This chapter discusses the characteristics, diagnosis, and management of fixational, saccadic, and pursuit eye movement disorders. We use the term ocular motor dysfunction to refer to the condition in which there are problems in all three areas of eye movement function. In our experience, this is the most common clinical presentation. It is unusual to find saccadic dysfunction in isolation of fixational or pursuit anomalies, or pursuit dysfunction in isolation of fixational or saccadic anomalies.

Eye movement disorders are a diagnostic and management concern for optometrists because of the effect such problems may have on the functional capability of an individual. Unlike accommodative¹ and binocular vision skills,² which reach adult levels of development very early in infancy, clinical assessment indicates that eye movement development is considerably slower, continuing through the early elementary school years.^3,4 The clinical picture of slow development is not consistent with the basic research data that suggest the presence of normal eye movements by approximately age 1 or 2 years. This apparent difference is likely to be related to cognitive and attentional factors affecting eye movements through about age 12. Because of the long developmental process for eye movement control, slow development can leave a child with inadequate skills to meet the demands of the classroom.⁵ Saccadic and pursuit dysfunction, therefore, primarily interfere with performance in schoolchildren, although some authors have reported the presence of these problems in adults as well.^6,7

Much of the emphasis of both researchers and clinicians has been on the relationship between eye movements and reading. During reading, the three important components of eye movements are saccades, fixations, and regressions. Saccades take up approximately 10% of the reading time. The average saccade is about eight to nine character spaces, which is about a 2-degree visual angle.⁸ The duration of the saccade is a function of the distance covered. For instance, a 2-degree saccade takes about 25 to 30 ms, and a 5-degree saccade takes about 35 to 40 ms.⁸ Between saccades, the eye is relatively still in a fixational pause. For normal readers, the average duration of the fixation is 200 to 250 ms. An important characteristic of normal reading eye movements is the great variability both within and between subjects. Saccade lengths can vary from 2 to 18 character spaces, and fixation duration values can range from 100 to over 500 ms, for a single reader within a single passage.⁸ The third important characteristic of reading eye movements is the regression. A regression is a right-to-left movement, and it occurs 10% to 20% of the time in skilled readers. Regressions occur when the reader overshoots the target, misinterprets the text, or has difficulty understanding the text.

Because eye movement deficiencies intuitively seem to be so closely linked with reading, there have been numerous studies investigating this relationship. Unfortunately, the results of these investigations are equivocal and, at times, confusing. Limitations and differences in experimental design, methodology, statistical procedures, and assumptions among these studies have created difficulty in their interpretation.⁵ Two basic viewpoints have evolved about the relationship between eye movements and reading. The first suggests that eye movement disorders can cause below-average reading ability.^{9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27} Investigators, using a variety of methods to assess eye movements, have found that poor readers tend to make more fixations and regressions than normal readers.^{10,11,12,13,14,15,16,17,18,19,20,27} The second view is that the random and unskilled eye movement skills observed in poor readers are secondary to deficient language skills that cause reading disorders. Thus, the reading difficulty itself leads to erratic and inconsistent eye movements.^{26,28,29,30,31,32}

A third perspective is probably most likely to be correct and is essentially a combination of the first two viewpoints. This alternative⁴ suggests that, in some cases, problems with fixation and saccadic abilities may be a primary factor interfering with a child’s ability to read quickly, comfortably, and with adequate comprehension. In other cases, the eye movement deficiencies observed during reading may simply be a reflection of poor reading ability.

Another important background issue is that during reading, eye movements are integrated with higher cognitive processes such as attention, memory, and the utilization of the perceived visual information.^{3,33,34,35,36,37,38,39,40} Some optometrists believe that there is a relationship between poor ocular motor skills and attentional problems.⁴¹ When such a relationship exists, treatment of eye movement disorders may lead to improvement in attention and concentration.^37,38,39

There have been few studies of the prevalence of eye movement disorders, particularly in the population of normally achieving children and adults. In children with reading and other learning difficulties, several studies have found a very high prevalence of eye movement anomalies.^42,43,44 In a sample of 50 children between the ages of 6 and 13 years with learning disabilities, Sherman⁴² found that 96% had problems with ocular motor inefficiency (saccadic and pursuit problems). He did not state how eye movements were evaluated or his criteria for establishing a diagnosis of ocular motor inefficiency. Hoffman⁴³ reported on a sample of 107 children between the ages of 5 and 14 years with learning problems. He evaluated pursuits and saccades using the qualitative scales described in Chapter 1 along with an objective assessment using the Eye Trac. The criteria for a diagnosis of ocular motor dysfunction were performance below a 3+ on the subjective clinical observation or 2 years below age-expected values on the objective assessment. His results revealed that 95% of the sample had ocular motor problems. He also reported on the results of 25 children without learning problems and found that 24% had ocular motor problems. It is interesting to note that both Hoffman and Sherman found that ocular motor dysfunction was the most prevalent vision disorder in their samples of learning disabled children. Lieberman⁴⁴ studied the prevalence of vision disorders in 55 children between the ages of 8 and 10 years at a school for children with emotional disorders. He used observational scales and the New York State Optometric Association King-Devick (NYSOA K-D) test to evaluate saccades. The NYSOA K-D test is similar to the developmental eye movement (DEM) test described in Chapter 1. Pursuits were evaluated using subjective observational scales; 53% of the children had saccadic dysfunction and 43% had pursuit anomalies. In this same study, Lieberman reported that in a sample of 1,681 children in a normal population, the prevalence of saccadic dysfunction (using the NYSOA K-D test) was 22.6%.

Jainta and Kapoula⁴⁵ examined the relationship between saccades and vergence control during real text reading. Thirteen dyslexic and seven non-dyslexic children read the French text “L’Allouette” in two viewing distances (40 cm vs. 100 cm), while binocular eye movements were measured with an objective eye-tracking system. They found that the binocular yoking of reading saccades was poor in dyslexic children (relative to non-dyslexics) resulting in vergence errors and fixation disparity. The fixation disparity was larger for dyslexic children, making a larger demand on their sensory fusion processes. The authors concluded that visual/ocular motor imperfections may exist in dyslexics that lead to fixation instability and thus, to instability of the letters or words during reading.

In our experience, and in the three studies described earlier,^42,43,44 eye movement disorders are rarely present in isolation. Rather, they are generally found associated with accommodative, binocular, and visual perceptual dysfunctions. As a result, treatment of eye movement deficiencies generally occurs within the context of an overall treatment approach designed to deal with other problems as well.

It is clear that more investigation is needed into the prevalence of eye movement disorders to clarify their role in reading and other areas of function. Regardless of these shortcomings in the literature, clinicians are regularly faced with children and adults presenting with signs and symptoms suggestive of fixation, saccadic, and pursuit disorders. Psychologists and educators often refer children with concerns about poor tracking, skipping words and lines, and losing place when reading. In addition, Solan⁶ has demonstrated that there are adults with eye movement problems that interfere with their performance in school and at work. Although these individuals often achieve at satisfactory levels, they complain of slow and inefficient reading.⁷ It is important for clinicians to be able to evaluate eye movement function and to prescribe appropriate treatment if a disorder is detected. An important concept that must be emphasized is that optometrists treat eye movement disorders to normalize these functions and eliminate the patient’s symptoms. We are not directly treating the reading disorders, although in some cases more accurate and efficient eye movements may lead to better reading performance.