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ARTICLE
Year : 1967  |  Volume : 15  |  Issue : 2  |  Page : 41-53

Adenwalla oration- heterophoria and convergence insufficiency


Medical College and M. Y. Hospital, Indore, India

Date of Web Publication18-Jan-2008

Correspondence Address:
B K Dhir
Medical College and M. Y. Hospital, Indore
India
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Source of Support: None, Conflict of Interest: None


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How to cite this article:
Dhir B K. Adenwalla oration- heterophoria and convergence insufficiency. Indian J Ophthalmol 1967;15:41-53

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Dhir B K. Adenwalla oration- heterophoria and convergence insufficiency. Indian J Ophthalmol [serial online] 1967 [cited 2020 Jul 10];15:41-53. Available from: http://www.ijo.in/text.asp?1967/15/2/41/38680

Table 13

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Table 11

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Table 10

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Table 1

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I am deeply conscious of the honour that you have done to me in inviting me to deliver this Adenwala Oration. I am most grateful to you for this, and in preparation of this oration, I have tried to deserve your confidence. An orthoptic clinic has been functioning since 1955, under my supervision, at the place of my work, i.e., the depart­ment of Ophthalmology, M.G.M. Me­dical College, Indore. During this pe­riod, several surveys and problems have been undertaken, and most of the subject matter of this oration is based upon the following material on:­[Table - 13]

In the preparation of this paper, I gratefully acknowledge the help ren­dered by Prof. L. P. Agarwal of New Delhi, Miss Seila Mayou of Moorfield Eye Hospital, London, Dr. Miss Ha­mida Said-ud-zafar of Aligarh and Dr. J. M. Pahwa and Dr. Miss S. Awasthi of Sitapur. I may be allowed to mourn­fully remember the late Mr. Adenwala, who died so young and in whose memory, this foundation of oration is established.

The advent of binocular vision in humans has been an event of funda­mental importance in the phylogene­tical order for the purpose of import­ing clear vision. The presence of bino­cular vision pre-supposes the harmo­nious working of the two eyes, not only in conjugate movements, but also in disjunctive movements, e.g., converg­ence. Several reflexes in the body e.g., postural reflexes are unconditional, i.e., they are predetermined. On the contrary, the binocular reflexes, being a later acquisition in phylogeny, are conditioned, i.e., they are developed post-natally. Being young phylogeneti­cally, they are very prone to give rise to anomalies, which may be manifest giving rise to heterotropia, or which may be latent and give rise to hetero­phoria and convergence insufficiency.

Before the consideration of hetero­phoria, I would like to pay great em­phasis on the consideration of two phenomena of accommodation and convergence, which have an intimate relationship in the causation and treat­ment of heterophoria ad convergence insufficiency. In an emmotropic person, these two are related to each other so that on seeing an object, a proportionate amount of accommoda­tion and convergence are exerted. But this relationship is flexible, so that for a fixed amount of accommodation, more or less than normal convergence can be exerted, and vice-versa. This is known as relative accommodation and convergence. The following figures based on 250 emmetropic persons of different age groups illustrate some salient features.

It shows that whereas the accom­modation falls rapidly with age, con­vergence shows very insignificant fall. Therefore, just as prebyopia occurs on account of decrease of accommodation, no such proportionate changes in con­vergence occur.

[Table - 2] shows the relative positive and negative accommodation at vari­ous age groups, i.e., at a fixed distance, how much more or less accommodation can be exerted. Both the negative and positive components are comparatively small.

This table shows the relative conver­gence. It illustrates that the positive component of relative convergence is fairly large, i.e., increasingly stronger prisms with base out can be put before the eye or eyes, without causing diplo­pia. It also shows that the relative convergence does not markedly de­crease with age.

These observations are important in the consideration of the exercises given in heterophoria or convergence insufficiency. The dissociation be­tween convergence and accommodation is brought about to make this relation­ship more elastic. The present day con­ception is that convergence can be easily moulded and not accommoda­tion. So exercises are based on creation of relative positive and negative con­vergence in relation to accommoda­tion.

