Nail patella syndrome (NPS), sometimes called Fong syndrome or hereditary osteoonychodysplasia (HOOD), is a rare genetic disorder. It commonly affects fingernails. It may also affect joints throughout the body, such as your knees, and other body systems, such as the nervous system and kidneys. Keep reading to learn more about this condition.

Symptoms of NPS sometimes are detectable as early as infancy, but they may emerge later in life. Symptoms of NPS are often experienced in the:

  • nails
  • knees
  • elbows
  • pelvis

Other joints, bones, and soft tissue can also be affected.

About 98 percent of people with NPS have symptoms that affect their fingernails. These symptoms may include:

  • absent fingernails
  • unusually small fingernails
  • discoloration
  • longitudinal splitting of the nail
  • unusually thin nails
  • triangular-shaped lunula, which is the bottom part of the nail, directly above the cuticle

Other, less common symptoms, may include:

  • disfigured small toenail
  • small or irregularly shaped patella, which is also known as the kneecap
  • knee displacement, usually laterally (to the side) or superiorly (to the top)
  • protrusions from bones in and around the knee
  • patellar dislocations, also known as kneecap dislocation
  • limited range of motion in the elbow
  • arthrodysplasia of the elbow, which is a genetic condition that affects joints
  • dislocation of the elbows
  • general hyperextension of the joints
  • iliac horns, which are bilateral, conical, bony protrusions from the pelvis that are usually visible on X-ray images
  • back pain
  • tight Achilles tendon
  • lower muscle mass
  • kidney problems, such as hematuria or proteinuria, or blood or protein in the urine
  • eye problems, such as glaucoma

Additionally, according to one study, approximately half of people diagnosed with NPS experience patellofemoral instability. Patellofemoral instability means that your kneecap has moved out of proper alignment. It causes constant pain and swelling in the knee.

Low bone mineral density is another possible symptom. One study from 2005 suggests that people with NPS have 8–20 percent lower levels of bone mineral density than people without it, especially in the hips.

NPS is not a common condition. Research estimates that it’s found in 1 out of 50,000 individuals. It’s a genetic disorder and more common in people who have parents or other family members with the disorder. If you have the disorder, any children you have will have a 50 percent chance of also having the condition.

It’s also possible to develop the condition if neither parent has it. When this happens, it’s likely caused by a mutation of the LMX1B gene, though researchers don’t know exactly how the mutation leads to nail patella. In about 20 percent of people with the condition, neither parent is a carrier. That means that 80 percent of people inherit the condition from one of their parents.

NPS can be diagnosed at various stages throughout your life. NPS can sometimes be identified in utero, or while a baby is in the womb, using ultrasound and ultrasonography. In infants, doctors may diagnose the condition if they identify missing kneecaps or bilateral symmetrical iliac spurs.

In other people, doctors may diagnose the condition with a clinical evaluation, an analysis of family history, and laboratory testing. Doctors may also use the following imaging tests to identify abnormalities in the bones, joints, and soft tissues affected by NPS:

NPS affects many joints throughout the body and can lead to many complications, including:

  • Increased risk of fracture: This is due to lower bone density coupled with bones and joints that usually have other problems, such as instability.
  • Scoliosis: Teenagers with NPS are at increased risk of developing this disorder, which causes an abnormal curve of the spine.
  • Preeclampsia: Women with NPS may have an increased risk of developing this serious complication during pregnancy.
  • Impaired sensation: People with NPS may experience reduced sensitivity to temperature and pain. They may also experience numbness and tingling.
  • Gastrointestinal problems: Some people with NPS report constipation and irritable bowel syndrome.
  • Glaucoma: This is an eye disorder in which increased eye pressure damages the optic nerve, which can lead to permanent vision loss.
  • Renal complications: People with NPS frequently have problems with their kidneys and urinary system. In more extreme cases of NPS, you may develop renal failure.

There’s no cure for NPS. Treatment focuses on managing symptoms. Pain in the knees, for example, can be managed by:

  • pain-relieving medications such as acetaminophen (Tylenol) and opioids
  • splints
  • braces
  • physical therapy

Corrective surgery is sometimes needed, especially after fractures.

People with NPS should also be monitored for kidney problems. Your doctor may recommend yearly urine tests to monitor the health of your kidneys. If problems develop, medication and dialysis may help manage and treat kidney issues.

Pregnant women that have NPS carry a risk of developing preeclampsia, and rarely this can develop postpartum. Preeclampsia is a serious condition that can lead to seizures and sometimes death. Preeclampsia causes increased blood pressure and can be diagnosed through blood and urine testing to assess end organ function.

Blood pressure monitoring is a regular part of prenatal care, but be sure to let your doctor know if you have NPS so they can be aware of your increased risk for this condition. You should also talk to your doctor about any medications you’re taking so they can determine which ones are safe to take while pregnant.

NPS carries the risk of glaucoma. Glaucoma can be diagnosed through an eye exam that checks the pressure around your eye. If you have NPS, schedule regular eye exams. If you do develop glaucoma, medicated eye drops can be used to reduce pressure. You may also need to wear special corrective eye glasses. In some cases, surgery may be needed.

Overall, a multidisciplinary approach to NPS is important for the treatment of symptoms and complications.

NPS is a rare genetic disorder, often inherited from one of your parents. In other cases, it’s a result of a spontaneous mutation in the LMX1B gene. NPS most commonly causes problems in the nails, knees, elbow, and pelvis. It can also affect a variety of other body systems including the kidney, nervous system, and gastrointestinal organs.

There’s no cure for NPS, but symptoms can be managed by working with a variety of specialists. Consult with your primary care doctor to find out which specialist is the best for your specific symptoms.