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|Year : 1965 | Volume
| Issue : 3 | Page : 105-108
Factors influencing the result of strabismus surgery
PN Srinivasa Rao
Kasturba Medical College and Hospital, Manipal, India
P N Srinivasa Rao
Kasturba Medical College and Hospital, Manipal
|How to cite this article:|
Srinivasa Rao P N. Factors influencing the result of strabismus surgery. Indian J Ophthalmol 1965;13:105-8
The aim of surgery in strabismus is mainly physiological--that is restoration of binocular vision. The surgical correction of the deviation is the most important step in that direction. This anatomical correction aims at obtaining parallelism and a symmetrical appearance of the eyes in all directions of gaze. This enables normal retinal correspondence to develop and so in the case of concomitant squint aids the development of stereoscopic vision. The purpose is also to prevent visual abnormalities and structural abnormalities such as ocular torticollis. Because of this aim of functional cure, surgical work in strabismus in many cases, is intimately associated with pleoptic or orthoptic exercises, either before or after the operations, according to the requirements of individual cases. In many cases especially in our country, this idealism is not obtained so that the result of surgery tends to be mainly cosmetic.
Many times it is not possible to get an ideal result with a single operation. Hence an ophthalmic surgeon is wise in informing the party earlier that more than one operation may be necessary at times. Many tables of figures in terms of degrees of squint and the millimeters of recession and resection to correct them are available and this very fact indicates that the results are rather unpredictable and that each case is a problem by itself. The purpose of this article is essentially to recollect these unpredictable factors, which materially affect the results of the operation.
One such factor is the tone of the muscle. There seems to be individual variation of the tonus not only for every individual, but also for every muscle. State of nutrition of the patient and nervous stress seem to influence this tonus. Clinically it is noted that the ocular deviation tends to increase when the body is weak or when the patient is more nervous. These variations of tonicity may produce the post-operative irregularity of muscle correction. This is especially liable to occur if there is failure of binocular vision. The muscle tonus increases on shortening; greater the shortening, greater the increase of tonus. Conversely, the tone of the muscle decreases on tenotomy and recession. Again for the rotation of the eye ball the muscle should be attached tangentially to the eye in a line parallel to the direction of its pull. In other words, there is a limit bounded by the equator beyond which certain extra-ocular muscles cannot be recessed.
For the rest, factors influencing the results of strabismus surgery can be best evaluated by correct diagnosis in a given squint case. The various steps of investigation for a case of squint are-(1) History (2) Inspection including cover test (3) Ocular movements- including the cover test in the cardinal directions of gaze. (4) Visual acuity (5) Measurement of deviation (6) accommodation and convergence (7) Tests of binocular functions (8) Field of binocular fixation (9) Photographic diagnosis (10) Occlusion (11) Estimation of refraction (12) Routine examination of the eye.
(1) History: Squint of recent onset can be more easily cured than long standing cases. Thus those cases which have an onset early in infancy are more difficult to treat.
The accommodational convergent squint cases are in most cases easily amenable to refraction correction and occlusion, if necessary. If there is any residual squint, that can be corrected surgically, higher the hypermetropia, greater the effect of operation, so that here tendency should be to avoid over correction.
In purposive squint, the effect of operation becomes less, if there is marked difference of vision between two eyes. The effect becomes still less, if the duration and constancy of the deviation is great.
(2) Cover test :-The purpose of cover test is especially to establish squint. While performing the cover test, if the fixing eye wanders aimlessly, it is grossly amblyopic and hence operation here tends to be purely cosmetic. In cases of convergent squint with gross amblyopia, under-correction is indicated and in the cases of divergent squint, an over- correction. The reason is, the amblyopic eye eventually tends to deviate outwards.
If there is a difference in the angle of deviation when each eye fixes in turn, the eye which shows the greater angle of deviation, when the other eye fixes, must be selected for operation.
Latent nystagmus : -Cover test may reveal latent nystagmus. A manifest squint with latent nystagmus may be corrected for cosmetic purposes. In some cases, it is said, it has reduced the intensity of nystagmus. In such cases, occlusion, and orthoptic treatment are obviously contra-indicated.
(3) Ocular movements and the cover test in the cardinal directions of gaze: -Limitation of ocular movement may be due to paretic element, or musculofascial anomalies like Duane's lid retraction syndrome, or in long standing cases of strabismus where the deviating eye has become grossly amblyopic.
The effect of an operation tends to be greater, if there is a paretic element. There again, effect on secondary deviation is greater than the effect on primary deviation.
Duane's lid retraction syndrome:- Here abduction or adduction of the eye is defective and retraction of the affected eye occurs with movement in certain directions of gaze. If in the primary position there is no squint, the patient is probably best left alone. If there is squint in the primary position, an operation is indicated. If the adduction is more affected than abduction recession of the external rectus of the same side and the resection of the medial rectus of the same side of considerable degree are indicated. In other cases, if the squint is very severe, reinforcement of internal rectus such as Hummelsheim's operation is indicated.
Strabismus fixus:-Here the eye is held strongly converged by a mass of fibrous tissue behind the internal rectus insertion. This is probably the only indication for tenotomy of internal rectus and should be accompanied by the division of the fibrous adhesions. Here results are not satisfactory.
