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Retina can influence refractive outcome
SLO can separate components of corneal aberration from retinal physiology.
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The SLO focuses on the retina through the ocular media. Any disturbances in the ocular media are projected onto the retinal image. Above left shows results with a cornea treated with a Gaussian excimer laser system (Inpro Intraokulare Prothetik, Norderstedt, Germany) with a very smooth transition zone. Above right is the second eye of the same patient, treated with a diaphragm-based excimer laser ablation on the corneal surface. The delivery system of the excimer laser is thus very important in photorefractive keratectomy treatment.
SLO image similar to the one seen above, but in this case the annular shadow is caused by a re-proliferation of capsular epithelial cells around a YAG laser capsulotomy.
EDINBURGH — Researchers are applying scanning laser ophthalmoscopy (SLO) to assess retinal and other causes of poor visual outcomes after refractive surgery.
Optical density of visual pigment or abnormal orientation of photoreceptors causes poorer visual performance even in the presence of a well-centered corneal ablation, said Marie Jose Tassignon, PhD, MD, of Antwerp, Belgium.
She conducted a small trial of SLO to demonstrate its use in detecting wave front aberrations and to justify starting a prospective study.
Quantifying aberration
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---A complicated case of radial keratotomy, where epithelial ingrowth was present. 
---Striking SLO images of the same complicated RK patient. Subjective visual comfort and objective visual performance of post-refractive patients have been poorly evaluated until now, Dr. Tassignon said. Clinical studies often report good results for visual acuity and corneal topography, but a number of unaccounted for complaints such as glare, halos, monocular diplopia and decreased sensitivity can reduce best corrected visual acuity.
“It is remarkable that a disagreement often exists between the subjective visual comfort of the patient and the outcome of topographical examinations. The correspondence between SLO retinoscopy results and subjective outcomes is much better,” she said.
The retinoscope images indicate exactly the corneal areas most responsible for glare and other adverse events, Dr. Tassignon said.
For the pilot study, she examined 60 post-refractive patients to assess what conditions she could detect with SLO. Only a minority of patients had both preoperative and postoperative examinations.
New uses
---Cataract also causes an annular disturbance on the SLO image in this example.
Scanning laser retinoscopy allowed researchers to see patterns directly on the retina. The patterns revealed the wave front aberrations caused by refractive surgery, the optical density of the visual pigment and the alignment of the visual photoreceptors.
SLO demonstrates wave front aberrations in qualitative fashion, she said. The fundus may appear out of focus because the local wave front disturbances are seen as grayish-dark areas within the confines of the pupil for any given degree of spherical correction.
“This is both the strong point and weak point of the technique. One can have an idea of the quality of the eye optics by simply looking at the fundus with the SLO under specific conditions,” Dr. Tassignon said. “It would be much harder to quantify the amount of aberration. To do this, we are developing another technique also based on scanning laser ophthalmoscopy.”
Researchers aim to package new capabilities of the SLO into a dedicated instrument for refractive surgery.
“Any institution with an SLO and using the appropriate parameters can obtain such images that will give the user a fast, qualitative idea of the optical qualities of the eye,” she said. “We also are working on a quantitative method. This latter approach will be used to create capabilities in which the profiling of the cornea is guided in real-time by the outcome of the quantitative SLO test.”
Retinal effects
---Although corneal topography showed a good result in this PRK patient, SLO demonstrates a small useful optical zone and even a peripheral annular optical zone, untreated - all of this explaining the patient's subjective complaint of diplopia.
The SLO method can diagnose diseases of the anterior and posterior segments, as well as neuro-ophthalmic conditions.
The SLO allows for the evaluation of cataract and corneal changes. Keratometry can show a perfectly centered ablation, but the corresponding SLO can reveal a small optical zone, large transition zone and peripheral annular optical zone, which could trigger complaints of diplopia. It also can assess keratoconus.
When combined with automated keratometry, a subtraction technique can be created to separate qualitatively the corneal and lens contributions to the wave front aberrations.
“The correlation of retinal imaging techniques that measure wave front aberrations and photoreceptor directionality with subjective outcomes of refractive surgery suggests that even in refractive surgery, the retina will have the last words to say,” Dr. Tassignon said.
For Your Information:
- Marie Jose Tassignon, PhD, MD, can be reached at University Hospital Antwerp, Wilrijkstraat, 10, 2650 Antwerp, Belgium; +(32) 3-821-3377; fax: +(32) 3-825-1926; e-mail: oftalmo@uia.ua.ac.be. Dr. Tassignon has no direct financial interest in any of the products mentioned in this article, nor is she a paid consultant for any companies mentioned.