected individuals over age 40. That rate is significantly higher than in the general population. It typically presents as open-angle glaucoma, the same type most common in the general public, but it tends to show up at a younger age. The median age at diagnosis in NPS-related cases is around 38.
Because glaucoma can cause irreversible vision loss before you notice any symptoms, annual eye pressure screening is recommended for everyone with NPS starting in childhood or early adulthood. Less common eye findings include unusually small corneas, cataracts present from birth, and a distinctive darkening at the edge of the iris known as Lester’s sign.
How It’s Diagnosed
Diagnosis is primarily clinical. A doctor who sees the characteristic combination of nail changes, knee problems, and elbow abnormalities will typically suspect NPS, and a pelvic X-ray showing iliac horns can confirm it. In families with a known history, the diagnosis is often straightforward. For cases without a clear family connection, genetic testing for LMX1B mutations can provide a definitive answer. The condition can also be identified prenatally through ultrasound findings or genetic testing when a parent is known to carry the mutation.
Living With Nail Patella Syndrome
There is no cure for NPS, so management focuses on monitoring and treating specific problems as they arise. For joint issues, physical therapy can help strengthen the muscles around unstable knees and maintain elbow mobility. Some people eventually need surgery to stabilize or replace kneecaps, particularly if dislocations are frequent or arthritis becomes severe.
Kidney health requires ongoing attention. Regular urine tests to check for protein and blood, along with blood pressure monitoring, help catch nephropathy early. If kidney function starts to decline, medications that lower blood pressure and reduce protein loss can slow progression. Annual eye exams to screen for glaucoma round out the core monitoring plan.
Most children and adults with NPS lead full, active lives. The condition’s impact ranges from cosmetic nail changes that require no treatment at all to significant joint and kidney complications that need sustained medical attention. Because the same mutation can produce such different outcomes, even siblings with NPS may have very different experiences with the condition.