12.2Compensatory Mechanisms

Several compensatory mechanisms exist in congenital nystagmus which tend to decrease the nystagmus and 12 thus improve the visual acuity. These compensatory mechanisms are achieved with superimposed vergence

and version movements.

Di erent compensatory mechanisms may coexist in the same patient with congenital nystagmus. These mechanisms need to be analyzed carefully both to plan the treatment and also to make prognostic predictions.

12.2.1Dampening by Versions

Version eye movements are used in some patients as a compensatory mechanism to reduce congenital nystagmus. Sustained contractions of yoke muscles help maintain the eyes in a peripheral lateral, vertical, or oblique gaze, depending on the position of the null region, leading to dampening of nystagmus. These versions are often accompanied, and consequently identified, by an AHP. An eccentric horizontal null zone leads to horizontal head turn and an eccentric vertical null zone leads to chin elevation or depression. For example, in a patient who has null position in the laevoversion, the compensatory head position is face turn to right, for null zone in elevation the compensatory mechanism is chin down position. Compensatory cycloversion leads to head tilt. A right head tilt corresponds to blocking incyclotorsion of the right eye and excyclotorsion of the left eye.

12.2.2Dampening by Vergence

There are two distinct clinical conditions which use dampening by convergence as a compensatory mechanism to reduce the amplitude and frequency of nystagmus. These are MLN and nystagmus blockade syndrome (NBS).

Adelstein and Cüppers [6] coined the term “nystagmus blockage syndrome” as having the following clinical features:

■Esotropia with sudden onset in early infancy, often preceded by nystagmus

■Pseudoparalysis of both abducens nerves

■The appearance of manifest nystagmus as the fixating eye moves from adduction toward abduction

■Increase in the angle of the convergent squint when a base-out prism is put in front of the fixating eye

NBS occurs with the waveform characteristics of increasing velocity slow phase and variable angle esotropia (Fig. 12.7a–d). MLN is also frequently associated with infantile esotropia. Most cases diagnosed as nystagmus blockage syndrome in the past probably corresponded to infantile esotropia associated with MLN.

Summary for the Clinician

■Compensatory mechanisms are seen in patients with congenital nystagmus to increase visual acuity by decreasing the intensity of nystagmus.

■Compensatory mechanism can be achieved by convergence or version movements in case of eccentric null region. Compensatory mechanisms by versions lead to AHP.

■Several compensatory mechanisms usually exist in the same patient.

12.2.3Anomalous Head Posture (AHP)

AHP in children could be due to abnormalities of the oculomotor system, neck muscles, or the central nervous system. The ocular causes of AHP include strabismus, nystagmus, refractive errors, and ptosis. Although clinical di erentiation of these disorders is accurately accomplished after thorough history and ocular examination, the exact mechanism of AHP is often di cult to determine in patients with combination of strabismus and nystagmus. It is important to delineate the cause of AHP and the amount of AHP before considering treatment in patients with congenital nystagmus.

12.2.3.4Measurement of AHP

An AHP typically becomes progressively larger with increased visual e ort. Hence, quantification of the surgery must be based on an appropriate e ort of fixation, usually achieved by testing visual acuity at distance and near. Figures 12.5a, b show a child with no AHP when no visual e ort is needed. However, when he identifies a stereoptic stimulus on the Lang test at near he is using a head turn to the right. Similarly, Figs. 12.5c, d show a patient with no head turn without visual e ort. However, he uses a very large chin elevation and head turn to the right when he is asked to read small letters at distance.

AHP can be measured objectively, while reading small optotypes at distance and near, using calipers or the Harms wall (Fig. 12.5k). For di erential diagnosis, it is important to record visual acuity with both eyes open as well as with each eye occluded. It is also useful

Fig. 12.7 A patient with nystagmus blockage syndrome (a) with straight eyes, (b) when dampening nystagmus with right esotropia, (c) wearing Fresnel prisms for surgical evaluation, which showed dampening of nystagmus and (d) after bimedial medial rectus recessions. Original eye movement recordings show periodic convergence to dampen the nystagmus before surgery and quieter eye movements after surgery (e)

clinically to look at the e ects of straightening the head on nystagmus.

12.2.3.5E ect of Monocular and Binocular Visual Acuity Testing on AHP

Testing Visual Acuity with Both Eyes Open

AHP should be first assessed testing visual acuity with both eyes open to determine the existence and the type of AHP naturally adopted by the patient. The patient could have one of the following:

■No AHP: This could indicate that either the patient is using vergence as a compensatory mechanism, that the null region is in the primary position, or that no compensatory mechanism is being used by the patient

■A horizontal AHP consisting of a face turn to the right or left

■A vertical AHP consisting of a chin elevation or depression

■A bidirectional or alternating AHP

■A head tilt to the right or left

■A combination of AHP in di erent planes