Another factor, the importance of which has somehow not received suf­ficient attention, is the angle kappa, which is slightly positive in normal persons, i.e., the visual axis cuts the cornea on the nasal side of the optic axis. The presence of a large positive or negative angle kappa produces the appearance of an apparent divergent or convergent squint. In such an instance, the absence of true squint can be seen by the cover test, in which there is no deviation of the covered eye. In hetero­phoria, the degree of deviation is too small to be consistent with the appearance of the eyes.

[Table - 4] shows that the angle be­comes more positive as the hyper­metropia increases. [Table - 5] shows that in smaller amounts of myopia, the angle is still positive though less than normal. In higher degrees of myopia, it becomes negative.

Etiology of Heterophoria

1. Relationship with error of refrac­tion.

2. Anatomical causes like orbital asymmetry and abnormality in inter­pupillary distance etc.

3. Traumatic causes producing dis­placement of the visual axes.

4. Uniocular activity as in watch­makers.

5. Age factors.

1. A survey of 800 cases of error of refraction (504 mypia and 296 hyper­metropia) is given in [Table - 6],[Table - 7].

This shows that a large majority of myopia cases are associated with exo­phoria and especially exophoria for near. In hypermetropia, though a majority of cases are associated with esophoria, the number of exophoria cases is only a little less. Most of the exophoria or esophoria cases in hyper­metropia are for near.

Incidence of Heterophoria

"While orthophoria is a physiologi­cal ideal, approached after years of practice, heterphoria is the physical reality".

These words of Chavasse are very true with regard to the incidence of heterophoria. Various workers have given the incidence in the neighbourhood of 75% in all individuals. It varies a good deal, because different workers have taken different amount of deviations as orthophoria.

Very few references are available with regard to the incidence of hetero­phoria with symptoms, no matter what the deviation may be.

This gives the incidence of hetero­phoria with symptoms. This shows a percentage of 8.3 from amongst cases, which came for eye testing in the M. Y. Hospital, Indore, and 16.6 of cases, who complained of headache.

Hencell and Rober (1912) observed that heterophoria is more common upto 30 years. (53%), and less fre­quent after that age. Rober further noted that 70% of the phorias are re­lieved of their symptoms after the cor­rection of refractive error. Sheard (1920) found exophoria of 2° for dis­tance in 70% of normal individuals and 2.7° for near in 70% normal indi­viduals. Bannister (1920) observed hyperphoria of 2° in 7% of the normal healthy individuals. He also empha­sised the presence of spurious hyper­phoria, which disappears after pres­cription of correcting glasses or after correction of exo or esophoria.

Incidence of Convergence Insufficiency

Bannister (1912) emphasised that cases with convergence insufficiency may show perfect orthophoria, some degree of exophoria or even esophoria.

The exophoria is explained on the basis of spastic contraction of internal rectus to avoid heteronymous diplopia.

Dobson (1941) is of the opinion that all the lateral imbalances are the errors of convergence test. While Prognon (1961) believed that there exists separate mechanisms for con­vergence and divergence and thought that exophoria is due to dominant di­vergence mechanism.

Capron (1957) emphasised that con­vergence insufficiency is infact not a muscle imbalance but a marked dyna­mic weakness.

Ponmore (1957) observed that con­vergence insufficiency is more common in males than females (62% and 38% respectively). The incidence is highest in the age group of 19-30 yrs. (158%). It was noted in 52% of hypermetropia, 34% of myopia and 14% of emmeto­ropia.

Carmer et al (1957) noted that con­vergence insufficiency is the common­est of all the phorias and those with more marked findings are more com­fortable than those with moderate de­gree.

Martins (1957) observed that exer­cises to improve convergence are only helpful in young persons and prisms should be considered for those of more than 40-45 yrs. of age. If there is no exophoria for distance the prism should be added to the near correction glasses only. Surgery is reserved for those who have large exophoria for distance and those who are not relieved by prisms.

