ASSESSMENT OF BINOCULAR VISION

5.1Relevant case history information 151

5.2Relevant information from assessments of other systems 153

5.3Classification of heterophoria 153

5.4Classification of comitant heterotropia (squint or strabismus) 155

5.5The cover test 157

5.6Hirschberg, Krimsky and Bruckner tests 167

5.7Modified Thorington test 169

5.8Maddox rod 171

5.9Maddox wing 174

5.10von Graefe phoria technique 176

5.11Modified gradient AC/A ratio test 178

5.12Fusional reserves (fusional vergences) 180

5.13The Mallett fixation disparity unit 184

5.143 base-in/12 base-out prism flippers 187

5.15Near point of convergence 188

5.16Jump convergence 190

5.17Push-up/push-down amplitude of accommodation 191

5.18Nott and MEM dynamic retinoscopy 194

5.192.00 DS flippers 195

5.20Worth 4-dot test 197

5.214 base out (BO) test 199

5.22TNO stereo test 201

5.23Titmus fly test 204

5.24Classification of incomitant heterotropia 206

5.25The motility test (broad H test) 207

5.26Pursuits (as part of motility testing) 211

5.279-point cover test or 9-point Maddox rod/modified Thorington 212

5.28Park’s 3-step test 214

5.29Saccades 215

5.30Bibliography and Further reading 217

5.31References 217

Tests that assess the binocular vision system are included in this chapter. Rather than group these tests in terms of preliminary or pre-refraction tests and post-refraction or functional tests, the tests are grouped together depending on the aspect of binocular vision that they help to assess. This is because the organisation of the book is directed towards the assimilation of a problem-oriented approach (section 2.1.3) built upon a systems examination (section 2.1.2). In addition, this grouping of tests may help students to better appreciate the relationship between preliminary tests, such as the cover test (section 5.5) and post-refraction or functional binocular vision tests, such as subjective assessments of heterophoria (sections 5.7 to 5.10).

5.1 RELEVANT CASE HISTORY

INFORMATION

The case history can provide significant information about binocular function and can help the practitioner decide, for example, that particular tests of binocular vision are not appropriate (e.g. near point of convergence with a near heterotropia) or, alternatively, that other tests, not routinely used, are warranted (e.g. determination of accommodative facility). The following areas are of relevance during examination of all patients but they may be of particular importance in a patient with a known binocular vision anomaly or in whom an anomaly of binocular vision is suspected.

5.1.1 Observations and symptoms

1.Observation of strabismus and head tilt: Simple observation of the patient as case history is being taken can highlight a strabismus or head tilt. Parents or carers may also inform you that they have noticed that their child occasionally has an ‘eye turn’ or perhaps, a head tilt. Any suggestion of a strabismus requires a careful cover test and stereopsis testing in addition to looking for amblyopia and possible causes of the strabismus such as hyperopia or anisometropia.

2.Symptoms of blurred vision, headaches or asthenopia at distance and/or near can indicate a decompensated heterophoria at the pertinent distance.

3.Complaints of ‘double vision’ could suggest a heterophoria breaking down into a heterotropia (typically horizontal diplopia, occurring especially when tired), a remote near point of convergence (section 5.15), the angle of strabismus changing so that the retinal image falls out of the suppression area or an incomitant deviation (section 5.24). An appropriate line of questioning during the case history will help in this differential diagnosis. Note that cortical cataract and occasionally posterior subcapsular cataract can cause monocular diplopia (or polyopia) and should be considered in elderly patients by determining if the diplopia persists if one eye is covered.

4.Fluctuations in distance vision and particularly distance blur after near work, suggest problems of accommodation and tests that assess accommodative function should be employed (sections 5.17–5.19).

5.Although the above symptoms may alert the practitioner to a possible anomaly of binocular vision, it is worth remembering that a lack of symptoms does not, in itself, mean that the binocular system is normal. For example, patients with suppression or long-standing heterotropia almost certainly will not experience binocular vision symptoms.

