Side Effects of Radiations from Ophthalmic Instruments

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There is concern that radiation from ophthalmic instruments may cause damage to the eye being examined, especially the retina. The proceedings of a symposium on intense light hazards in ophthalmic diagnosis and treatment have shown that various ophthalmic instruments are capable of causing damage to the retina.

Estimates were made of the critical exposure time for the retina with the indirect ophthalmoscopy, slit lamp, operating microscope and overhead surgical lamp.

Ophthalmoscopes

Indirect ophthalmoscopes can produce levels of retinal irradiance up to five times greater than that produced by a direct ophthalmoscope, for example the Keeler Indirect produces nearly 3.6 times more irradiance than the American Optical at maximum voltage setting. The power of the condensing lens also affects the retinal irradiance, which increases as the power of the condensing lens decreases. Therefore, an indirect ophthalmoscope is considered to be unsafe, when compared with the laser standard, after 23 seconds exposure in a normal patient with clear media and dilated pupils. The time taken for fundus examination should not, therefore, be prolonged unnecessarily.

The light sources in ophthalmoscopes are usually an incandescent bulb (tungsten halogen) and are composed of one-third visible and UV radiation and two-thirds IR radiation. It has therefore been suggested that an IR filter should be incorporated in all ophthalmoscopes to avoid thermal demage from extensive viewing.

The higher the voltage at which the bulbs are operated, the greater the amount of UV radiation emitted. As UVA and blue light can cause damage to the retina, especially in aphakes (no crystalline lens) whose natural filter has been removed, it has been suggested that a cut-off filter at 450nm should also be incorporated.

Slit lamps

Slit lamp examination of the retina produces up to three times more irradiance than indirect ophthalmoscopy. The level of irradiance obviously depends upon the lamp voltage and can range from 140mW/cm2 to 358mW/cm2 for a 5 and 7.5V lamp, respectively. The safe durations for retinal examination are also shorter than for indirect ophthalmoscopy, being as little as 8seconds. It has therefore, been suggested that medium voltage settings should be used and that short examination times of 10seconds should be employed. This applies particularly to the examination of patients with macular or retinal degenerations.

Operating microscopes

These can produce up to ten times more retinal irradiance than indirect ophthalmoscopes, i.e. up to 970mW/cm2. The retinal irradiance and hence safe times are seen to vary with the patient's refractive error. The safe time varies from 1.8 to 49seconds, which is still relatively short when considering the time taken during operation procedures. It has therefore been suggested that corneal occluders should be used during prolonged procedures with operating microscopes.

Although microscopes do not produce much UV radiation it is still advisable to incorporate a pale yellow filter to absorb blue light and UVA. Care should be taken in certain conditions, such as retinitis pigmentosa, when light exposure may accelerate the disease. Light sources such as the surgical illuminating lid speculum provide a safe retinal irradiance. It may provide 900 times less retinal irradiance than conventional operating microscopes.

To summarize:

1. Use IR filters to absorb wavelengths longer than 700nm.
2. Absorb wavelengths blow 450nm to eliminate blue light and UVA. This will improve image quality by reducing the light scattering and chromatic aberration.
3. Use the minimum amount of light and time necessary for examination.
4. In some cases corneal occluders can be used to prevent unnecessary exposure from operating microscopes