Kent and Steeva (1954) in a series of 4461 military personnel noted converg­ence insufficiency in 3.15% to 4.9% of the cases. Out of these, 60 were having headache, 49% blurring of vision, 34% ocular fatigue and 21% oc­casional diplopia. The benefit of orthoptic exercises was noted only in few cases. Bunan does not agree with this view and thinks that orthoptic ex­ercises are of definite value in these cases.

Convergence insufficiency was noted in variable percentages by various au­thors 8% by Darries (1956) 10% by Cushan (1956) and 25% by Kratka (1956), of the cases seen in private practice. The frequency depends mainly on the efficiency with which the cases are seen.

Symptoms of Heterophoria and Convergence Insufficiency

Carig (1963) feels that all symptoms produced by heterophoria reflect misuse or disuse of the faculty of vo­luntary convergence.

Symptoms in heterophoria do not depend upon the degree of hetero­phoria, but depend on the amount of fusional power available to compen­sate the beterophoria (Duke Elder). This factor is not only variable from person to person but from time to time in the same individual. He further emphasised that symptoms of hetero­phoria may serve as a barometer of physical and mental health of the patient.

Heterophoria may be even asympto­matic and may be present in normal individuals. In a series of 7070 cases Grieves and Archibald (1942) noted heterophoria in 53% of cases, of more than 4 prism diopters. Out of these only 1.3% were having symptoms. Adler emphasised that exophoria of 1-5 diopters and esophoria of 1-2 diopters is physiological, while hyper­phoria of only 1/2 diopter is physiolo­gical.

Kearney while describing various types of headache described that oc­cipital headache is the type of head­ache usually associated with hetero­phoria.

Van den Burg (1918) was of the opi­nion that ocular headache is mainly (in 70%) due to lower degree of eso­phoria.

H. H. Briggs (1920) observed that 25% to 33% of all the cases of asthe­nopia are due to anomalies of muscle balance, which cannot be cured by mere correction of refractive error. Of the symptoms complained of by the patients of heterophoria and converg­ence insufficiency, by far the majority had headache. Duke Elder emphasises that symptoms come on with fatigue and may even have periodic incidence appearing at the end of day's work or appear suddenly when work of great intensity is undertaken. He stated that most of the symptoms can be induced artificially by wearing of prisms by a normal person. He stated further that non-symptomatic heterophoria may be­come symptomatic in conditions of reduced vitality or increased strain. Headache due to convergence insuffi­ciency has definite periodic onset and is found only in a group of people, who have to use their eyes for near, while persons whose work does not demand the use of their eyes for near may go undetected even with gross deficiency of convergence. The other symptoms may be due to failure to maintain constant binocular vision, or due to defective stereoscopic vision or due to defective reception of postal sensations transmitted from the ocular muscles as a result of alteration of muscle tonus. Our observations on the various symptoms in heterophoria and convergence insufficiency are charted out in [Table - 9],[Table - 10].

Management of Convergence Insuffi­ciency and Heterophoria Cases

The following findings are based upon the work done in the Orthoptic section of the Dept. of Opthalmology, Medical College and Hospital, Indore. The figures are taken from the records of eight years from 1956. Only those cases are taken here, where the follow up could be managed for at least one year. Thus only 500 cases are consi­dered, although the attendance of such cases was much larger.


  Convergence Insufficiency Top


In the modern civilisation, when near work has assumed immense im­portance, it is but natural, that conver­ence becomes of great significance. Phylogenetically, it is a very young faculty, and hence is prone to great disorders, especially where needs of near work are highly exaggerated. Convergence is of two varieties, invo­luntary and voluntary. The involuntary one depends upon fixation and refixa­tion reflexes and its centre lies in the peristriate area of the occipital cortex. Voluntary convergence means that one has to converge the eyes without the help of the fixation object. Its cortical centre lies in the second frontal convo­lution. Clinically relative convergence has great significance. It is the amount that is exerted or relaxed, while the accommodation is kept constant. Nor­mally accommodation and convergence are intimately related, but while ac­commodation becomes less and less with increasing presbyopia, converg­ence remains almost the same.