6.Poor reading ability and poor progress at school could also be due to a binocular vision problem.

5.1.2 Ocular history

The ocular history may indicate that the patient, or perhaps someone in the family, has a ‘weak’ or ‘lazy’ eye and/or strabismus. This should be followed up by asking if ‘patching’ or spectacles or any eye exercises have been prescribed or if ‘eye muscle’ surgery has taken place. Any positive response to these questions should lead to further questioning regarding the age when these interventions happened, when they stopped, their success and if the patient is still under any care for the amblyopia/ strabismus. If the latter is the case, you should be careful not to change an optical prescription or alter the current therapy in any way without permission/ agreement from the other practitioner treating the patient.

5.1.3 General medical history and family history

General health questions may indicate a systemic condition that can lead to binocular vision problems, such as diabetes, or systemic medications that can affect accommodation or binocular vision. When examining young children with or without a binocular vision abnormality it is useful to ask whether any member of the patient’s family has a strabismus or ‘lazy eye’ as there appears to be a hereditary link, particularly for esotropia. In children diagnosed with strabismus and/or amblyopia, you should ask whether the child’s siblings have been examined and, if not, inform the parent/carer that the other children should be examined to avoid the possibility of amblyopia developing.

5.1.4 Birth history

It is also useful to ask the child’s parent/carer about the pregnancy and birth history. There is a high prevalence of ocular abnormality, in particular strabismus, in children born prematurely, those with low birth weight or disorders of the central nervous

system, and in children with significant birth complications (e.g. forceps delivery). It is, therefore, recommended that the following questions be posed to the parent/carer during the case history examination:

■Was the child a full-term baby or were they born prematurely?

■What was the birth weight? (less than 2500 gr or 5.5 pounds is a significant risk factor for strabismus, in particular esotropia; Mohney et al. 1998).

■Were there significant complications at the child’s birth?

■Is the child’s current and past general health good?

■Since birth, has the child been investigated or received treatment for any medical condition?

5.2 RELEVANT INFORMATION FROM ASSESSMENTS

OF OTHER SYSTEMS

5.2.1 Binocular visual acuity

In cases where the acuities in the right and left eyes are similar or identical, it is usual to find that binocular visual acuity (VA) is typically between half a line and a line better than monocular acuity (Pardhan & Elliott 1991). Of course, it is not possible to find this improvement if monocular VA equals the ‘bottom line’ of the Snellen chart you are using. When using a non-truncated chart, a binocular VA that is equal to or worse than the monocular VA can indicate a binocular vision problem. A poor patient reaction to the restoration of binocular vision after an occluder has been removed following monocular subjective refraction can also indicate a binocular vision problem.

due to variable fixation, but also possible latent hyperopia or pseudomyopia that should be investigated using assessments of accommodation (sections 5.17 to 5.19) and/or cycloplegic refraction (section 4.20).

2.Differences between retinoscopy and subjective refraction: A retinoscopy result that is significantly ( 1.50 D) more positive than the subjective result could indicate latent hyperopia or pseudomyopia that should be investigated using assessments of accommodation (sections 5.17 to 5.19) and/or cycloplegic refraction (section 4.20).

3.Fluctuations in subjective refraction: Fluctuations in spherical power during subjective refraction could suggest poor control of accommodation. These could be due to the use of monocular refraction and/or poor technique, but may need to be investigated using assessments of accommodation (sections 5.17 to 5.19) and/or cycloplegic refraction (section 4.20).

5.2.3 Systemic and ocular health assessment

Information provided by the patient about systemic or ocular disease, previous or current, may explain signs or symptoms that are of a binocular vision nature. For example, diabetes or thyroid disease can lead to binocular vision problems. Similarly, particular signs or symptoms may prompt the practitioner to ask again about systemic health and/or to seek explanation within the eye. For example, a newly acquired divergent heterotropia (section 5.4) and ptosis may be observed in a palsy of the third cranial nerve and is suggestive of diabetes. Finally, cortical cataract and occasionally posterior subcapsular cataract can generate diplopia that is monocular in origin (i.e. it persists even when one eye is covered).

