Optical Coherence Tomography Imaging of the Posterior Segment in High Myopia.

Overview

The prevalence of myopia in East Asia and Singapore in particular is amongst the highest in the world, with estimates ranging from 30-70% of the general population. Up to 30% of these are high myopes. High myopia is associated with degenerative changes in the fundus. It may also be associated with vision-threatening complications such as macular holes. The pathogenesis of macular holes in high myopes is not completely understood but is postulated to include a combination of anterior vitreous traction and posterior staphyloma formation and axial elongation. These forces lead to degenerative changes at the macula, including foveal detachment and retinoschisis that precede the formation of lamellar or full thickness macular holes. These changes are difficult to detect either clinically or by conventional imaging such as ultrasound, making efforts to correct them in the early stages with surgery difficult. High myopia is also associated with a two- to threefold increase in risk of developing glaucoma. However, the diagnosis of glaucoma in high myopes is difficult as many of the pathological changes in the myopic eye mimic those seen in glaucoma. The myopic optic disc in particular is notoriously difficult to differentiate from the glaucomatous disc. Currently, the diagnosis is highly subjective, relying on observations of the clinical appearance of the disc or on disc photos.Optical coherence tomography (OCT) is an evolving technology that relies on time delays of reflected or backscattered light and interferometry to yield cross-sectional images of the retina and optic disc. The Stratus OCT is the latest model and has been demonstrated to be able to yield images with a resolution comparable to that of histology. It is thus potentially useful in assessing degenerative changes occurring in the myopic fundus, in evaluating the early changes preceding macular hole formation, and in providing objective measures of various disc parameters to aid in diagnosing glaucoma in high myopes. This study aims to recruit 150 healthy, young, ophthalmologically normal males from the SAF and to examine them with OCT. High myopes (≤-8D) will be selected and compared with a control group of low myopes. The performance of the OCT will be evaluated against current diagnostic methods.

Primary AimThe aim of the study is to evaluate the retinal and macular topography, vitreomacular relationships and optic nerve head changes in highly myopic eyes of young adults. Specific aims include:1) To determine the prevalence of vitreo-macular traction and macular degenerations (macular thickening, detachments, schisis or lamellar holes) in asymptomatic, highly myopic young Asian males using OCT.2) To characterize and measure optic nerve head and peripapillary retinal nerve fibre layer changes in high myopia using OCT.3) To compare OCT findings with current methods of investigation and diagnosis SubjectsSubjects will be drawn from in-service personnel as well as recruits awaiting enlistment. They will be identified based on their refraction through the SAF's computerized medical records system.Informed consent will be sought from the subjects before commencement. All subjects will be healthy young males and will be ophthalmologically normal apart from myopia. The main exclusion criteria are:1) Best corrected visual acuity \<6/92) Previous intraocular surgery3) Intraocular pressure \>21mmHg4) Gonioscopic findings of angle closure5) Clinical evidence of pseudo-exfoliation, uveitis or pigment dispersion syndrome, corneal or media opacities, retinal pathology or neurological diseases6) Family history in a first degree relative of glaucoma or other optic neuropathy.Excluded subjects will be replaced by the subject next-in-line in the cohort.ProcedureAll subjects identified will be examined at SNEC/SERI by one of the investigators. The assessment for each subject will include:1) Autorefraction performed with a non-accommodating target and recording of best corrected visual acuity. 2) Slit lamp examination, Goldmann applanation tonometry, gonioscopy, examination with a Goldmann three-mirror lens, and a dilated fundus examination with a 78 diopter fundus lens. Retinal findings are recorded in standard Amsler Dubois retinal diagrams. Grading of background myopic chorioretinal changes will be in accordance with the scheme proposed by Avila . 3) Axial length measurements. Ultrasound A scan is performed for each eye. Axial length is ascertained from the average of six consistent recordings. Care is taken to locate the fovea especially in cases with posterior staphyloma, by ensuring fixation of the A scan probe light. 4) Fundus photography with the Topcon camera. Stereoscopic disc photographs will also be taken for comparison with OCT scans of the optic nerve head.5) OCT using the Stratus OCTOCT measurements will be performed at one sitting by a trained technician. The commercially prescribed Optic Disc, RNFL and Macular Thickness scanning algorithms will be used. Each subject will fixate on an internal target presented by the computer where possible, or an external fixation target for cases unable to cooperate. Subjects will be encouraged to blink between the acquisition of each radial scan to minimize discomfort and minimize the effect of an irregular tear film or corneal desiccation on the tear film. Each scan will take approximately 2.5s, making for a total image acquisition time of less than a minute. The optic nerve head scan consists of six radial scans centered on the optic nerve head. The computer generated disc margin will be used as the reference for optic nerve head measurements. The macular thickness protocol uses six radial scans centered on the fovea. The retinal thickness is measured automatically as the distance between the vitreoretinal interface and the junction between the inner and outer segments of photoreceptors.