Automatic exposure control in retinal laser therapy

Retinal laser spots are often over- or underexposed due to inhomogeneities of fundus pigmentation or variations in the radiation power impinging on the retina. We developed a method to control the exposure time of each individual spot of retinal laser photocoagulation in real time. The laser light reflected from the spot area is recorded by a detector. The intensity of this light increases during the exposure due to bleaching of the retinal pigment. If the reflected light reaches a predefined level, the laser apparatus is immediately stopped by a computer, which controls the whole process. The procedure was tested in enucleated porcine eyes. After verification of the proper function, a pilot study in a small collective of 19 patients (20 eyes, 25 sessions) with clear optical media was performed. In all cases, the apparatus worked safely. Visible indications for overexposed spots, particularly bleeding in extreme cases, did not occur. However, about 2%-5% of the spots were underexposed, mainly due to eye movements of the patient during the coagulation. Large variations in the automatically controlled exposure time confirm the difficulty in properly adjusting this parameter without such an automatic device. However, to be applicable to patients with opaque optical media, the method must be improved.