It is frequently asked as to what is the criterion of convergence insuffi­ciency. It has been advocated that if the near point of convergence recedes more than a certain distance, say 9.5 cm, there is convergence insufficiency.

It is quite alright, if only absolute values are considered. But clinically, it is the relative convergence, which is of more importance. It is known on the synoptophore as the fusional am­plitude. It can also be measured by the patient fixing an object and the observer putting before his eyes in­creasingly stronger prisms base out till the patient gets diplopia. There is no doubt, that a marked receding of the near point of convergence will produce symptoms on near work, but it has been observed that a small variation of the receding of the near point is com­patible with normalcy. The measure­ment of the near point gives the idea of the absolute convergence, while the fusional amplitude gives assessment of the relative convergence. In a total of 160 cases of convergence insufficiency studied in this series, the near point varied between 12 cm to 36 cm, where­as the fusional amplitude was uniform­ly poor in all the cases.

A few observations in this series are of interest. Only 18% of the cases had voluntary convergence, others had either difficulty or inability to perform voluntary convergence.

With regard to association with error of refraction, 72% had emmetropia, 18% myopia and 10% hypermetropia.

The following table shows the asso­ciation of horizontal deviations with convergence insufficiency:-

This shows that majority of the cases are associated with exophoria for near. But 13% cases showed asso­ciation with esophoria, proving that convergence insufficiency can be as­sociated with esophoria as well.


  Treatment Top


It is taken for granted, that any error of refraction present has been corrected and any general disease like anemia has been treated. The princi­ples of treatment are to increase both the involuntary and voluntary conver­gence, which can be done by orthoptic exercises both at the clinic and at home.

In the clinic, increase of adduction on the synoptophore should be carried out to 35 to 40, first with the aid of concave glasses and then without them. In the later stages, voluntary converg­ence should also be accomplished on the synoptophore. Diploscope can be introduced say after the third exercise. All the positions should be exercised, so that patient learns to change posi­tions rapidly from the most converg­ent to the most divergent.

Home Exercises as they are ex­tremely important, not only in conver­gence insufficiency, but in different phorias as well, they are being taken in detail here and they will be only referred to, while describing the pho­rias. The principle is to bring about elastic dissociation between converg­ence and accommodation. Present day conception is that convergence can be easily moulded but not accommoda­tion. So exercises are meant for in­creasing positive and negative relative convergence in relation to fixed accom­modation. Usually gross objects are used, as they do not require much ac­commodation.

Pen exercise ask the patient to fix the nib of the pen. This should be brought nearer and nearer till he should appreciate diplopia. If he does not, he is using facultative suppression. In that case, use red and green glasses and a small torch. This exercise is very useful in any condition, in which near point is more than 10 cm.

Physiological diplopia. Through the window, the patient sees a distant pole as double (homonymous diplopia), while he is actually converging by look­ing at a pencil about ten inches away.

Stereograms. Now the distant ob­ject can be a smaller object like a stereogram kept at an arm's length. They may be:­-

i. simple objects like cats

ii. simple objects like tubes with stereoscopic effect

iii. objects which require accommo­dation.

Voluntary Convergence. To accom­plish this, the patient has to imagine to be looking at a near object like pencil, when actually he sees the dis­tant pole as double. Stereograms can be substituted for the pole.

Relaxation exercises should be start­ed from the very begining to avoid spasm of convergence. Simple exer­cise is looking at the distant pole and seeing the heteronymous diplopia of the pencil kept at about ten inches. These exercises can be carried out with the stereograms also.


  Heterophoria Top


Heterophoria may be defined as the condition wherein the eyes in their conjugate movements are maintained on the fixation point only under stress with the aid of corrective fusional re­flexes (Duke-Elder). In other words, it is the condition in which there exists a latent squint, which is counteracted and kept masked by the corrective fusional activity, once the reflex activity is avoided by means of the cover, the eye deviates and the squint be­comes manifest.

