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Outline notes to accompany City University 2^nd year undergraduate Clinical Practice course lecture

Students are recommended to Evans BJW (1997) Pickwell’s Binocular Vision Anomalies. Investigation & Treatment. 3^rd Edition, Butterworth-Heinemann, Oxford.

The near point of convergence (NPC) of the eyes can be investigated by placing a fixation object at 30 to 40 cm in the mid-plane of the patient’s head. The patient asked to maintain fixation on the object as it is brought towards the patient until one of the eyes loses fixation and dissociates from the fellow eye (von Noorden, 1990). This has also been termed the push-up test and tests a combination of both reflex and voluntary convergence (Mallett, 1988). It has also been termed as tonic convergence (Daum, 1983b). According to von Noorden (1990), a normal NPC should be 8 - 10 cm with anything more remote than this being ‘defective’ or ‘remote’. The push up test can also be carried out subjectively with the practitioner asking the patient to report when the target is first seen in diplopia. The target should be a bold vertical line (Mallett, 1988).

Jump convergence (phasic or step convergence) is tested by asking the patient to change fixation from a distant target to a near target positioned in the mid-plane 15 cm from the patient’s head. The position of the near target can be varied to determine the near point (Rashbass & Westheimer, 1961; Pickwell & Stevens, 1975). This test is almost entirely voluntary in nature. The practitioner may observe either a smooth convergence of both eyes from distance to near or a variety of abnormal responses including a slow or hesitant movement; an over-convergence; versional movements followed by convergence; or no movement of either eye or movement of one eye only (Pickwell & Hampshire, 1981).

Whilst it is not possible to isolate completely reflex convergence, the Capobianco test, which employs a deep red filter placed before one eye to produce ‘partial dissociation’, is purported to provide a measure of reflex convergence with little or no voluntary input (Capobianco, 1952).

There are a number of ways of classifying vergence anomalies (von Noorden, 1990; Evans, 1997). A useful, but simplistic method is to classify anomalies according to the fixation distance at which they are manifest. The earliest description of this type is the Duane-White classification of divergence and convergence malfunction (Borisch, 1970). This classification applies to strabismus rather than heterophoria, and therefore, when used in the context of heterophoria tends to imply that heterophoria is a form of latent strabismus. Tait (1951) modified the original classification to incorporate the concept of accommodative convergence involvement and his concepts are included in the discussion to follow.

Divergence insufficiency (DI) describes the condition where the patient has a decompensated esophoria for distance vision with either no or a lesser amplitude of well compensated esophoria at near (Evans, 1997). Tait (1951) further categorised DI as either primary or secondary, and their characteristics are described in Table 1. Both types may be associated with convergence excess.

The term divergence excess (DE) describes a decompensated exophoria for distance, with any exophoria at near being of less amplitude and well compensated (Evans, 1997). Tait (1951) described this as a marked exophoria at far with equal or less exophoria at near. If the NPC is normal this may be described as primary DE and if deficient, it may be described as secondary DE.

Convergence excess (CE) describes a decompensated esophoria at near with any esophoria for distance being of less amplitude and well compensated (Evans, 1997). Tait (1951) defines CE in similar terms, there being orthophoria or moderate esophoria for distance and more marked esophoria for near, with the cause being an excessive ACA ratio.

A common cause of CE is a high AC/A ratio and/or uncorrected hypermetropia e.g., the classical Donder’s type squint. Another cause can be of a psychosomatic nature.

Symptoms may be particularly associated with close work and may include frontal headache, ocular fatigue, blurred vision, inertia of accommodation (see notes on Anomalies of Accommodation).

Investigation may include a cycloplegic refraction, determination of AC/A ratio

This is very rare and may be associated with ciliary spasm. Can be psychosomatically induced. Refer any new cases.

Convergence cannot be elicited even with a prism. Causes include 3^rd N anomalies, head trauma, Parinaud’s syndrome (convergence paralysis + vertical gaze palsy).

All new cases require referral for neuro-ophthalmological opinion.

Convergence insufficiency (CI) is a condition in which the patient has an inability to sustain sufficient convergence for comfortable near vision (Evans, 1997). Despite this simple definition, CI may probably be more correctly described as a syndrome. Although CI is frequently associated with convergence weakness exophoria, which presents as a decompensated exophoria for near with any distance exophoria being both well compensated and of less amplitude than for near (Evans, 1997), von Noorden (1990) pointed out that CI can occur in the presence of esophoria at near.

