Optics for transscleral laser applications

Transscleral photocoagulation of the ciliary body is a potential method in glaucoma therapy. However, in order to reach the ciliary body for treatment, the laser beam has to pass through the sclera. Spectral dependance of the optical properties of the human sclera was investigated both globally in terms of total integrated transmission and reflection, and directionally in terms of angular and linear intensity profiles. It was shown that long wavelengths such as those emitted by the Nd:YAG laser present the greatest total integrated transmission as well as the narrowest angular scattered intensity profile. In contrast, short wavelengths present a lower transmission through the sclera and a larger scatter pattern. Contact and non contact applications are discussed. In particular, the crucial role of pressure exerted on the sclera is analyzed. It is shown that pressure exerted on the sclera locally increased the transparency of the sclera. The total integrated transmission raises to more than 90% for infrared wavelengths. The scatter patterns are also modified, becoming closer to that of the free beam. This effect can explain the lower energy dose which were reported in clinical applications of the contact mode as compared to the non contact method. Therefore, possibilities for microfocusing the laser beam with distal tips are investigated (piano-hemispherical and ball lenses, microlens-ended fibers, and cones). Due to their small diameter and the intensity pattern obtained in an aqueous environment, fibers having a hemispherically melted distal end give the best compromise between beam divergence and pressure which can be exerted on the sclera. A pressure-controlled delivery system was developed in order to increase the reproducibility of the treatment. However, as the transparency changes gradually with time, an on-line monitoring of the energy reflected by the sclera was also developed in order to specify the energy to be delivered. These two systems can be used whatever the irradiating wavelength was. Although the Nd:YAG laser is most widely used at this time, the laser diode emitting in the near infrared seems also to be a good compromise between high transparency of the sclera, narrow scattering pattern, and absorption of the targeted tissue, the pigmented layers of the ciliary body.