ARTICLE ON STRABISMUS-SHORT REVIEW (By S.A. Patney)

In this issue of the short review article on strabismus we go on to part III of the subject of "Strabismus resulting from ocular surgery" (iatrogenic?). The reason for the choice of this subject is obvious. Maintenance of goodwill between the patient and the physician is vital. By anticipating certain complications in the postoperative phase we can warn the patient in advance about the possibility of their occurrence so that he is mentally prepared and will not tend to blame the surgeon if some difficult to manage complications do take place. We also have to know how others have managed those complications so that we do not have to go through a long trial and error period.

NOTE: A mentioned earlier, this subject has been divided into 3 parts. Part I dealt with "Strabismus after cataract surgery" in the InteRyc volume 1, 2000. Part II described "Strabismus after retinal reattachment surgery with implants". Part III appearing in this edition of InteRyc will deal with complications following glaucoma surgery with implants. A complete list of references has been included in this part. It covers all the three parts of this series.

Strabismus after ocular surgery: Part III

Strabismus after glaucoma surgery with implants

In India many patients are illiterate and do not notice problems that are not significant or they carry on with them. This holds true of cases of strabismus after any type of ocular surgery including that for cataract, retinal detachment or glaucoma procedure involving implantation of drainage devices.

Strabismus and diplopia after glaucoma surgery using the various drainage devices/implants have been reported in literature often enough. However, I have come across hardly any case that I can remember the reason being that implantation of these devices is not in vogue in the part of our country where I live.

I have seen cases of strabismus after usual glaucoma surgery but the reason in these cases was either a preexisting heterophoria that became decompensated due to monocular bandaging etc. or loss / diminution of vision due to complications. These patients may complain of diplopia. Some of these cases are due to severe diminution of vision in one eye causing a loss of fusion and the consequent strabismus. These latter patients do not complain of diplopia.

Incidence

Various reports in literature give varying figures (6% to 100%) according to the type of the implant used. The range is given in the following table (No. 44-1):

Table 44-1:

Name of the glaucoma implant

Incidence of strabismus
Molteno valve implant62, 63 6%-47%
Baerveldt implant62, 64 10%-88%
Krupin disc implant64, 65 100%


Mechanism of production of strabismus

Strabismus seems to be related to the type and size of the implant and the technique of implantation. The main reason is scarring and adhesions resulting from the implantation as most devices are implanted between the rectus muscles with their tubes or part of the plate extending under or over the muscles.

The mechanism is similar to that in cases of strabismus after scleral buckling procedures (involving scleral implants or exoplants) for retinal reattachment.

We have to remember that drainage devices are only used in difficult cases of glaucoma where previous operations have failed or they do not stand a chance. It is usual to find scarring that resulted from previous surgery. These cases are at greater risk of severe scarring and adhesions. Taking into consideration all the above mentioned facts the following factors seem to be involved in varying degree in different cases:

  1. Scarring and adhesions causing restriction of the muscle that has scars and/or adhesions. Ocular motility is restricted in the direction opposite to that in which the scarred muscle functions, e.g., if superior rectus is scarred depression of the eye is limited causing a hypertropia of that eye. Similarly, if lateral rectus is affected, adduction is limited and there is an exotropia. These deviations cause diplopia in the postoperative period. The adhesions and the resulting restriction lead to a leash effect (or a reverse leash effect) that has been blamed for the strabismus.

    2. Fat adherence seen after retina surgery has also been observed in some cases36.

  2. Mass effect: The device along with the bleb forms an added mass for the extraocular muscles to overcome when the implant lies under the muscle. When the bleb is large and the muscle passes over it, the length/traction ration of the muscle is affected. Consequently, the effect of the implant/bleb-mass is to cause a resection like effect.
  3. Muscle factors: Placement of the drainage device under the muscle belly can cause direct trauma, ischemia or necrosis.

    4. Use of immunosuppressive drugs (like mitomycin-c) can cause ocular motility problems in different ways, e.g., By causing toxicity, ischemia and necrosis leading to scarring and adhesions of the extraocular muscles (as detailed earlier in this chapter).

  4. Loss or severe diminution of vision in one eye can lead to loss of fusion and strabismus. Severe constriction and loss of peripheral field also causes the same effect.

Symptomatology, investigations and diagnosis are similar to those of strabismus resulting after scleral buckling procedures for retinal reattachment surgery (See Strabismus after ocular surgery: Part II: Strabismus after retinal reattachment (detachment repair) surgery)

Treatment
  1. Preventive treatment is important as once strabismus takes place with diplopia and a lot of scarring the treatment becomes difficult.

    2. A thorough orthoptic examination is a must in every case in which ocular surgery is being considered.

  2. Prismotherapy: Is not very effective, as the deviation is usually incomitant.
  3. Botulinum toxin: So far I have not found any reports in the literature.
  4. Surgery: If the implant is bulky or large, its removal may be necessary but this is a serious matter as the vision is at stake. The control of glaucoma is very difficult in these cases and if there still is some useful vision, the glaucoma surgeon should be consulted. It is a question of weighing in the pros and cons of operating and taking a risk with the vision or leaving the strabismus alone in order to leave the drainage device that is effectively controlling the intraocular pressure, alone. In the latter case one can try to give the patient as large a diplopia free field as possible with the prisms.

If surgery is considered, the same basic principles apply as those in cases of strabismus after scleral buckling procedures.


References

(NOTE: These references cover all three parts of this series of "Strabismus after ocular surgery". The material for this series is taken from chapter 44 of the "Strabismology Desk Reference", the fellowship course reading material, published by The JKAI Publications. The first two parts of this series appeared in the InteRyc volumes 1 and 2, 2000).

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