Whilst the categories above may be useful for descriptive purposes, in practise clinical signs are not always clear cut and Evans (1997) also defines basic (or mixed) esophoria and exophoria in which the amplitude of heterophoria does not differ from distance to near.

CI is one of the most common causes of ocular discomfort. Von Noorden (1990) suggested that it is the most common cause of muscular asthenopia and that it frequently has an aetiological connection with accommodative difficulties.

von Graefe (1855), (cited by von Noorden, 1990), described symptoms arising from CI. Mentions of CI can also be found in textbooks published towards the turn of the 18^th century (Berry, 1893; Clarke, 1893). Duane (1897) subsequently provided a clinical description of CI.

The description of Rouse et al (1998) further developed that of Duane (1897) and Tait (1951). For far there is either orthophoria or a slight (~ 2 to 4 D ) exophoria, normal versions, frequently subnormal abduction (~ 8 to 10 D and not more than ~ 15D ), with prism induced convergence often decreased to ~ 14 to 20 D or less. For near vision there is a marked exophoria of 12D or greater, normal versions, and a NPC of 7.5 cm or greater.

Daum (1986a), using a criterion of a larger exo deviation for near compared to distance, reported other correlates which essentially concur with Duane’s group of diagnostic signs. However, despite these precise diagnostic descriptions, and the observation that most patients do exhibit exophoria at near, the disorder can occur in the presence of orthophoria or even esophoria (von Noorden, 1990). It is not clear whether Duane required asthenopia to be present to make a diagnosis of CI and studies have suggested that the presence of symptoms is not essential for a diagnosis of CI (Capobianco,1952; Cooper & Duckman, 1978). However, it is of interest to note that the clinicians that presented the classic descriptions of CI were not in a position at that time to differentiate between well compensated and poorly compensated heterophoria. Hence the definitions of Evans (1997) described above add a useful dimension to categorisation of vergence anomalies.

Some authors have defined CI in terms of a single sign such as a remote NPC. For example, Letourneau et al (1979) diagnosed CI when the NPC was more remote than 10 cm when measured with a penlight. Pickwell & Stevens (1975) made an initial diagnosis of CI if a standard NPC measurement was remote or jump convergence was inadequate. Others such as Letourneau & Ducic (1988) and Scheiman et al (1996) used more than one sign to diagnose CI. In a most comprehensive review, Daum (1988) analysed 58 published papers and noted considerable variation in the criteria used to define CI. He found that symptoms and decreased positive fusional reserves at near were the only criteria named in more than one half of the studies reviewed. An extended NPC and an exophoria that was larger for near were criteria in about one-third of the papers.

A review of the literature by Rouse et al (1998) reported that estimates of the prevalence of CI vary between 1.75 to 33.0% (Norn, 1966; Dwyer, 1992). This variability may be attributed to variations in the definition of CI and to differences between samples. Table 2 is adapted from Rouse et al (1998) and summarises some of the studies of CI reported in the literature.

In a study of 11,600 subjects, White & Brown (1939) suggested a 7.5% prevalence of CI. Although they used Duane’s criteria for diagnosing CI, they failed to provide a population description.

Kratka & Kratka (1956) examined 500 patients and found a prevalence of 25% (n = 125) manifesting at least one finding of CI and 12.5% exhibiting exophoria at near, remote NPC, and reduced positive fusional reserves. They found that 75% of the 125 were symptomatic and reported that in the asymptomatic patients, the large exophoria and remote NPC were accompanied by excellent positive fusional reserves.

Using the criterion of an NPC of > 9 cm, Norn (1966) found a prevalence of CI of 1.75% in a population of 10,022 aged from 6- to 70- years.

Mahto (1972) reported that 11% of his 310 patients under the age of 40 showed an NPC greater than 10cm. This study must be regarded with great caution as the target used to assess convergence was the examiner’s fingertip.

Using either a remote NPC (break > 10 cm) or a poor jump convergence as diagnostic signs, Pickwell & Stephens (1975) reported a CI prevalence of 36% in 200 consecutive patients aged 8- to 83- years. The prevalence of patients presenting solely with a remote NPC was 12%.