5.2.2 Retinoscopy and subjective refraction

5.3 CLASSIFICATION OF

HETEROPHORIA

1.Fluctuations in retinoscopy: Fluctuations in spherical power during retinoscopy indicate changes in accommodation. These could be

Binocular vision requires that the eyes move together so that the visual axes intersect at the object of regard.

The eyes are held in alignment by a combination of the sensory and motor fusion mechanisms. If sensory fusion is prevented (for example, by occluding one eye as during the cover test), only the motor fusion mechanism is operational and a misalignment of the visual axes will occur in many patients. This misalignment is sometimes referred to as a latent deviation but is more commonly known as a heterophoria. Video clips of the cover test being used to assess a variety of heterophorias are provided on the website .

5.3.1 Direction

ORTHOPHORIA is present if the visual axes remain correctly aligned when sensory fusion is prevented. Heterophorias can be defined in terms of the direction of the misalignment when sensory fusion is prevented:

■ESOPHORIA: Convergence of the visual axes

■EXOPHORIA: Divergence of the visual axes

■HYPERPHORIA: One visual axis higher than the other

■HYPOPHORIA: One visual axis lower than the other.

Classification of vertical heterophorias is rather artificial in the sense that if the right visual axis is higher than the left this may be classified as a right hyperphoria or, alternatively, as a left hypophoria. In practice, it is usual to classify vertical heterophorias in terms of which eye is the hyperphoric eye; thus vertical heterophorias are normally described as either right hyperphoria or left hyperphoria in order to indicate the higher visual axis.

A relative rotation of the vertical poles of the cornea is called a cyclophoria, which can be further categorised into:

■EXCYCLOPHORIA: Outward rotation of the upper poles

■INCYCLOPHORIA: Inward rotation of the upper poles.

Cyclophorias are seldom investigated in primary eye care examinations. If a cyclophoria is present it is likely that it will be accompanied by other types of heterophoria (e.g. vertical heterophoria) and in most cases it will be possible to explain its presence

by considering the actions of the elevating and depressing extraocular muscles of the eye (von Noorden 2002; e.g. the intorting actions of the superior oblique muscles and extorting actions of the inferior oblique muscles). Given the rarity with which cyclophorias alone are diagnosed in primary eye care and the fact that no treatment exists, cyclophorias will not be discussed further.

5.3.2 Magnitude and stability

Most patients have a small amount of heterophoria, especially at near. The magnitude of heterophoria is estimated or measured in prism dioptres ( ). At distance, between 2 of esophoria and 4 of exophoria is considered normal. At near, between 3 and 6 of exophoria is considered normal. The tendency for the eyes to exhibit a small amount of exophoria at near is referred to as physiological exophoria. The tolerance to vertical misalignments is less than horizontal with greater than 0.5 vertical heterophoria considered abnormal. While heterophoria over the course of a lifetime remains fairly constant (although physiological exophoria shows a small increase with age; Freier & Pickwell 1983), the ability of the patient to cope with their heterophoria can be influenced by stress on the visual system (e.g. excessive workload), by fatigue or by the patient’s general health.

5.3.3 Comparing heterophoria at distance and at near

Heterophoria is usually evaluated with distance (6 m or 20 ft) and near (40 cm or 16”) viewing because the amount of heterophoria exhibited at the two distances is often quite different. This is because of the accommodation/convergence relationship. When a near target is viewed the eyes converge as well as accommodate. Depending upon the amount of convergence that accompanies each dioptre of accommodation (the magnitude of the AC/A ratio, section 5.11), the heterophoria at near may be very different from that which exists at distance. The following names are used to describe the possible conditions that may be present when a large difference exists between the distance and near heterophoria and where the patient is experiencing