Idiopathic infantile nystagmus (IIN) is an oculomotor disorder characterized by bilateral and involuntary oscillations of the eye.
IIN is the most common type of nystagmus, and its prevalence is estimated to be 1/3,000 in United Kingdom.
IIN is usually present at birth, or develops within the first 3 months of life. IIN is characterized by bilateral involuntary, periodic, and predominantly horizontal ocular oscillations and mild visual impairment (usually better than 0.3 LogMar (0.5 Snellen) acuity. Abnormal head postures are common. Affected individuals maintain normal color vision. Nystagmus often changes throughout life, the nystagmus being initially pendular changing later in life to jerk nystagmus. The movements are mainly in horizontal plane, although there may be vertical or torsional components. Nystagmus may increase with fixation or reading small print. Astigmatism and strabismus may be observed. Periodic alternating nystagmus (PAN) is observed on eye movement recordings in approximately 25% of patients. IIN is independent of any other ocular or neurological pathology and is characterized by the absence of oscillopsia. The null region corresponds to the region of fixation for optimal vision and the head position maintains vision in this region. In approximately one third of IIN patients, an anomalous head posture (AHP) may be observed if the null region is not in the primary position of gaze.
The molecular mechanism for IIN is unknown but mutations in FRMD7 (Four-point-one ezrin, radixin, moesin (FERM) domain-containing-7) (Xq26-q27) have been identified as disease-causing in 85% of IIN families and in 7% of IIN single cases. An additional locus has been mapped to chromosomes Xp11.3-p11.4. FRMD7 patients have more frequently the null zone in primary position, thus exhibit less head turn, moderate visual acuity loss of 0.5 Snellen VA or better and more commonly pendular waveforms than nystagmus associated with retinal diseases.
Diagnosis of IIN is based on clinical features. 50% of female carriers are affected. They show slightly better visual acuity than affected males and asymptomatic women show a loss of the optokinetic nystagmus. The eyes are structurally normal and electrophysiologic studies (visual evoked potential (VEPs) and electroretinograms (ERG)) are normal. Median visual acuity, ocular alignment and binocular vision are good. The diagnosis is confirmed by the genetic screening of FRMD7.
Differential diagnosis includes oculocutaneous albinism, X-linked recessive ocular albinism, Chédiak-Higashi syndrome, congenital retinal dystrophies such as achromatopsia, blue cone monochromatism, congenital stationary night blindness, Leber congenital amaurosis, congenital low vision such as optic nerve hypoplasia, retinopathy of prematurity (see these terms), congenital cataracts, infantile esotropia syndrome (manifest latent nystagmus), nystagmus associated with PAX6 mutations and isolated foveal hypoplasia. Nystagmus is also associated with neurological disease for example Pelizaeus-Merzbacher disease, Joubert syndrome, Down syndrome, multiple sclerosis (see these terms), microcephaly in childhood and stroke and cerebellar degeneration are more common in adult patients.
Carrier testing for at-risk female relatives and prenatal testing for pregnancies at increased risk are possible once the disease-causing mutation has been identified in the family.
IIN may be inherited as autosomal dominant, autosomal recessive or X-linked manner. However, the most common mode of inheritance is X-linked with variable expressivity and approximately 50% penetrance in females.
There is no cure for IIN but therapeutic options include magnifying visual aids or eye muscle surgery to correct abnormal head positions (the Anderson-Kestenbaum procedures). Drugs for adult patients with nystagmus include memantine, gabapentin and baclofen.
Nystagmus usually persists but its amplitude recedes over the first 2 years of life.
Last update: January 2014