Letourneau et al (1979) examined 735 children aged 7- to 14-year-old. Using the single criterion of an NPC > 10 cm, they found that 8.3% of children

had CI. Letourneau & Ducic (1988) subsequently assessed 2054 children aged 6- to 13- years and found that 2.3% manifested at least two signs of CI.

Pickwell & Hampshire (1981) assessed 505 consecutive patients presenting to an optometric practice. 50 of these were excluded from the study because of manifest strabismus or active pathology. NPC was measured using a line on a black card mounted on a near point rule to obtain the precise distance. In addition, jump convergence movements were observed whilst the patient changed fixation from 6 m to 15 cm. Of the 455 patients included in the study, 110 (24.2%) showed poor convergence by one or both methods. 20% showed an abnormal jump-convergence response. Further analysis of the prevalence of various jump convergence responses is shown in Table 3.

Dwyer (1992) used Sheard’s criterion and characteristics of fixation disparity curves to diagnose CI and reported a prevalence of 33% of 144 consecutive patients, aged 7- to 18- years, presenting to his optometry practice. It should be noted that such a sample is likely to be highly selective.

Scheiman et al (1996) reported the prevalence of CI to be 5.3% of 1650 consecutive patients’ aged 6 to 18-years seen in an optometry clinic. The criterion used for diagnosis was based on the presence of multiple clinical signs.

Rouse et al (1998) examined the prevalence of convergence insufficiency amongst 8 to 12-year-old children in optometry clinic settings. They found that 17.6% of the children were either definite or high suspect CI according to their criteria that are shown in Table 4.

In a retrospective study of 179 patients, aged from 2 to 56-years, with binocular visual dysfunction and an exo-deviation for distance, near or both, Daum (1986a) categorised the anomalies into three classes namely (a) CI, (b) equal exo-deviation at distance and near and ( c) divergence excess. The patients were classified according to the relation between the distance and near angles of deviation. CI was found to be the most prevalent condition. This group was notable in that the amplitude of accommodation was reduced by about 3.0D below the expected average value. In addition, Daum reported that CI occurred primarily in females. It was also more prevalent in young patients and was correlated with reported headaches, blur, and asthenopia. Other associated signs were a remote NPC and poor positive vergences for near. These subjects exhibited good stereo-thresholds.

As has already been observed, patients with CI may not necessarily suffer symptoms. von Noorden (1990) observes that " to prevent asthenopic symptoms and diplopia, human have a built-in mechanism - suppression". He suggests that suppression is most active for patients with heterophoria and that it is for this reason that heterophoric patients "rarely complain" of symptoms.

While von Graefe supposed that CI was myogenic (von Noorden, 1990), there is anecdotal evidence of a psychological, or psychosomatic input as a causative factor in some cases (Taylor, 1990). Knapp (1958) suggested that if asthenopic symptoms are of recent onset, a possible psychological cause should be investigated and mention is made of a psychiatric intervention if necessary.

Hugonnier & Clayette-Hugonnier (1969) classified the aetiology of CI into five categories:

1. Anatomical (e.g. large inter-pupillary distance) or paretic origin (e.g.

myasthenia)

(b) Retardation of development

(c ) Ocular causes. A "dyssynergy" of accommodation and convergence in

uncorrected myopia, anisometropia, very high hypermetropia, high

4. General physical causes. These include "intoxications" and diseases of

endocrine gland (e.g. Moebius’ sign in thyroid ophthalmopathy).

5. Psychological causes. These have been alluded to above and may

include anxiety and neurosis.

Lyle & Bridgeman (1959) suggest there are seven types of CI

1. primary

2. secondary to primary divergence excess

3. secondary to vertical muscle imbalance

4. refractive

5. associated with general debility or stress

6 presbyopic

7. post-operative, for example, secondary to a tenotomy of the medial rectus muscle

More specific aetiologies have also been reported including an association with "whiplash injuries" caused by road traffic accidents (Anderson, 1961), anaemia (Manson, 1962) and head trauma (Carroll & Seaber, 1974).

The use of prisms may be considered when orthoptic treatment is inappropriate because of ill-health, or due to lack of time or incentive on the part of the patient. There are a variety of ways of determining the amplitude of prism. A common method of prescribing in the UK is to determine the amount of prism required to eliminate a fixation disparity. The minimum prism required to neutralise the fixation disparity may be prescribed (Mallett, 1966).

