Nail patella syndrome is a genetic disorder which results in abnormal development of an individual’s nails, kneecaps, elbows and pelvis. There is a one in fifty thousand chance of a person being born with this disorder. The abnormal development or dysplasia is either evident at birth or manifests itself in early infancy.
Since the most visual abnormalities are seen on the nails, it is commonly referred to as nail patella syndrome or NPS. This can be a bit of a misnomer as besides the nails, bones, joints, kidney and eyes can also be affected by this disorder. Other names for this disorder include Fong disease, Hereditary Onychoosteodysplasia or HOOD disease, and Turner- Kieser syndrome.
NPS is caused by mutations in the LMX1B gene which encodes a binding protein for specific areas of the DNA. The protein is especially important during the time the fetus develops its limbs, kidneys and eyes. The mutation produces short and non-functional protein that affects its ability to bind to the DNA. More studies are required to find out exactly how the mutations in the LMX1B gene lead to NPS.
Common features of nail patella syndrome
Abnormal nail growth:
Individuals with NPS likely suffer from some form of abnormality associated with their nails. The abnormalities could range from having missing nails right from birth, to having nails that are split, ridged or pitted. Symptoms are more likely to be seen on finger nails than on the toes nails.
The lunulae or area at the base of the nails, is sometimes misshaped as a triangle rather than the natural crescent shape of those with healthy nails. The nails of the individual may also be discolored. Thumbs are often the most affected with the degree of abnormality reducing from the index finger to the small finger.
Issues in the knees:
These can range from missing patellas (kneecaps) to misshaped patellas. The severity of the abnormality may differ from the kneecap simply being misshaped to those which are prone to easy dislocation. The problem with the kneecaps often leads to difficulty in walking normally for an individual suffering from NPS.
Motion of the elbow joints:
The portion of bone meeting at the elbows may be underdeveloped or hypoplastic. This causes limited motion and mobility of the elbows, affecting the ability of the arms to extend completely. The individual suffering from NPS may not be able to turn the palm upwards if they keep their elbows straight.
When keeping the arms by the side of the body, the elbows may protrude outwards at a slight angle. There may also be abnormal webbing present at the elbows in some cases.
Growth on the pelvis:
The pelvic bone may have bony growth known as iliac horns - they are called this as they usually protrude from the iliac bones. These can be seen on X-rays and are present in the majority of patients suffering from this disorder. At times these bony projections may be felt through the individual’s skin. Such skeletal abnormalities are considered a pathognomonic characteristic for the disorder.
Small amounts of blood may be seen in the urine due to the degeneration of the renal tubules in the kidney. These tiny tubes collect, conduct and secrete urine. Their degeneration can lead to kidney disease, high blood pressure and eventually renal failure. An early sign that kidney problems may be present is signalled by protein found in the urine.
The eyes have a tendency to suffer from glaucoma, or increased fluid pressure. The individuals may stay asymptomatic for a long while. Then the fluid pressure increases enough to cause blurry eyesight, headaches and a generation of distorted light halos around things seen. Glaucoma can set in at an early age for individuals suffering from NPS.
Other ocular abnormalities associated with NPS include a dark ring around the iris. The hyper pigmentation may be in the shape of a clover leaf. There can also be early formation of cataracts clouding the lenses of the eyes. Some individuals may be born with extremely small corneas causing various degrees of visual impairment.
Other associated features of NPS
Individuals with NPS tend to have small lean builds and are usually born with thin bones. A general difficulty in putting on weight is owed to issues such as constipation or irritable bowel system affecting the digestive system.
The circulatory system doesn’t work efficiently causing hands and feet to often be cold even during the summers. Nerves at the extremities of the limbs also cause numbness or tingling sensations. Some individuals may even have high hairlines especially around the temples. Not all symptoms featured here may actually be seen in individuals suffering from NPS.
Reviewed by Afsaneh Khetrapal, BSc (Hons)