Slight amount of heterophoria is so common, that orthophoria is only a physiological ideal, while heterophoria is a physiological reality (Chavasse). Only when heterophoria is marked or is associated with symptoms, it be­comes pathological.

The following routine examination is done in every case of heterophoria:­-

1. Recording of visual acuity

2. Correction of error of refraction, determined by retinoscopy under cycloplegia

3. Maddox wing and Maddox rod test to assess the degree of hetero­phoria for near and distance

4. Testing the accommodation and convergence on the Livingston binocular gauge

5. Cover Test, the deviation of the eye under cover as well as the quickness or slowness of its re­turn on removal of the cover was noted in each case.

6. Range and grade of fusion were recorded on synoptophore.


  Treatment Top


ALL the cases were treated on the following lines of treatment:­-

A. Correction of error of refraction

B. Orthoptic exercises

C. Surgery or prescription of prisms


  Esophoria Top


Refraction

Hypermetropia should be fully cor­rected. If the patient does not tolerate the full correction, then under correc­tion for distance and full correction for near should be given.

In myopic persons, the error must be corrected and reading glasses for near should be advisable, even if the patient is not of the prebyopic age.

Any significant amount of hyper­phoria should be corrected by prisms.

Orthoptic Exercises

These should be tried. Several au­thors have stated that esophoria is not much benefited by orthoptics. It has been seen that the increase of fu­sional amplitude both for adduction and abduction has been followed with good results in a large number of cases. It can be helped by exercises with base in prisms and by bar reading and home exercises.

Surgery and Prisms

Relieving prisms, base out, are of little help in the treatment of eso­phoria.

Surgical relief is more effective in esophoria, especially for near. Reces­sion of internal rectus of one or more sides gives good results. Esophoria for distance requires resection of external rectus of one or both sides.


  Exophoria Top


In this the treatment is most satis­factory of all types of heterophorias, as it can be cured by improving con­vergence by orthoptic exercises.

Refraction

Hypermetropia and prebyopia should be under-corrected to encourage (but not to over strain) accommodation and convergence.

Any significant hyperphoria must be fully corrected.

Orthoptic Exercises

Orthoptic exercises to increase the fusional amplitude, first with and then without concave glasses should be given at the clinic and be augmented with the home exercises. Suppression, if present, should be treated.

Surgery and Prisms

Relieving prisms, base in, may be of considerable value for near work in presbyopic persons. The prescription should not correct more than half of exophoria by prism, so that the stimu­lus for convergence is maintained.

Surgery is rarely needed. Principle is to increase the efficiency of internal rectus by resection, tucking or ad­vancement.


  Hyperphoria Top


Eerror of refraction should be cor­rected.

Orthoptics has no place in its treat­ment, except in very small degrees.

Prisms are of great benefit when hyperphoria is static, i.e., due to some anatomical factor or is constant. Upto 10 prism diaptres can be prescribed, half on each side.

Surgery affords good results in higher degrees of static hyperphoria or most of the paretic or spastic hyper­phorias. If muscular actions are nor­mal, reinforcement operation (resection or tucking) of the inferior rectus of the hyperphoric eye or similar opera­tion on the superior rectus of the hypophoric eye, give very satisfactory results. If muscular actions are abnor­mal, the choice lies between reinfor­cing the paretic muscle or weakening the antagonistic muscles.


  Cyclophoria Top


It is characterised by a tendency of the eyes to rotate around their sagital axis. This rotation is held in check by the fusional impulses. According to deviation, cyclophoria is of two types, outward rotation of the upper poles of the eyes is termed ex-cyclophoria and inward rotation as in-cyclophoria. Etiologically cyclophoria is of two types:­-

Accommodational cyclophoria : It is also known as pseudo-cyclophoria. It is produced due to the effort to over­come refractive error and usually ceases with the correction of the refrac­tive error. It is mainly due to the presence of oblique astigmatism, which is overcome by the corrective overac­tion of the oblique muscles. This, in turn, gives rise to headache and other symptoms. The main muscles concern­ed are superior and inferior obliques.