Evans (1997) suggested an objective method of determining the prism power whereby the practitioner determines the power of the weakest prism required to produce a quick and smooth recovery following the cover test. This method presupposes that a poorly compensated heterophoria does produce a slow and jerky recovery during the cover test. This supposition is based on anecdotal evidence and will be investigated in this thesis.

Prisms may also be used in the treatment of divergent strabismus (Ravault et al, 1968; Berard, 1968). Ravault et al suggested that the elimination of suppression is a major effect of constantly wearing a prismatic correction in those patients for whom prismatic treatment eliminates a manifest deviation.

The relief of vertical heterophoria with prism has been reported to aid compensation of exophoria (London & Wick, 1987).

Prism adaptation commonly occurs in patients with normal binocular vision. However, most patients with binocular vision anomalies that are causing symptoms exhibit abnormal prism adaptation (North & Henson, 1981). Whilst this may be an indication for prescribing prisms, North & Henson (1982; 1992) showed that some patients who had abnormal adaptation to prisms before receiving treatment had normal prism adaptation after treatment.

Winn (1994) demonstrated that prism adaptation reduces with age and suggested that this may at least partly explain a claim that older patients do not respond as well as younger patients to orthoptic exercises.

Consideration should be made to whether correcting a patient’s ametropia or anisometropia may facilitate an improvement in binocular function. Blurring of one or both eyes may precipitate poorer compensation of a heterophoria. Whilst Ukwade & Bedell (1993) found that small degrees of blur and reduction of contrast would have inconsequential effects on fixation ability or vergence, Dwyer & Wick (1995) found that even correcting low degrees of ametropia can dramatically improve vergence and accommodative function for some patients.

Modifying the spectacle prescription may alleviate oculomotor stress. This effectively employs the relationship between accommodation and accommodative vergence to either increase or decrease convergence. For a patient with a poorly compensated esophoria at near or a convergence excess, an additional positive lens may be used to reduce accommodative demand and thus reduce accommodative convergence. This will have the effect of reducing the amplitude of an esophoria. An additional positive lens will cause blurring of distance vision and so a bifocal type lens is often employed. The addition in the near segment provides the patient with the benefit of a reduced accommodative requirement for near vision. Similarly, incorporating a binocular negative lens addition (Caltrider & Jampolsky, 1983) may alleviate a poorly compensated exophoria. This will produce an additional accommodative demand and so stimulate accommodative vergence. The fact that this technique requires additional accommodation means that it may only be used for patients with adequate accommodation. In young patients this technique may be employed to help control associated exophoria at distance and near as well as divergence excess and convergence insufficiency.

It should be noted that patients may adapt to spherical lenses just as they can adapt to prisms (North & Henson, 1985). Patients should therefore be allowed to wear the proposed prescription for 3-5 minutes and the ocular motor balance re-assessed to ensure that adaptation has not taken place (Evans, 1997).

Dwyer (1992) assessed the refraction, vergence, and accommodative status of 144 consecutive patients and found that 39% presented with ametropia. Of these, nearly 80% manifested a vergence and/or accommodative anomaly. Dwyer & Wick (1995) recommended that refractive correction should be used for such patients and suggested there was a possibility that this type of intervention will cause passive resolution of the binocular disorder.

In the UK the term ‘orthoptics’ has been used to describe eye exercises to treat ocular motor disorders such as vergence anomalies. Orthoptics is a term used both by the ‘optometric profession’ and by ‘hospital orthoptists’. In the USA the term ‘orthoptics’ tends to describe treatment offered by the separate orthoptist profession and ‘vision therapy’ (or ‘visual therapy’) is used to describe exercise regimes prescribed by optometrists. For most exercise regimes the two terms probably describe very similar treatment philosophies. For the purpose of this discussion the two terms will be used interchangeably to allow for the various usage’s in the international literature.

The term ‘orthoptics’ is derived from the Greek or o s meaning "straight", and op t i k o z meaning "pertaining to sight". Orthoptics has been used for many centuries to treat ocular motor disorders. Flick (1937) (cited by Giles, 1943) traced a reference to treatment of strabismus using a mask as far back as the Ebers papyrus dated about 1650 BCE. Flick suggested that the first orthoptic treatment was carried out by Aeginets (AD 625-690).

The types of vision training or orthoptic exercises are very wide and varied. Some of the most commonly prescribed exercises for convergence insufficiency and exophoric conditions are "push-up", "jump convergence" and "physiological diplopia techniques".

