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Stefan Otto Johannes Koinzer, Mark Joachim Saeger, Konstantine Purtskhvanidze, Jan Heckmann, Kerstin Schlott, Alexander Baade, Amke Caliebe, Ralf Brinkmann, Johann Roider; Accurate Definition of Photocoagulation Intensity by Optical Coherence Tomography (OCT), Microperimetry and Temperature. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6350.
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Clinical laser photocoagulation dosage still relies on immediate ophthalmoscopic visibility, while new imaging techniques would allow more accurate lesion definitions. We demonstrate the high variability of standard ETDRS lesions and suggest objective, OCT-based lesion classes with related mean end temperatures, and we show microperimetrical side-effects of the lesions.
We used a universal OCT-based lesion classification that we had developped in a previous study, based on images and retinal peak end temperatures of 504 lesions (100 or 300 µm, 20-50-100-200 ms) from 20 patients. Now we compared 532 nm continuous wave lesions of ETDRS intensity (moderate grey) from 12 patients applied by 4 experienced physicians (300 µm, 20 or 200 ms) in a non-interventional clinical trial. Ophthalmoscopic and OCT characteristics (Heidelberg Spectralis), and microperimetric findings (Nidek MP1) of the lesions were compared over 3 months. Per patient, we imaged 40-60 lesions clinically and 8 lesions microperimetrically (> 6 stimuli/lesion, minimal and mean sensitivities).
Lesion variability between patients and physicians measured up to 177% in diameter, or 313% in area. Variability of 20 ms lesions was greater than of 200 ms lesions (both p < 0.001). The majority of 200 ms lesions had strong OCT classes 5 and 6 (end temperatures > 90°C), while the classes of 20 ms lesions were highly variable from 2 to 6 (end temperatures ~ 65 to > 100 °C). 37% (20 ms) or 71% (200 ms) caused absolute scotomas after 1 day, but only 4% or 32% after 3 months. Mean retinal sensitivities were initially reduced by -3 dB (20 ms) or -6 dB (200 ms), but recovered fully (20ms) or partly (200ms: -2 dB) after 3 months.
Lesions aiming at the same ophthalmoscopical endpoint - moderate grey discoloration - are extremely variable, which limits the predictability of the overall clinical treatment effect. The microperimetric damage per lesion is less pronounced than generally believed, even in lesions with obvious photoreceptor scarring. In conventional photocoagulation, 200 ms lesions were more homogenous, more destructive and caused more pronounced microperimetrical damage than 20 ms lesions. Our findings imply that temperature-controlled 20 ms lesions with peak end temperatures around 90 - 100 °C would be safe, effective and microperimetrically innocuous.
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