Key words: contact/fundus lens, cataract, diabetic retinopathy, retinal photocoagulation, tech support forensics
Diabetic retinopathy, a condition where abnormal new blood vessels grow within the retina or on its surface, is the leading cause of new cases of legal blindness in the U.S. for patients aged 20-64 years. Fifty percent of diabetics (an estimated 16 million in the U.S.) will develop retinopathy after 7 years, and 90% within 17-25 years. Laser photocoagulation (vs. cryotherapy) of these vessels has become the treatment of choice due to its superior ability to produce localized ablation. During this procedure, the physician views the retina and directs the laser beam through a fundus contact lens which is held against the cornea. A potential complication associated with this procedure is the production of instantaneous burns in the cornea or lens of the patient.
OST scientists, in collaboration with ophthalmologists at the Bethesda National Naval Medical Center, have evaluated the profile of the laser beam as it propagates through the different regions of the eye. This was performed using a model eye with several different contact lens designs, and beam sizes were determined both at the lens and the retina. Photographs taken through the viewing port of the model eye were digitized using a low-power microscope coupled to a CCD camera.
Results indicated that for some of the higher power contact lenses, the recommended spot-size setting (200 (m) produced more intense treatment beams in the lens of the eye than in the retina. In fact, the relative energy density (which is inversely proportional to the square of the beam diameter) at the model eye lens was estimated to be more than an order of magnitude greater for the higher power indirect lenses than for the standard direct lens. This finding was unexpected as these lenses typically are shipped with warnings to avoid spot-size settings larger than the recommended size. For all lenses tested, the smallest beam waist at the model eye lens was produced when the 200 (m spot size setting was chosen. The higher power, indirect contact lenses produced smaller beam waists at the model eye lens than the standard direct lens at all spot size settings tested.
These findings do not negate the use of either the high power treatment contact lenses nor the recommended spot-size settings provided by the manufacturers. Rather, they define conditions where unwanted laser injury can occur if a physician is unaware of the properties of the treatment system. A simple beam adjustment can prevent this injury in those cases where higher treatment beam energies are required. This work may serve as a basis for examining labeling which could be provided by both the manufacturers of the laser and the contact lens to warn physicians about potentially unsafe treatment parameters.