The patient is asked to look at a fine tip of a pencil placed at about 50 cm or well outside the range of NPC. The pencil is then moved towards the eyes until it appears to double, or the practitioner observes that one eye has ceased to converge (Evans, 1997). The exercise is repeated a number of times with the aim of developing convergence and reducing the NPC. If the patient does not appreciate diplopia then the practitioner should assist the patient to develop awareness of physiological diplopia before proceeding. Once the patient understands the exercise, instructions can be given for the patient to carry out the exercises at home on a daily basis.

The patient moves a small, detailed target in as close as possible before it becomes blurred, double, or one eye diverges. The target is held stationary and the patient relaxes accommodation and convergence to view a distance target. Fixation is then changed from the distance to the near target. The patient should be aware of physiological diplopia (Evans, 1997). This cycle is repeated a prescribed number of times.

There are a number of variations of this category of exercise. For example, the patient can be instructed to hold two different colour pencils at different distances on the median line against a plain background. The patient is instructed to look at the further pencil that should be seen singly. The nearer pencil should appear double. The patient is then instructed to view the near target that should then be single. The further target should then be seen in diplopia. This procedure is then repeated with the pencils being brought closer to the patient.

Another variation includes the use of a string on which a number of small beads have been threaded. The patient is instructed to hold the string so that one end is touching the tip of the nose and the other end is at arm's length. The patient fixates on the farthest bead that should be seen singly. The patient should also perceive the string and the remainder of the beads in diplopia with the separation of the two images becoming greater as the string gets closer to the nose. The patient then re-fixates to the next bead. Now this one should be seen singly, with the furthest bead and the nearer beads all being seen in diplopia. The diplopic images of the string should appear to cross through the single bead. The patient is instructed to change fixation and advance from bead to bead until the nearest bead is seen singly.

A similar technique uses a card on which a line and a series of dots have been drawn as an alternative to the string and the beads.

Development of fusional reserves may be carried out using haploscopic instruments or with vectographs (Rosner & Rosner, 1990; Evans, 1997). The Institute Free Space exercises (Evans, 1998) are specifically designed for patients to carry out at home and may be obtained from the Institute of Optometry, London.

Afandor (1982), Goldrich (1982), Scheiman et al (1983) and Letourneau & Giroux (1984) have all reported success in treating large exo-deviations using oculomotor biofeedback therapy. Computerised vergence therapy has also been employed in the treatment of oculomotor disorders (Sommers et al, 1984; Daum et al, 1987).

Orthoptic exercises have been employed as a conventional or "orthodox" method of treatment for many years. It is widely accepted that fusional reserve training exercises can be an effective method of treating positive fusional reserve dysfunction such as convergence insufficiency (Davies, 1956; Kratka & Kratka, 1956; Passmore & MacLean, 1957; Norn, 1966; Wick, 1977; Cooper & Duckman, 1978; Kertesz, 1982; Daum,1982; Pantano, 1982; Cooper et al, 1983; Cohen & Soden, 1984; Daum, 1984a; Daum, 1984b). Other studies purport to show that patients receiving vergence training are effectively relieved of difficulties associated with other oculomotor anomalies (Ludlam & Kleinman, 1965; Sanfillipo & Clahane, 1970; Hoffman, 1973).

Goodson & Rahe (1981) studied the effects of vision training on various optometric parameters, including vergence ranges, in a group of US Air Force pilots with normal binocular vision. There was a control group who did not undergo any vision training and the study was single (practitioner) masked but there was no placebo control in operation for the treated group. They reported significant (p < 0.05) increases in the trained group for base-out vergence to blur and break and base in vergence to recovery.

Daum (1982) studied the time course and the magnitude of changes in the horizontal vergence system of subjects with normal binocularity following regimes of commonly used vergence training techniques. Thirty-five healthy adults aged 22 to 28 years underwent a full examination of their refractive, binocular and oculomotor systems. They all then underwent visual training which consisted of two sessions (totalling 4 min/day) of "push up" exercises, flip prism or jump vergence exercises (2 min), variable vectograph exercises (2 min) and vergence training on a major amblyoscope (2 min). All but the latter exercise were carried out at home and for five days a week for three weeks. Analysis of the results showed positive fusional ranges to be significantly increased after 1week of training with an even greater effect measurable after three weeks. Six months after training ceased, the effect of the exercises was still apparent but the vergence facility was found to have decreased. Whilst negative fusional ranges were shown to have increased, this training was less effective.