Essential cyclophoria: This is due to anatomical or innervational abnorma­lities.


  Symptoms Top


Symptoms are more significant than in other heterophorias. How much of the cyclophoria can be corrected by the overaction of the muscles, without pro­ducing symptoms, varies from indivi­dual to individual. General symptoms like headache, nervous upset, nausea or even vomiting may be produced. Opti­cal symptoms may be marked. While walking about, floor appears tilted, houses on either side of the road ap­pear to lean over or fall upon the un­fortunate patient, right angles appear acute or obtuse, letters in book are dis­torted and tables tipped. Near work is more difficult, as torsion becomes more accentuated on convergence.

Diagnosis can be made on testing by Maddox double prism. Cyclophoria can also be detected and measured roughly on the Maddox Wing. But accurate measurement in degrees can be made on synoptophore.

Treatment

1. Correction of ametropia usually results in cure in the accommoda­tional type of cyclophoria. Asso­ciated hyperphoria should first be corrected by prisms. It helps in reduction of the amount of cyclo­phoria.

2. Orthoptic exercises can be carried out to improve cycloduction by torsional exercises on the synop­tophore. It may be useful in small degrees of cyclophoria.

3. Surgery is the only effective treat­ment in the high degrees of essen­tial cyclophoria. The following guiding principles may be help­ful:-­

a. surgery on the vertical recti should be performed on un­complicated cases of cases as­sociated with hyperphoria.

b. surgery on the horizontal recti is also performed, if there is associated esophoria or exo­phoria.

c. surgery on the obliques of the same or opposite side can be performed in all cases.

d, surgery should be done as a last resort.


  Heterophoria and Aviation Top


According to T. G. Jones (1962), the limits of heterophoria in India Air Force are 6 prism D of eso and exo­phoria and 1 prism D of hyperphoria at 6 meters distance, and 6 prism D of eso and 16 of exophoria and 1 prism of hyperphoria at 30 cm. The main object of these limitations is to ensure against decompensation during flying stress. Hence the importance of rou­tine muscle tests, which evaluate the fusion sense. He also emphasised the importance of convergence in evaluat­ing fusion sense. The difference be­tween subjective and objective conver­gence is inversely proportionate to fu­sion, i.e. smaller the difference, better the fusion. If the difference is more, especially if the objective convergence is more, it indicates poor fusion.

He concludes that to land high speed aircraft, a very fine degree of depth perception is required. This ability bears no relation with heterophoria, and can be improved with training. The most important monocular cues in flight and landing are motion parallax, retinal image and linear perspective. The binocular cues only operate at limited distances only, but good bino­cular vision is necessary to provide a potentially wider visual field for target detection and for visual tasks within the cockpit. Therefore the main ob­ject of muscle balance examination of the aircrew should be to establish bino­cularity with good fusion to ensure against decompensation in the face of flying stress. If proper importance is given to tests like Bishop-Harman test, cover test and tests for subjective and objective convergence, the require­ment of modern flying will be fully met.

Effect of fatigue and anoxia upon heterophoria

Weldon pointed out that ocular muscle balance is affected by anoxia in most cases. Beyne confirmed the data published upto 1933, stating that heterophoria tends to become hetero­tropia under conditions of anoxia. Borges Diaz reported increased hetero­phoria resulting from fatigue. As fatigue and anoxia cause adverse effect upon muscle balance, earlier workers in this field felt to enforce some res­trictions for muscle balance tests, but due to new flying techniques and rou­tine carrying of oxygen for a flight above 10,000 feet, these limitations have become unnecessary.

Relationship between heterophoria, Fusion and Stereopsis to Flying Performance

Elliot (1942) noted that failure in landing was due to air sickness or some other causes. He could not find any relationship between landing ability and heterophoria. He further could not find any substantiation for the statement made by Clement and Livingston that "while landing, exo­phorics tend to level off too high and esophorics land to fly into the ground."

