Testing Visual Acuity with Either Eye Covered

Testing AHP under monocular conditions using occlusion helps to di erentiate between congenital nystagmus and 12 MLN,since in congenital nystagmus the AHP is usually concordant (i.e., usually does not change position when covering one eye), whereas in MLN nystagmus the AHP is discordant. This is because in MLN the intensity of the nystagmus tends to be least in adduction. Consequently, in MLN the head turn and the nystagmus direction reverse when fixation shifts from one eye to the other (Fig. 12.5e, f).

12.2.3.6Testing AHP at Near

Since convergence has an e ect on nystagmus, AHP should also be tested when measuring visual acuity or reading at near (e.g. at 33 cm). All the observations noted regarding the position of AHP and the nystagmus intensity for distance should also be evaluated for near vision.

12.2.3.7The E ect of Straightening the Head in Patients with AHP

On straightening the head, if the nystagmus increases, then the cause of the AHP is almost certainly due to the nystagmus. If there is no change in the nystagmus, the AHP is either due to other ocular causes, a structural anomaly of the head or neck, CNS anomalies, or because of strabismus. Since strabismus in presence of nystagmus can be responsible for AHP thorough examination for comitant or incomitant squint is important in all patients with nystagmus. If the strabismus increases with head straightening, it indicates that an incomitant deviation is responsible for the AHP. However, if the strabismus improves with straightening of the head, the AHP is more likely associated with the nystagmus or some other cause.

12.3 Treatment

Various modes of treatment are available for patients with congenital nystagmus. However, it is necessary to decide the best method to treat these patients in the light of understanding the type of congenital nystagmus and the compensatory mechanism being used. Sometimes a combination of treatment options might be needed to achieve a better outcome.

The main aim of treatment of congenital nystagmus is:

1.To improve visual acuity

2.To diminish the amplitude and frequency of nystagmus

3.To shift the null position to primary position with the aim of correcting an AHP

4.To correct the strabismus if present

The main categories of treatment of nystagmus are optical, medical, and surgical although other forms of treatment have been attempted such as acupuncture, biofeedback, and use of botulinum toxin-A.

12.3.1Optical Treatment

The incidence of significant refractive errors in patients with congenital nystagmus has been estimated to be as high as 85% [7]. The importance of correcting refractive errors besides improving visual acuity is to prevent ambylopia and to treat the associated strabismus, commonly seen in patients with congenital nystagmus. Optical treatment can involve spectacles, contact lenses (CL), or low visual aids.

Summary for the Clinician

■It is important to delineate the cause of AHP and the amount of AHP before considering treatment in patients with congenital nystagmus.

■AHP typically becomes progressively larger with increased visual e ort. Hence quantification of the head turn for surgical assessment must be based on measurement during maximal visual e ort.

■In patients with combination of strabismus and nystagmus, the cause of AHP needs to be carefully analyzed.

12.3.1.1Refractive Correction

A full cycloplegic refraction should be performed in children. A simple correction of refraction is the easiest way of improving the visual acuity in congenital nystagmus. Hence, all patients with congenital nystagmus should have precise refraction with appropriate correction before attempting other modalities of treatment.

12.3.1.2Spectacles and Contact Lenses (CL)

Several studies have suggested that CL improve visual function better than spectacles in patients with congenital nystagmus [8, 9]. The possible mechanisms underlying this are that CL reduces the chromatic and spherical