The long-term effect of exercises reported in normal subjects (Daum, 1982) has also been demonstrated for CI by Pantano (1982) whose study suggested a long-term cure for convergence insufficiency. Two hundred and seven patients with CI were compared on the basis of their success category immediately after training. Those that had been released as "cured" maintained the same result after six months and two years. Of those patients in the "partially cured" group, 79% remained asymptomatic after six months, but only 11% were asymptomatic after two years. The "failure" group received no symptomatic relief and even the improved convergence skills were not maintained six months post-therapy. Only those patients who were able to achieve both voluntary and fusional convergence and who had learned to relax accommodation adequately while converging so that they had altered their AC/A ratio could maintain their status over time.

Daum (1983b) compared the effectiveness of step and jump (phasic) vergence training with sliding and push-up (tonic) vergence procedures in a group of thirty-four healthy asymptomatic adults who were divided randomly into two groups. One group carried out phasic vergence training and the other group tonic vergence training. Whilst both groups made substantial gains in convergence and divergence ranges, the phasic group showed the greatest improvement. It is worth noting that Pickwell & Stephens (1975) suggested that jump (phasic) convergence may be of more clinical significance than a poor NPC and Pickwell & Hampshire (1981) demonstrated a stronger association between symptoms and poor jump convergence than with poor NPC.

Daum (1982) and Vaegan (1979) have also demonstrated the efficacy of repeated, short training periods.

However, there are surprisingly few studies that look critically at the efficacy of the various techniques. There are even less that utilise double masked placebo controlled trials such as employed by Daum (1986c) and the matched-subjects control group crossover design used to reduce placebo effects in the study described by Cooper et al (1983).

Cooper et al (1983) used a matched-subjects control group crossover design in a study of the effects of automated fusional reserve convergence training on a group of seven patients with CI. All patients showed significant increases in vergence ranges with concurrent marked reduction of symptoms after training. Their results clearly demonstrated the effectiveness of fusional reserve training in reducing asthenopia in these patients. Subsequent accommodation and vergence training using traditional orthoptic procedures yielded further reduction of asthenopia, as well as an increase in the positive fusional ranges.

Daum (1986c) conducted a double-blind placebo-controlled study to investigate the effects of positive fusional reserve training. Four experimental protocols were selected and five subjects were assigned randomly to each of the four experimental groups. Each subject in each group spent a total of 120 minutes over a three week period doing fusional reserve training. Group A trained in twelve 10-minute sessions; Group B in six 20-minute sessions; Group D was the control group and carried out version rather than vergence exercises. All of the training sessions took place within the optometry clinic and consisted of positive fusional vergence training on the synoptophore. Each of the test groups showed increases in their positive fusional vergence ability at both distance and near with the group undertaking the shorter sessions demonstrating the largest overall increases. This group was also the only group to show significant increases in positive blur findings and negative fusional reserves. The control group did not show a significant change in vergence amplitudes. It is worth noting that whilst this study suggested that it is possible to increase the positive fusional vergences it did not aim to show that increasing positive fusional vergence is effective in dealing with the asthenopic symptoms produced by vergence dysfunction.

Although younger patients may tend to respond more positively to such exercises than older patients (Winn et al, 1994), Wick (1977), using well defined and rigorous criteria for success, reported a 93% cure rate following the treatment of 191 presbyopes aged from 45 to 89, having asthenopic symptoms associated with convergence insufficiency, near point exophoria, and/or fusional vergence deficiencies. Similar results have been obtained for adults over the age of 60 years with convergence insufficiency (Cohen & Soden, 1984). This suggests that practitioners should not rely on patients’ ages in determining whether orthoptic exercises should be prescribed. Indeed, Grisham (1988) suggested that because of their maturity, older patients might demonstrate the self-discipline necessary to follow through with visual therapy.

Surgery may be required for large angle deviations, as well as various incomitant anomalies that have not or are unlikely to respond to other treatment methods. A fuller description of these procedures is outside of the scope of this thesis and the reader is recommended to von Noorden (1990) and Coffey et al (1992).

Contact Author: e-mail - sb@eye-spy.co.uk