Concomitant strabismus



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Prisms

Immediately after the equalization of the visual acuity in both eyes and achievement of the central fixation, strabismus and anomalous retinal correspondence are treated. If the angle of strabismus is not large and does not exceed 20 PD, nonsurgical treatment may be started. To equalize the angle of strabismus, prisms of the strength equal to the angle of strabismus are used, with bases always directed inversely to the angle of deviation. In esotropia base-out prisms, in exotropia base-in prisms are used. Reduction of the angle of strabismus allows symmetrical stimulation of both retinas and formation of the normal binocular vision with normal straight binocular localization. As in the strabismus anomalous retinal correspondence is fixed, to dissociate fixed cortical pathways, prism hypercorrection is used. Hypercorrection prisms of the strength 2 – 6 times higher than the angle of strabismus are prescribed to the treated children. Alternatively covering the eyes, the child trains the vision at home. In such a way, pathological reflex leading to the persistent recurrence of strabismus is dissociated (Baranowska-George). However, the treatment is long-lasting and requires patience that is difficult to obtain in children.



Orthoptics

Binocular vision may be improved with the aid of orthoptic training. The treatment aims at restoring normal retinal correspondence and simultaneous foveal perception, fusion range, and sometimes stereoscopy. This training concerns first of all CNS. At the same time, exercises of the extraocular muscles are performed. Before the start of exercises, the eyes should be straighten with prisms, botulinum toxin injections or surgery. Straight eyes allow constant, symmetrical stimulation of the retinas of both eyeballs and fixation of the exercises effect in a free space under normal living conditions.



Simultaneous perception may be trained on synoptophore, using special images . Fusion amplitude is also trained with special, so-called fusional, images on synoptophore. Prism-bar may also be used for this purpose. Increasing the strength of prisms, diplopia is produced and so that fusion range in both convergence and divergence is widened (see Fig. 48)

Fig .48 The horizontal and vertical prism bar and separate prisms.



. Other exercises, producing diplopia, also serve to fusion improvement, decrease in accommodation spasm and dissociation of accommodation and convergence, which is of importance in the treatment of accommodative esotropia. Stereograms , diploscope or reading with a rod (see Fig.49) are used for this purpose.

Fig.49.Diploskop



Exercises on cheiroscope are performed by the patients with suppression and normal retinal correspondence (see Fig. 50) to remove suppression and stimulate binocular vision.

Fig.50 Cheiroskop using to remove suppression and improve binocular vision.

Stereoscopic exercises may be carried out on the synoptophore and various stereoscopes. Orthoptic exercises are indicated in case of all three stages of binocular vision insufficiency as well as the treatment of intermittent exotropia, deviations of the variable angle, in heterophorias. They are excellent visual rehabilitation following surgery.

Botulinum toxin

Action of the botulinum toxin A, produced by anaerobic Clostridium botulinum, means relay on the blockade of acetylocholine release in presynaptic nerve endings in the neuro-muscular junction, paralyzing the muscle. In the eighties of the last century, Alan Scott introduced a method of concomitant and paralytic strabismus treatment. Injection of botulinum toxin into overactive muscle produces its temporary paresis or paralysis. Due to relaxation and elongation of the muscle overaction of its homolateral antagonist develops, leading to the positioning the eye in the contraposition to that in strabismus and temporary dissociation of the pathological reflex pathways with anomalous retinal correspondence. Scheme of the botulinum toxin action is shown in Tab.3. This method is effective, especially in the young children, as the achievement of eyes alignment in the binocular vision development process in infants facilitates fusion. These method of treatment is quick and safe for patient. There is always possibility to repeat injections or to perform surgery in cases of poor or without effect.



TABLE 3



SURGICAL MANAGEMENT

The purpose of the concomitant strabismus surgical treatment is orthoposition with normal eyeballs motility. It allows single binocular vision with good fusion amplitude, reducing asthenoptic symptoms. Surgery frequently reduces abnormal head posture and improves patient’s esthetic appearance. In the majority of cases, the objective of surgery is reduction of the constant static angle of strabismus with the aid of classic surgical techniques. Special surgical approach is needed in case of less frequent variable dynamic angle. To the surgical treatment of strabismus are qualified the patients in whom abnormal eyes position makes single binocular vision in the valuable segment of the visional field impossible and these in whom all nonsurgical methods of reducing eye deviations have failed.