Adawson (1942) also could find no relationship between flying perform­ance and depth perception.

It was found that with Bishop-Har­man diaphragm test or with near point convergence test, there was no relation­ship between the degree of hetero­phoria or fusion with flying success.

Nicholls from a large study sum­marized that flying performance show­ed no relation to fusion or stereopsis. By and large heterophoria apparently did not affect flying performance, though in a minority of those with convergence insufficiency and hyper­phoria greater that one prism dioptre for near, these faults had an adverse effect. He further, in assessing the va­lue of orthoptics in pilots, said that it gives very small success, but it may be of value in an experienced pilot, who develops eye strain later on.

In respect of depth perception, he divided pilots into four groups as fol­lows:­--

Group I

Those with good muscle balance, who are good landers. In this group there is good co-ordination of ocular muscles and thus good depth percep­tion is possible. This is the ideal sought for the present visual standards.

Group II

Those with good muscle balance, but who are poor landers, it is pre­sumed that they can have some degree of binocular depth perception and yet they are poor landers. Hence the dif­ficulty in such cases is in the inter­pretation of the brain of the informa­tion obtained from the eyes. In such cases judgment is affected by fear, tensiveness, low intelligence level, air sickness and a number of other fac­tors.

Group III

Those with poor ocular muscle ba­lance but who are good landers. In this group, the persons may have poor binocular vision possibly due to poor co-ordination of eyes. Good landing in these cases is possible due to accurate monocular depth perception, based upon size, motion, parallax and pers­pective.

Group IV

Those with poor ocular muscle ba­lance and who are poor landers.[18]

 
  References Top

1.
Banister, J. M., (1920), Am. J. Ophth. 3: 878.  Back to cited text no. 1
    
2.
Chavasse, B., (1939), Worth & Chavass's Squint 9th Ed. Balliere Tindall & Co., London, p. 73.  Back to cited text no. 2
    
3.
Cridland Nigel, (1941), Brit. J. Ophth. 25: 141.  Back to cited text no. 3
    
4.
Cushman, B. and Burri, C., (1942), Am. J. Ophth. 4: 1944, 1941.  Back to cited text no. 4
    
5.
Cushman, B., (1944), Am. J. Ophth. 27:75.  Back to cited text no. 5
    
6.
Dhir, B. K., (1961), J. All Ind. Ophth. Soc. 9: 33.  Back to cited text no. 6
    
7.
Dobson, M., (1941), Brit. J. Ophth. 25: 66.  Back to cited text no. 7
    
8.
Duane, A., (1933), Binocular Move­ments Arch. Ophth. 9: 579.  Back to cited text no. 8
    
9.
Duke Elder. S. W., (1938), Text Book of Ophth. Vol. IV. St. Louis 1949 The C. V. Mosby Co., p. 3952.  Back to cited text no. 9
    
10.
Foster, C. B.. (1950), Am. J. Ophth 33: 773.  Back to cited text no. 10
    
11.
Jackson, S. R. S., (1960), Trans. Ophth. Soc. U. K. 80: 49.  Back to cited text no. 11
    
12.
Khosla, P. K., Bhatia, R. K., Agarwal, L. P. (1964), Orient Arch. Ophth. 2: 114.  Back to cited text no. 12
    
13.
Mann Ida, (1940), Brit. J. Ophth. 24: 373.  Back to cited text no. 13
    
14.
(1960), Brit. Med. J. 1: 208.  Back to cited text no. 14
    
15.
Stutterhein, N. A., (1938), Am. J. Ophth. 21: 77.  Back to cited text no. 15
    
16.
-, (1942), Brit. J. Ophth. 26: 216.  Back to cited text no. 16
    
17.
White, J. W. and Brown, H. W., (1939), Arch. Ophth. 21: 999.  Back to cited text no. 17
    
18.
Williamson Noble, F. A., (1941), Pro­blems of Asthenopia, Brit. J. Ophth. 25: 397.  Back to cited text no. 18
    



 
 
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