The role of the optometrist in dyslexia

would be present when the patient is reading. The test therefore has good ‘face validity’ for identifying binocular vision anomalies which are likely to interfere with the usual reading process. Recent research has shown that the questions the practitioner asks during this test are crucially important. Quite commonly, the patient is just asked whether the Nonius markers (green strips) are lined up. Interestingly, many who answer yes to this question also answer yes to a supplementary question: “Do one or both of the lines ever move?” This latter question is the best way of identifying patients who are likely to have symptoms. If one of the Nonius markers moves, then the direction of movement is identified, and the aligning prism or sphere is determined. This is the prism (base-in for an exo-slip) or sphere (negative for an exo-slip) which eliminates movement of the strip. The aligning sphere corrects a fixation disparity through influencing accommodative vergence. In cases where exercises are not appropriate, this can be a useful treatment. The Mallett unit OXO test has been shown to have good sensitivity and specificity for detecting symptomatic heterophoria at near. It should be stressed that the test does not detect the full heterophoria, but rather the element which is decompensated. In prepresbyopes, if an aligning prism of 1∆ or more is detected then there are likely to be symptoms. If this is the minimum interval for correction, then it could be argued that the minimum step size for measurement should be less than this, i.e. 0.5∆. The sensitivity and specificity of the Mallett OXO test is good, but not perfect. So, other clinical tests are useful to help