Preoperative evaluation and diagnostic approach

The most important examination is assessment of the angle of strabismus in the primary position when the patient looks at distance and near in both correction with spectacles and without it. Difference of the angle of strabismus at distance and near indicates anomalous AC/A ratio in the accommodative strabismus and the nystagmus blockage syndrome by convergence. After correction of the refractive error, indications to the surgery extend is frequently being decreased (accommodative esotropia, intermittent divergent deviation). Significant information is gained with measurements of the angle of strabismus at different positions of gaze. Increase or decrease in the upgaze or downgaze allows A-pattern or V-pattern syndromes diagnosis and taking it into consideration, when planning the surgery. Very important is also examination of eyeballs motility and deviation degree at nine diagnostic positions of gaze. Another examination is the forced duction test. Possible muscular contracture may thus be evaluated, allowing the choice of appropriate surgical technique. In case of the medial rectus muscle contracture, surgical strengthening lateral rectus muscle is required ,followed by large recession of medial rectus muscle.

Evaluation of the lid slit is of esthetic importance. In case of narrow lid slit of the deviating eye, oculomotor muscle is slightly more weakened, leading to the protrusion of the eyeball, whereas at the wide lid slit an appropriate muscle is strengthen, and the eyeball is retracted.

Assessing indications to the surgery, binocular vision status is also taken into consideration. If after an operation there is possibility of the single binocular vision, a goals of it are straight eyes. Correction of small deviation or strabismus in adult patients may lead to the postoperative diplopia. Therefore, prism or biological test should be performed prior to surgery to exclude diplopia.

All diagnostic examinations are the base of patient qualification for the surgical treatment. Technique and extend of surgery should be established individually, basing on the clinical findings.

How to establish the extend of surgery

There are various ways of establishing the extend of surgery. All provide only indicatory information. There are empirical rules and ways of calculation by how many millimeters muscles must be recessed or resected, depending on the angle of strabismus. Simple Stallard principle is frequently used: each millimeter recession or resection of the medial rectus muscle reduces the angle of strabismus by 4º, whereas each millimeter of the lateral rectus muscle recession or resection reduces the angle of strabismus by 2º. Combined surgery on two antagonist muscles is to increase the result by 20%. Cuppers thinks that muscle resection-recession ratio should be 2:1. Size of the eyeball should also be considered. If the eyeball is smaller, the same recession or resection gives considerably larger shortening of the arch of muscular contact with the eyeball in comparison with the eye of longer diameter.



In concomitant strabismus the best result of surgery with preservation of movements symmetry is, when the sum of retraction and pull to the opposite direction should be zero, and the same muscular tone as that before surgery is maintained.

Table 4. Indications to the surgery

Esotropia

Symetrical surgery

One eye surgery

ET angle Recess MR OU or Resect LR OU

Recess MR and Resect LR

15 PD 3,omm 4,0mm

3,0mm 4,0mm

20PD 3,5mm 5,omm

3,5mm 5,0mm

25PD 4,0mm 6,0mm

4,0mm 6,0mm

30PD 4,5mm 7,0mm

4,5mm 7,0mm

35PD 5,0mm 8,0mm

5,0mm 8,0mm

40PD 5,5mm 8,0mm

5,5mm 8,0mm

50 PD 6.0mm 9,0mm

6,0mm 9,0mm



Exotropia

Symetrical surgery

One eye surgery

XT angle Recess LR OU or Resect MR OU

Recess LR and Resect MR

15PD 4,0mm 3,0mm

4,0mm 3,0mm

20PD 5,0mm 4,0mm

5,0mm 4,0mm

25PD 6,0mm 5,0mm

6,0mm 5,0mm

30PD 7,0 mm 6,0mm

7,0mm 6,0mm

35PD 7,5mm

7,5mm 6,0mm

40PD 8,0mm

8,0mm 6,5mm

XT with profund amblyopia

50PD 9,0mm 7,0mm

60PD 10,0mm 8,0mm

70PD 10,0mm 9,0mm

80PD 10,0mm 10,0mm



Strabismus surgery techniques

Position of the eyes, dependent on the oculomotor muscles action, may be changed in two ways: muscle strengthening or weakening.

1. Weakening of the muscles:

 Recession of muscle.

 Elongation.

 Myotomy.

 Myectomy.

 Posterior fixation suture.

2. Strengthening of muscle:

 Resection of muscle.



 Anteriorization.

 Plication.
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