A heterophoria is compensated when the vergence system can adequately overcome the heterophoria. Yet there are subjects with a negligible heterophoria and whose opposing fusional reserves meet the conventional criteria for compensation discussed in Chapter 4 , and yet who have symptoms associated with poor binocular coordination. These patients may be best described by the term binocular instability , also known as fusional vergence dysfunction .
Binocular instability is characterised by low fusional reserves and an unstable heterophoria. The fusional reserves are usually low in both directions (divergent and convergent), so the fusional amplitude is lower than 20Δ, which is more than 1 standard deviation below normal ( Evans et al., 1994 ). The unstable heterophoria can be detected with a Maddox wing test, but is likely to be more significant if it is present with more naturalistic tests, such as the Mallett fixation disparity test when it will manifest as movement of the Nonius strips. A movement of the arrow in the Maddox wing test of ±1Δ is normal, but ±2Δ or more is abnormal. Binocular instability may be associated with suppression (possibly, transient) with the Mallett foveal suppression test.
Nearly 70 years ago, binocular instability was defined as ‘uncertainty in the collaboration of the vision of the two eyes’, ‘a kind of binocular anarchy’ ( Cantonnet & Filliozat, 1938 ). These authors said that the condition was common and often associated with symptoms of asthenopia, blurring, and reversals of letters and numbers. It could be diagnosed using an early precursor of the Maddox wing test (Cantonnet’s test of binocular vision), where there was an inability to maintain the arrowhead in a fixed position. Cantonnet stressed that a test for binocular instability should require precise focusing. He looked upon binocular instability as a different condition to the cases of heterophoria and strabismus that required treatment.
Gibson (1947) also considered binocular instability to be a separate entity from strabismus and symptomatic heterophoria. He said binocular instability often caused a maladjustment of letters and numbers, causing complaints of reversals and of the eyes jumping from one line to another when reading. He noted that the condition was sometimes associated with foveal suppression and with anisometropia, unequal acuities, and unequal accommodation. Gibson (1955) stated, binocular instability is often associated with low fusional reserves. He advocated the Turville Infinity Balance test, which was, in this context, a precursor of the Mallett foveal suppression test.
Giles (1960) considered that there were two types of binocular instability. The first, a ‘fusion deficiency’, was regarded as midway between heterophoria and strabismus. The second was caused by poor general health associated with neurosis, fatigue, debility, or toxaemia. In the latter type, symptoms may be much worse in the evening when the patient is tired.
Mallett (1964) noted that binocular instability was sometimes associated with decompensated heterophoria when there would usually be variation in the amount of prism or sphere required to eliminate a fixation disparity. He advocated his foveal suppression test for detecting suppression in binocular instability, noting that treatment involved correction of refractive error, alleviation of gross decompensated heterophoria, and antisuppression exercises.
More recently, fixation disparity techniques have investigated binocular instability under normal binocular viewing conditions. Jaschinski-Kruza and Schubert-Alshuth (1992) found a range of variability of fixation disparity in different subjects and Cooper et al. (1981) suggested that variability of fixation disparity might be a useful clinical measure. Duwaer (1983) found the stability of fixation disparity to be a useful predictor of symptoms.
Binocular instability can co-occur with dyslexia ( Evans et al., 1994 ) and the clinician should always consider this when examining children or adults who report difficulty with reading or spelling. This study found that about 15% of people with dyslexia had a fusional amplitude less than 20Δ, compared with 5% of good readers ( Evans, 1991 ). It seems likely that a test of vergence facility will also detect binocular instability in children with educational difficulties ( Dusek, Pierscionek, & McClelland, 2010 ; Quaid & Simpson, 2013 ).
It should be noted, however, that nearly all dyslexic people reverse letters and words, probably owing to problems in the higher cortical processes of decoding sequential material stored in short-term memory. Optometrists should not necessarily expect to correct reading difficulties, or even reversals, by treating binocular instability. However, in some cases treatment of binocular instability may help by reducing symptoms and, possibly, by improving the perception of text.
In those with symptoms of asthenopia or perceptual distortions, or in reading disabled children who may be too young to recognise these symptoms, it is advisable to carry out a fixation disparity test even if no movement has been seen on the cover test. When carrying out the fixation disparity test it is not enough to simply ask whether the strips appear to be aligned ( Karania & Evans, 2006b ). The patient should be asked whether one or both strips ever move ( Fig. 4.4 ). If the patient can discern the movement as being predominantly in one direction, the effect of prisms or spheres can be investigated in the usual way. If there is no aligning prism but there is binocular instability (a movement) on the fixation disparity test, this can be investigated further with the Maddox wing test and by measuring fusional reserves.
Diagnostic Occlusion and Investigative Occlusion
Since the 1920s, it has been suggested that prolonged occlusion (known as ‘Marlow occlusion’, or diagnostic occlusion) for up to 14 days can be useful in investigating asthenopic symptoms from binocular vision anomalies. It was originally thought that the increase in the deviation that occurred after this time was meaningful, but it is now known that most symptom-free patients show a large increase in horizontal and vertical heterophoria after occlusion ( Duke-Elder, 1973 ; Neikter, 1994a ). Diagnostic occlusion has been suggested for cases where symptoms suggest a hyperphoria but one cannot be found on normal testing ( Surdacki & Wick, 1991 ).
An alternative use of occlusion, ‘investigative occlusion’ can be helpful in rare cases where there are vague signs and symptoms of binocular instability or decompensated heterophoria. In a few cases it can be unclear whether the patient would benefit from treatment of the ocular motor problems, especially when the symptoms have another potential cause, such as sensory visual stress (pp. 64–66) or general fatigue. The patient can be asked to occlude one eye for the tasks when the symptoms occur and report whether this reduces symptoms. If it does help, treatment of the binocular instability is warranted; if not, another cause for the symptoms should be sought.
Caution is necessary, because it is conceivable that investigative occlusion could cause a decompensated heterophoria to break down into a strabismus. This is unlikely, however, and some studies suggest that even prolonged full-time occlusion does not adversely affect ocular motor function ( Holmes & Kaz, 1994 ; Neikter, 1994b ). Indeed, it has been suggested that occlusion can be used to treat sensory and motor factors in intermittent exotropia, reducing the frequency of strabismus ( Freeman & Isenberg, 1989 ; Jin & Son, 1991 ), possibly because it reduces amblyopia ( Santiago, Ing, Kushner, & Rosenbaum, 1999 ). A randomised controlled trial supports this view, with deterioration of intermittent exotropia in slightly fewer (0.6%) of those undertaking part-time occlusion (3 hours daily) compared with a control group ( PEDIG et al., 2014 ). However, even in the control (observation only) group, only 6.1% deteriorated.
Is Binocular Instability Different from Decompensated Heterophoria?
Decompensated heterophoria and binocular instability are contrasted in Table 5.1 . Binocular instability can be present in an orthophoric patient who, by definition, cannot have a decompensated heterophoria. If a patient is orthophoric, they only need negligible fusional reserves to satisfy Sheard’s criterion. To take an extreme example, an orthophoric patient with convergent and divergent reserves (to blur and break) of 3Δ and 2Δ respectively, will meet both Sheard’s and Percival’s criteria. A cover test will not detect any abnormality. In such a case, binocular instability may be detected as a movement of the Nonius strips during the fixation disparity test. The strips may move equally often in either direction, so there is an unstable fixation disparity without there being any aligning prism. Similarly, during the Maddox wing test the arrow may move over a large area, with the mean position orthophoria. Measurement of the fusional reserves would reveal them to be low, confirming the diagnosis of binocular instability.