Laser Safety

Laser damage to the inner ear structures or the facial nerve has been a concern since both before and after lasers proved clinically effective in otologic surgery. Silverstein et al.33 reported adverse results in their first two cases using an argon laser fiberoptic hand piece. In a revision stapes procedure, their first patient developed a discrimination loss of 40%. In a second case, a total hearing loss resulted when a large granuloma of the tympanomeatal flap filled the middle ear and grew into the oval window.

In a study using model vestibules and thermocouples to monitor temperature changes, Lesinski and Palmer,13 questioned the safety of both argon and KTP-532 lasers in primary and revision stapes surgery. Lesinski and Palmer's experimental model showed minimal (0.4°C) temperature elevations recorded by a large (0.5-mm) black thermocouple within a model vestibule with careful argon and KTP-532 laser stape-dotomies but marked temperature elevations with direct laser irradiation of the thermocouple.

In 1992, we repeated the experiments of Lesinski and Palmer, using a model vestibule with a small (0.025-mm) silver thermocouple and a 200-^m optical fiber delivery system for an argon laser.34 No significant changes in temperature within the vestibule were noted with vaporization of the stapes crura, footplate, or open vestibule. A temperature elevation was only obtained in an open vestibule when using a large thermocouple (0.5-mm) painted black and after saline was aspirated from the model vestibule. An interesting observation in our study was an 80°C temperature elevation in a second thermocouple placed at the level of the facial nerve. Kodali et al.35 recently performed a similar thermocouple study in the chinchilla and found no significant difference between fiberoptic KTP-532 and CO2 lasers. These investigators concluded that there was no contraindication, in terms of thermal injury to the inner ear, for the use of a visible light laser with a hand-held probe delivery system as compared to the CO2 laser for stapedotomy.

There is a theoretical advantage to an optical fiber visible light laser system for stapes surgery. A micromanipulator system produces a highly focused beam. When using a 250-mm lens, the laser beam in a micromanipulator system has an angle of convergence of approximately 3 degrees; beyond the focal point, the beam has an angle of divergence of approximately 3 degrees. The rate of power density fall-off on either side of the focal point is small, and structures both proximal and distal to the target tissue are at potential risk to laser damage. This also means that a micromanipulator system must be checked before each use to ensure that the laser beam is focused properly on the target tissue. By contrast, with a fiberoptic hand piece, the angle of divergence of energy at the tip of the optical fiber is approximately 14 degrees. Because of the larger angle of divergence and subsequent rapid power density fall-off inherent with a optical fiber, the risk of damage to a distal structure, such as the saccule in stapedotomy, is less likely to occur.

The safety of fiberoptic visible light laser use in an open vestibule is further implied by a recent series of publications on inner ear laser surgery. In 1990, Okuna et al.36 reported using the argon laser through the oval window after stapedectomy in guinea pigs and monkeys. These workers noted acute elevation of the supporting and sensory epithelium from the basement membrane in the irradiated area of the utricular macula. Two weeks after irradiation, the otoliths of the macula had disappeared; by 10 weeks, the entire macula had disappeared. In these experimental animals, there was no change in the structure of the membranous labyrinth, sensory epithelium of the cochlea or nonirradiated vestibular organs. Both Nomura et al.37 and Anthony38 have reported safe use of the fiberoptic argon laser for utricular macula ablation.

In the series of Anthony, 13 of 14 patients were treated without loss of auditory function with a power setting of 3.5 W for 0.5 s. In this procedure, the laser is directed at the utricule through a fiberoptic probe placed through the oval window. Anthony attributed the loss of auditory function in his one patient to opening the footplate in an ear with endolymphatic hydrops.

In summary, experimental data presented during the past decade do not support the concept of thermal damage to the inner ear using either argon, KTP-532, or CO2 lasers. Clinically, the use of otologic lasers over the past two decades has proved safe and effective for the surgical treatment of otosclerosis as well as other otologic disorders.

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