A-V Syndromes:-In A syndrome, convergence increases on looking up or divergence increases on looking down. In V syndrome, the converse holds good - divergence is greater on looking up or convergence is greater on looking down. These are usually associated with eso-, or exo-tropia. Sometimes, these may be associated with orthophoria in the primary position.
Rene Hugonnier of Lyon states that recession of the internal recti for esotropia must be combined with vertical displacement in the direction of the greatest deviation, i.e., upwards in the A syndrome or downwards in the V syndrome. The amount of this displacement may be up to 5 mm. Results are good in A syndrome, but good and variable in V syndrome.
The same rule applies for exotropia associated with A and V syndromes - the external recti are recessed and displaced upwards with the V syndrome, and downwards in the A syndrome. Results are not very consistent.
In long standing cases of squint, especially when it is marked, the eyes do not move as much as usual in the direction opposite to that of the deviation. This defective movement may be due to muscular contracture. The effect of operation is naturally less in these cases, and it becomes still less in those cases where the limitation of movement is absolute.
In long standing cases, when the fixing eye is covered, the squinting eye may not move at all, or it may move slightly. The condition is known as eccentric fixation. In presence of eccentric fixation, the effect of operation is always less. A type of ophthamoscope, known as visuscope is devised to detect this state of affairs.
(4) Visual acuity:-Subnormal vision in the squinting eye in spite of glasses and normal fundus indicates amblyopia. In the presence of amblyopia, the effect of operation is always less. The treatment of amblyopia is by occlusion of the normal eye, which is contraindicated in cases of eccentric fixation. An instrument, euthyscope, is devised to treat these conditions of neglected amblyopia and eccentric fixation by the after image method. The procedure requires an intelligent and co-operative patient. Hence it is rarely of use for a child under the age of 6 or 7 years. For a child over 12 years, the economic obstacle may be a major problem. Between the age of 7-12, the procedure may be of some use in the management of these cases. Both Euthyscope and Visuscope were introduced by Cuppers of Giessen.
(5) The angle of deviation :- Greater the angle of deviation, greater the result of the operation. Similarly, if the squint shows marked variation in the angle of deviation, the result of operation will be greater. Again if the squint is absent in deep sleep or deep anesthesia, the effect of operation becomes more.
(6) Accommodation and convergence:- As already mentioned in the accommodational type of convergent squint, higher the hypermetropia, greater the effect of operation. Hence it is probably wiser to slightly under- correct the residual deviation. If the case shows secondary convergence palsy, the effect of operation becomes more.
(7) & (8) State of binocular vision and field of binocular fixation : - Where the orthoptic treatment has attained normal retinal correspondence and the power of fusion before operation, the effect of operation is good and the squint can be cured functionally and cosmetically. Presence of abnormal retinal correspondence is necessarily an unfavourable factor.
Thus cases of congenital palsy or cases of congenital alternating squint which show abnormal retinal correspondence, the result of operation is poor. On the other hand, in cases of congenital palsy in which binocularity is preserved in at least certain directions of the gaze the effect of operation will be better. Greater the perversion quotient in the state of secondary sensory correspondence, lesser the effect of operation.
Other factors :-
(9) Type of the globe:-Larger the ocular globe, the greater is the effect.
(10) Type of muscles:-When a muscle is robust and elastic the effect is necessarily greater.
(11) Effect of an operation on each muscle:- The internal rectus is more sensitive to recession, and the external rectus is more affected by resection. In general, the vertical muscles behave more consistently, but respond less to surgery.
(12) Combined operations: -If the recession of the medial rectus is combined with the resection of the lateral rectus, the effect is 25% more than that achieved when individual muscle is tackled separately with an interval between them. This enhancing effect is not so striking when resection of the medial rectus is associated with recession of the lateral rectus.
(13) Repeated operations: -If the deviation has been undercorrected at the original operation, a second operation is likely to have a relatively greater effect. If the deviation is over-corrected, the general impression is the second operation should be more extensive. However, if the excessive effect is due to too large a resection and the same muscle is to be touched again, a very small operation is quite sufficient. Excessive first recession sometimes results in the contracture of the antagonist and in these cases if the operation is delayed action is necessary on this muscle, as well as on the palsied muscle.
(14) Passive duction during the operation may reveal capsulo-ligamental anomalies. If they are freed, as much surgery as originally planned may not be necessary.
(15) Due to its ex-ophthalmic effect, a large retrobular injection may favour the result of recession.
Thus, the factors influencing the results of strabismus surgery are many. The surgeon must assess these factors before deciding on operation. In addition, possibility of post-operative diploma must be thought of. Arruga anaesthetizes the conjunctiva slightly, grasps the eyeball with a forceps and by forced movement determines what angle it is possible to correct without provoking postoperative diplopia. It is always wise to explain to the patient or patient's relatives that more than one operation may be necessary. Each surgeon will find more than these factors influencing the final result. It depends essentially upon him and his technique - the manner of suturing, placing of sutures either nearer or further from the edge of the tendon, amount of liberation of check ligaments and prolongation of Tenon's capsule, etc. However, cosmetic result of the operation is ultimately always satisfactory, but the same may not apply to the functional cure.
| Summary|| |
Several factors influence the results of surgery in squint. The different factors which are variable and are individualistic are discussed with a view to secure the best possible results.