Namba and Nomura [7] irradiated the round window membrane of guinea pigs (Takei strain) with an argon laser after pentobarbital anesthesia. The probe had an outer diameter of 600 μm, in which a glass fiber 200 μm in diameter was encased. One watt of power was applied for 0.5 s. The tip of the probe was approximated to the round window membrane about 1 mm from the target.

The irradiated round window membrane showed no microscopic changes. No burned spot or perforation was observed after application of the argon laser (Fig. 7.1). However, various changes were found within the cochlea. When the organ of Corti was irradiated, an area 300 μm in diameter completely disappeared, with formation of three slits in the basilar membrane. The direction of the slits was the same, parallel to the fibers of the basilar membrane. It seemed that the slits occurred secondary to micro-explosion of the organ of Corti. The basilar membrane itself seemed to permit passage of the argon laser beam (Fig. 7.2).

When the osseous spiral lamina of the basal turn was lased, the irradiated area was destroyed and the associated venule vaporized and coagulated, resulting in congestion of its periphery (Fig. 7.3). In another specimen, partial loss of the lower shelf of the osseous spiral lamina and loss of nerve fibers were seen on cross section. The upper shelf of the osseous spiral lamina was partly fractured and elevated. The limbus spiralis, tectorial membrane, and organ of Corti had degenerated (Fig. 7.4). Cyst formation in the stria vascularis was observed 2 days after irradiation. Because anatomy prevents direct access to the stria vascularis through the round window membrane, cyst formation might be a secondary effect. Though the secondary spiral lamina appeared intact, the adjacent lower spiral ligament was widely destroyed (Fig. 7.5) [8]. The basilar membrane showed a break. Claudius’ cells near the break were lifted and attached to Reissner’s membrane. The round window membrane remained intact (Fig. 7.6).

The purpose of argon laser application to the bony wall of the cochlea is to produce localized loss or atrophy of the stria vascularis. In another study, the tip of the probe (diameter 200 μm) was advanced toward the otic capsule of the third cochlear turn, about 1 mm from the bony surface. The target was the stria vascularis [8].

The stria vascularis with spiral ligament was removed immediately after laser irradiation (800 mW × 0.5 s) of the otic capsule of the third turn. Alkaline phosphatase staining was performed to confirm the effect of lasing on the strial capillary network. A circular coagulated area was found in the stria vascularis within which the strial capillaries showed no enzymatic activity (Fig. 7.7).

A similar experiment was conducted using reduced laser power of 300 mW applied to the otic capsule [7]. This produced a charred area with an adjacent vaporized area in the capsule. The irradiated area showed a slight bulge covered by intact periosteum (Fig. 7.8). Strands connecting the remaining living tissues passed through the charred and lucent area. The strands may have been capillary remnants. Reissner’s membrane appeared to have extended to cover the defect of the stria vascularis and spiral ligament. It is not clear whether the altered marginal cells of the stria vascularis participated in this process (Fig. 7.8). In another specimen, altered marginal cells remained and connected to Reissner’s membrane. Several days later, cellular debris had cleared from the perilymphatic space, as well as from the scala media (Fig. 7.9).

Thirty-five days after laser irradiation (90 mW × 0.5 s), the stria vascularis was completely absent but the configuration of the scala media was preserved. The upper half of the spiral ligament was almost completely missing. The inner and outer hair cells had degenerated. Hensen’s cells appeared preserved. No cellular debris remained in the scala media (Fig. 7.10).

The otic capsule did not show any changes after laser irradiation (50 mW × 0.5 s). However, the spiral ligament was atrophic and acellular behind the stria vascularis, which persisted as a single layer of altered marginal cells. The outer hair cells were atrophic and some lacked nuclei. The outer hair cells and the phalanx of Deiters’ cells appeared to have fused. Hensen’s cell appeared unchanged 30 days after laser application (Fig. 7.11).

The average thickness of the otic capsule of the third turn was 102 μm (mean of 12 animals) and the capsule did not show morphologic changes. This suggests that the laser beam passed through the bony wall and led to destruction of the spiral ligament and disappearance of the stria vascularis. However, the primary goal of inducing localized strial atrophy was unsuccessful. The laser power needed to be reduced to preserve an intact organ of Corti.

Using the photochemical reaction between the systemic injection of Rose Bengal and controlled green light irradiation of the cochlea, Ocho et al. [9] produced a localized degeneration of the stria vascularis with preservation of the outer hair cells. The photochemical reaction produced a reactive oxygen species, resulting in damage to the endothelium. This damage triggered platelet adhesion and aggregation at the site of endothelial injury, producing thrombi and affecting microcirculation in the lateral wall of the cochlea at the site of irradiation. The direction of irradiation was tangential to the target in the cochlear wall. The outer hair cells survived for 24 h after irradiation, then disappeared with loss of the stria vascularis [9, 10].

The utricular macula is located on the floor of the utricle and its area can be seen through the oval window as a white plaque in the upper portion of the vestibule.

An argon laser passes through the utricular wall without damaging it, except in the dark cell area. Melanin in the dark cell area is susceptible to the beam, resulting in perforation [11]. Irradiation of the macular area is safe, because the connective tissue layer beneath the sensory epithelium is relatively thick and almost no pigments or pigment cells exist there.

When irradiating the utricular macula of guinea pigs, stapedectomy should be performed carefully to allow replacement of the stapes in the oval window after irradiation.

A single irradiation with an argon laser (1–1.5 W for 0.5–1 s) is enough to erase the entire sensory area, including the otoliths, sensory epithelium, nerve fibers, and capillary networks. The beam is directed toward the center of the white plaque (Fig. 7.12) [11].

Histological changes seen in the utricular macula differ depending on the power and duration of irradiation, and the interval between irradiation and sacrifice. The sensory cells and supporting cells are most vulnerable to irradiation.

Histological Changes 10 Weeks After Argon Laser Irradiation of the Utricular Macula (1 W x 1 s)

Complete loss of the macula was observed with preservation of the utricular wall, saccular wall, and membrana limitans. The utricle showed a mild hydrops (Fig. 7.13) [12]. The nerve fibers underneath the sensory epithelium were also missing. The crista of the lateral semicircular canal remained intact (Fig. 7.14) [11]. Caloric testing response had returned to normal.

Immediately after irradiation, the supporting cells were partly missing and partly remained on the basement membrane as destroyed cells with pyknotic nuclei. The sensory cells were destroyed and elevated. The top surface of the sensory epithelium appeared undulated. There were clear spaces between the sensory cells and the basement membrane and/or remnants of the supporting cells. The basement membrane did not show any clear rupture under light microscopy (Fig. 7.15) [13]. Capillaries beneath the basement membrane were intact. When viewed with electron microscopy, nerve fibers in the subepithelial connective tissue layer showed marked distortion of their myelin sheaths with loss of neurofilaments.