Norlelawati Z., Faridah H.A.
Corneal topography changes following strabismus surgery
Purpose - to determine whether strabismus correction surgery significantly influences cornea curvature.
Materials and methods. This was a prospective and observational study. Study subjects were strabismus patients under the care of Eye Clinic, Universiti Kebangsaan Malaysia medical centre, planned for strabismus correction involving horizontal rectus muscles and cooperative for corneal topography and objective refraction. Strabismus patient without age limit with correction involving horizontal muscles only and able to sit up for cornea topography were included. Patients with previous or recent eye injury or preexisting cornea disorder, previous squint surgery or glaucoma drainage devices implantation were excluded. Outcome measured was changes of the corneal topography before strabismus correction surgery and after the procedure.
All measurements were obtained with Haag-Streit corneal topographer. In this system, the corneal curvature was divided into 12 spots and measured in diopters (D) prior to the operation, 6 weeks postoperation and 12 weeks postoperation. Subsequently, the participants underwent squint correction surgery done by a single surgeon under general anesthesia. The statistical analysis in this study was done using the Statistical Package for Social Sciences (SPSS) version 16.0. The differences between corneal topography measurement was done using the repeated measure analyses.
Results. A total of 17 eyes from 12 patients were included in this study with equal number of male and female. Five patients had both eyes operated as their angles of strabismus were large. Majority of the patients belonged to the paediatric age group from 4 years old to 12 years old with only 2 adult aged 23 and 26 years respectively. Four patients had only right eye operated, 3 patients had only left eye operated and 5 patients had both eyes operated. The mean age was 11.65±7.79 years. Out of 12 patients, 7 had exotropia and 5 had esotropia. The angle of deviation ranges from 20 to 80 Prism Diopter.
The most frequent procedure performed was lateral rectus recession and medial rectus resection which involved 6 (35.3%) eyes followed by medial rectus recession and lateral rectus resection which included 5 (29.4%) eyes. Although the amount of lateral rectus underwent recession ranged from 5 to 10 mm, 3 out of 9 had 9mm lateral rectus muscle recessed. The range of corneal topography measurement (D) in this study was from 32.8D to 46.3D. The mean corneal topography measurement was 40.86±2.57D, 40.65±2.76D and 40.85±2.72D for preoperation, 6 and 12 weeks postoperation respectively.
Repeated measure analysis p=0.058
From Fig., we could observe that there were at certain points in which the corneal curvature decreased at 6 weeks postoperation but increased again after 12 weeks postoperatively however there was no significant difference in corneal topography changes over time after horizontal muscle strabismus correction surgery with Wilks’ Lambda = 0.971, F=2.895 and p=0.058.
There was statistically significant difference in corneal topography changes following 6 weeks postoperation but the effect was transient that lasts for 12 weeks as shown in Table.
Strabismus correction surgery is a common surgical procedure that is performed by general ophthalmologist. The aim of this study is to find out to what extent corneal topography is changed by strabismus surgery at 6 weeks post operation and 12 weeks postoperation. Changes in refraction following operation for strabismus had been reported and it appears to be due to cornea changes [1-5]. The cornea changes are related to the altered tension of the extraocular muscle on the sclera that is transmitted to the cornea following the surgery. Therefore, the factors evaluated in this study were cornea topography changes by corneal topography measurement.
The factors that influence a normal corneal topography are not well understood. Factors that may change the corneal curvature measured using corneal topography in a normal eye include accommodation and external forces such as those exerted by the eye lid . Suture and muscle placement are the mechanical forces that may alter the corneal curvature. If a muscle is placed too close to the limbus or tied to the sclera under tightly, the corneal curvature might be altered . Although it has been reported that there are changes of the corneal curvature following extraocular muscle surgery, the effect was also known to be transient [2, 7]. This study demonstrated that there are a corneal topography changes following horizontal muscle strabismus correction surgery, however the effect is transient. Corneal topography measurement 6 weeks after the operation was significantly different than that before operation (mean difference = 0.210±0.096, p=0.029). There was a significant difference in corneal topography measurement between 6 weeks post operation and 12 weeks post operation (mean difference = 0.195±0.093, p=0.032) however the corneal topography measurements were not significantly different after 12 weeks of operation (mean difference = 0.015±0.081, p=0.854) if compared to corneal topography measurements prior to operation.
The sample size of this study was very small compare to other studies. In this study only 17 samples were included due to several limitations including uncooperative paediatric patients and surgery involving vertical extraocular muscle. Corneal topography was more challenging in paediatric age group compare to adult due to difficulty in fixation during the procedure. As this study had majority of paediatric patient, therefore the reproducibility of the test had higher standard deviation. The wide range of age with most of the patient in paediatric group did not allow for comparison of persistent cornea curvature change as adult cornea was expected to be inelastic. The small sample size also limit the evaluation of different type of surgical procedure and the amount of muscle resected or recessed which may be statistically significant if the sample size is larger.
Conclusions. This study had achieved its objective in showing that there is a change in the refractive state of an eye which underwent horizontal muscle strabismus correction surgery as observed via corneal topography and spherical equivalent, however, the effect is transient and resolved by 12 weeks postoperation.
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