Nuevas contribuciones para la medida objetiva de la difusión intraocular y su aplicación clínica

Resumen

lmage form ation in the retina is the first stage in the visual process (optical stage), which is com pleted with photoreceptor stimulation and distribution of the generated signals (retinal stage), transmission to the visual cortex and, ultimately, neural interpretation of the signal, which leads to the final visual perception (cortical stage). Aberrations and light scattering are the two phenomena that affect the first stage of the process. Thus, quantifying ocular optical quality is of paramount importance to know the visual quality of the patient. Aberrometers can measure eye aberrations in a fast and objective way. However, intraocular scattering has traditionally been measured in clinical settings by means of subjective and often excessively com plex techniques. Double-pass (DP) is another alternative technique that provides information on both aberrations and intraocular scattering in an absolutely objective way.Despite the existence of a com mercial clinical instrument based on such technique, no clinical studies including the normal values for optical quality and intraocular scattering measured with this instrument for the diagnosis and follow-up of ocular pathologies had been published to date. This Ph.D. thesis is about the assessment of optical quality and intraocular scattering with such instrument for three groups of population. First, young healthy eyes of patients aged between 18 and 30 years have been tested, determ ining the normal values for optical quality and intraocular scattering useful for early detection of anom alies . Second, normal age-related impairment has been studied in patients aged between 30 and 70 years, the values obtained being a reference for healthy aging In this case, it has been found that worsening of optical aspects appears earlier than its effects on the visual function, which suggests sorne com pensation by means of neural processes. Third, patients with different types of cataracts have been analyzed, finding differences between them regarding their impact on aberrations and intraocular scattering and, consequently, on the worsening of the retinal image as well as on their effects on visual function .In this case, measurements of intraocular scattering by means of the DP technique and the psychophysical compensation-comparison method of fundus luminance have been compared, determ ining the correlation between both methods and the grade of cataract clinically diagnosed. In addition, it has been implemented an experimental high-sensitivity double-pass system which includes an EM-CCD camera to measure intraocular scattering using a new quantifying method based on the double information present in the DP image, that is, aberrations and intraocular scattering. Particularly, the parameter developed (FSl3 - Frequency Scatter lndex) is com puted in the frequency dom ain taking into account the response at very low spatial frequencies (up to 3 cpd) exclusively. lt has been shown that this new parameter can meas ure intraocular scattering caused by scattering filters as well as differentiate between patients with different grades of cataracts. On the other hand, the parameter exhibits very low sensitivity to the presence of aberrations. The device has also been used with three laser light sources - one with infrared radiation (780 nm wavelength) and the other two with visible radiation (630 nm and 520 nm respectively). This has made it possible to study the influence of wavelength on the measurement of intraocular scattering. The parameter has been compared with the OSI (Objective Scatter lndex) provided by the commercial DP instrument as well. The results obtained clearly show the usefulness of the parameter developed and its robustness against defocus.