The spinal column or backbone is made up of 33 irregularly-shaped bones known as vertebrae. The first seven vertebrae, beginning at the base of the skull, are known as the cervical vertebrae. Two overlapping classification systems exist for KFS.
The original classification of KFS subtypes (I, II and III) described by Maurice Klippel and Andre Feil and one more recent updated classification of KFS Classes (KF1-4) as described by Clarke and colleagues.
Various eye (ocular) abnormalities may also be associated with KFS, such as deviation of one eye toward the other (cross-eye or convergent strabismus); involuntary, rapid eye movements (nystagmus); or absence or defects of ocular tissue (colobomas).
These may include structural malformations of the heart (congenital heart defects), particularly ventricular septal defects (VSDs).
Klippel-Feil syndrome (KFS) is a rare skeletal disorder primarily characterized by abnormal union or fusion of two or more bones of the spinal column (vertebrae) within the neck (cervical vertebrae).
Some affected individuals may also have an abnormally short neck, restricted movement of the head and neck, and a low hairline at the back of the head (posterior hairline).
Approximately 25 to 50 percent of individuals with KFS also have hearing impairment.
Such hearing loss may result from impaired transmission of sound from the outer or middle ear to the inner ear (conductive hearing loss); failed transmission of sound impulses from the inner ear to the brain (sensorineural hearing loss); or both (mixed hearing loss).