Klippel-Feil syndrome is a bone disorder that is characterized by the abnormal joining (fusion) of two or more spinal bones in the neck (cervical vertebrae). The vertebral fusion is present from birth. Cases of Klippel-Feil syndrome may be classified using the Samartzis classification system. This system divides the condition into three types based on the number of fused vertebrae and whether the fused vertebrae are next to each other (contiguous). According to the Samartzis classification, type I involves a single fusion of two vertebrae, type II involves multiple fused vertebrae that are not contiguous, and type III involves multiple fused vertebrae that are contiguous.

Klippel-Feil syndrome has been described as having three classic features: a short neck, the appearance of a low hairline at the back of the head, and a limited range of motion in the neck. However, less than half of the individuals who have been diagnosed with Klippel-Feil syndrome have all three features. People with Klippel-Feil syndrome may have additional signs and symptoms, and these can vary greatly among affected individuals. Some people with Klippel-Feil syndrome do not have any signs or symptoms.

The fused vertebrae in people with Klippel-Feil syndrome can cause health problems. These include chronic headaches; muscle pain; and an involuntary tensing of the neck, which causes the head to tilt or turn (torticollis). Because some of the cervical vertebrae are fused, this region of the neck can become unstable, which increases the risk of damage to the spinal cord. The fused area of the spine may be more susceptible to trauma, such as a fall or car accident. 

Some individuals with Klippel-Feil syndrome have a narrowing of the spinal canal (spinal stenosis) in the neck, which can put pressure on the spinal cord and nerves. This may be more common in those with Klippel-Feil syndrome type III.

People with Klippel-Feil syndrome may have additional skeletal features, such as rib abnormalities or an abnormal side-to-side curvature of the spine (scoliosis). Approximately 20 to 30 percent of affected individuals have a skeletal condition called Sprengel deformity, in which the shoulder blades are underdeveloped and sit abnormally high on the back.

Additional signs and symptoms seen in people with Klippel-Feil syndrome may include a difference in the size and shape of the right and left sides of the face (facial asymmetry), hearing loss, eye abnormalities, or an opening in the roof of the mouth (cleft palate). Abnormalities of the heart or kidneys may also be present. 

Klippel-Feil syndrome may occur as a feature of other disorders, such as Wildervanck syndrome or hemifacial microsomia. In these instances, affected individuals have the signs and symptoms of both Klippel-Feil syndrome and the additional disorder.

Some people with Klippel-Feil syndrome have neurological issues, which can include a type of birth defect that occurs during the development of the brain and spinal cord (neural tube defect) and a condition in which intentional movements of one side of the body are mirrored by involuntary movements of the other side (synkinesia). [from MedlinePlus Genetics]