Nystagmus (sometimes called 'dancing eyes') in all its forms has been known since antiquity. It is a type of bilateral involuntary eye movement disorder usually manifest as a slow movement in one direction and a rapid motion in the other direction. It is present physiologically in everyone to a minor degree under certain conditions but it is often pathologic when exaggerated movements are present during normal visual activities. Movements may be in any direction but usually the motion is horizontal. Since the eye cannot fixate steadily, vision is reduced to some degree in pathologic conditions.
Involuntary, rhythmic eye movements occur in a wide variety of conditions such as central nervous system disease, drug toxicity, retinal disease, metabolic disease, and are a feature of many congenital disorders with developmental delays. However, it may also occur in isolation and can be hereditary in which case it is often called idiopathic congenital nystagmus. In such individuals it is usually noted shortly after birth and the abnormal movements are typically horizontal, often described as a 'to-and-fro' motion.
Once established, it is a permanent condition although frequency and amplitude of motion may change slightly. Because the eye cannot fixate steadily on objects, vision is below normal, usually in the range of 20/50-20/60 although it may be somewhat worse. Since visual acuity of 20/40 or better is required for driving and most reading tasks, some disability is present. Complete elimination of the involuntary movements is not possible with current treatments although improvement in vision has occasionally been reported following surgery on the eye muscles and sometimes correction lenses are helpful to some degree. It is not unusual for patients to find a 'null' point in certain directions of gaze that reduces the amplitude of movement resulting in a slight improvement in vision. This may lead to an evident head turn, especially when looking at near targets. Surgical attempts to move the eyes to a position of least motion have met with mixed results.
No systemic disease is associated with idiopathic congenital nystagmus.
No single gene mutation is responsible for the majority of cases with idiopathic congenital nystagmus. Rather, a variety of inheritance patterns and alterations in genes seem to be responsible. It is common for many individuals to have no family history of this disease. In the disorder, nystagmus 1, described here, the mutant gene responsible is on the X-chromosome and therefore only males are affected. Mothers of such sons are carriers and have a risk that half of their male offspring will inherit this condition. Half of their daughters will be carriers like their mother. Fathers with nystagmus 1 will have no affected children but all of their daughters will be obligatory carriers and half their grandsons can be predicted to inherit this form of nystagmus.
While no curative treatment is available for infantile onset idiopathic congenital nystagmus, it is important to obtain thorough ophthalmological, medical and neurological evaluations. In most individuals the eye movements are evident from simple inspection, but it is important to rule out causative neurologic and eye diseases which may be treatable. In the absence of such disorders, the nystagmus condition remains. Appropriate eye muscle surgery may be indicated when a significant head deviation is present to prevent secondary changes in neck muscles that may not be correctable later.