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| ARTICLE |
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| Year : 1967 | Volume
: 15
| Issue : 2 | Page : 41-53 |
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Adenwalla oration- heterophoria and convergence insufficiency
BK Dhir
Medical College and M. Y. Hospital, Indore, India
| Date of Web Publication | 18-Jan-2008 |
Correspondence Address:
B K Dhir
Medical College and M. Y. Hospital, Indore
India
 Source of Support: None, Conflict of Interest: None  | Check |
How to cite this article:
Dhir B K. Adenwalla oration- heterophoria and convergence insufficiency. Indian J Ophthalmol 1967;15:41-53 |
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 | |  |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
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 | | |
| 1. | Banister, J. M., (1920), Am. J. Ophth. 3: 878.  |
| 2. | Chavasse, B., (1939), Worth & Chavass's Squint 9th Ed. Balliere Tindall & Co., London, p. 73. |
| 3. | Cridland Nigel, (1941), Brit. J. Ophth. 25: 141. |
| 4. | Cushman, B. and Burri, C., (1942), Am. J. Ophth. 4: 1944, 1941. |
| 5. | Cushman, B., (1944), Am. J. Ophth. 27:75. |
| 6. | Dhir, B. K., (1961), J. All Ind. Ophth. Soc. 9: 33. |
| 7. | Dobson, M., (1941), Brit. J. Ophth. 25: 66. |
| 8. | Duane, A., (1933), Binocular Movements Arch. Ophth. 9: 579. |
| 9. | Duke Elder. S. W., (1938), Text Book of Ophth. Vol. IV. St. Louis 1949 The C. V. Mosby Co., p. 3952. |
| 10. | Foster, C. B.. (1950), Am. J. Ophth 33: 773. |
| 11. | Jackson, S. R. S., (1960), Trans. Ophth. Soc. U. K. 80: 49. |
| 12. | Khosla, P. K., Bhatia, R. K., Agarwal, L. P. (1964), Orient Arch. Ophth. 2: 114. |
| 13. | Mann Ida, (1940), Brit. J. Ophth. 24: 373. |
| 14. | (1960), Brit. Med. J. 1: 208. |
| 15. | Stutterhein, N. A., (1938), Am. J. Ophth. 21: 77. |
| 16. | -, (1942), Brit. J. Ophth. 26: 216. |
| 17. | White, J. W. and Brown, H. W., (1939), Arch. Ophth. 21: 999. |
| 18. | Williamson Noble, F. A., (1941), Problems of Asthenopia, Brit. J. Ophth. 25: 397. |
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7], [Table - 8], [Table - 9], [Table - 10], [Table - 11], [Table - 12], [Table - 13]
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