Local ocular hypothermia in experimental intraocular surgery

Research Progress of the Application of Hypothermia in the Eye

Hypothermia is widely used in the medical field to protect organs or tissues from damage. Different research fields have different explanations of the protection mechanism of hypothermia. Hypothermia is also widely used in the field of ophthalmology, for example, in the eye bank, the preservation of corneal tissue and the preservation of the eyeball. Low temperature can also be applied to some ophthalmic diseases, such as allergic conjunctivitis, retinal ischemia, and retinal hypoxia. It is used to relieve eye symptoms or reduce tissue damage. Hypothermic techniques have important applications in ophthalmic surgery, such as corneal refractive surgery, vitrectomy surgery, and ciliary body cryotherapy for end-stage glaucoma. Hypothermia can reduce the inflammation of the cornea and protect the retinal tissue. The eyeball is a complex organ, including collagen tissue of the eyeball wall and retinal nerve tissue and retinal blood vessels. The mechanism of low temperature protecting eye tissue is complicated. It is important to understand the mechanism of hypothermia and its applications in ophthalmology. This review introduces the mechanism of hypothermia and its application in the eye banks, eye diseases (allergic conjunctivitis, retinal ischemia, and hypoxia), and eye surgeries (corneal transplant surgery, corneal refractive surgery, and vitrectomy).

Hypothermia Protects Cultured Human Retinal Pigment Epithelial Cells against Indocyanine Green Toxicity

PURPOSE

The aim of this study was to determine whether indocyanine green (ICG) is toxic to cultured human retinal pigment epithelial (ARPE-19) cells, and whether hypothermia can protect the ARPE-19 cells against the ICG toxicity.

METHODS

Cultured ARPE-19 cells were exposed to 0.25, 0.5, 1, 2.5, and 5 mg/mL of ICG dye at 37 and 4 degrees C for 30 min. The percentage of ARPE-19 cells that survived was determined by resazurin 1 day after the exposure.

RESULTS

Exposure of the RPE cells to a hypotonic saline solution with an osmolarity equal to 5 mg/mL of ICG did not induce a statistically significant decrease in the percentage of RPE cells that survived. Exposure of the ARPE-19 cells to ICG induced a significant decrease in the percentage of cell survival at all concentrations of ICG (P<0.05), except in 0.25 mg/mL at 37 degrees C. At 4 degrees C, on the other hand, ICG induced a statistically significant decrease in the percentage of RPE cell survival only at 5 mg/mL of ICG (P<0.05).

CONCLUSIONS

These results indicate that ICG is toxic to human RPE cells in culture, and that cell death cannot be attributed to the low osmolarity. Hypothermia of 4 degrees C has a protective effect against ICG toxicity.

Hypothermia protects cultured human retinal pigment epithelial cells against trypan blue toxicity

PURPOSE

To determine whether trypan blue (TB) is toxic to cultured human retinal pigment epithelial (ARPE-19) cells, and whether hypothermia can protect ARPE-19 cells against TB toxicity.

METHODS

ARPE-19 cells were cultured and exposed to balanced salt solution as a control, while other cells were exposed to 0.05, 0.2, and 0.5% TB dye at 37 and 4 degrees C for 5 and 30 min. The percentage of ARPE-19 cells that survived was determined by resazurin 1 day after the exposure.

RESULTS

A statistically significant decrease in the percentage of ARPE-19 cells surviving was found after exposure to 0.2 and 0.5% TB at any temperature or for any exposure duration (p < 0.01). The percentage of RPE cells surviving at 0.05% was not significantly different from that of controls except for a 30-min exposure at 37 degrees C. The percentage of cells surviving at 4 degrees C for a 5-min exposure to 0.5% TB and a 30-min exposure to 0.2 and 0.5% was significantly higher than that at 37 degrees C under each condition (p < 0.01 for all).

CONCLUSIONS

These results indicate that TB is toxic to human RPE cells, and the toxicity is dose- and exposure duration-dependent. Exposing the cells at 4 degrees C had a protective effect against higher concentrations or longer exposure durations of TB compared to exposure at 37 degrees C.

Effect of cooled intraocular irrigating solution on the blood-aqueous barrier after cataract surgery

PURPOSE

To evaluate the effect of cooled intraocular irrigating solution during phacoemulsification on postoperative blood-aqueous barrier (BAB) disturbance.

SETTING

Department of Ophthalmology, University of Vienna, Austria.

METHODS

In a prospective, randomized clinical study, 40 eyes with senile cataract had phacoemulsification with irrigating solutions cooled to approximately 10 degrees C (n = 20) or at room temperature (n = 20). Surgical procedure and postoperative therapy were otherwise identical in both groups. Postoperative BAB disturbance was assessed with the laser flare-cell meter on days 1, 3, 7, 14, and 28.

RESULTS

Anterior chamber flare was significantly lower in the group with cooled irrigating solution on postoperative day 1. There was no significant between-group difference in flare on any other postoperative day.

CONCLUSION

Cooled intraocular irrigating solution reduced immediate postoperative inflammation compared with irrigating solution at room temperature. However, this effect was of short duration.

Protective effects of local hypothermia in vitrectomy under fluctuating intraocular pressure

The effects of local hypothermia on the blood-aqueous barrier and retina were investigated in experimental vitrectomy under wide-ranging fluctuations of intraocular pressure. After anesthetized albino rabbits had undergone closed vitrectomy, their vitreous cavities were continuously irrigated for 60 minutes. Perfusion pressures were fluctuated alternately between 10 and 80 mmHg. The rabbits were divided into two groups in accordance with their intraocular perfusion temperatures: 8 degrees C and 22 degrees C. Electroretinograms were taken both before and after the irrigation. Protein and prostaglandin E2 (PGE2) concentrations in the aqueous humor were measured on the 1st and 7th postoperative days. Histologic examination was performed on the eyes enucleated on the same days. There were no significant differences in either a- or b-wave amplitudes between the 8 degrees C and 22 degrees C groups. Both aqueous protein and PGE2 concentrations were significantly higher in the 22 degrees C group than in the 8 degrees C group on the 1st postoperative day. Histologically, the ciliary body showed remarkably more stromal edema in the 22 degrees C group than in the 8 degrees C group. The severity of the retinal damage did not seem to differ between the two groups. Local hypothermia during vitrectomy under fluctuating intraocular pressure inhibits the breakdown of the blood-aqueous barrier in the early postoperative stages.

Local hypothermia protects the retina from ischaemic injury in vitrectomy

AIMS

Hypothermic irrigating solutions were used during vitrectomy in pressure induced ischaemic eyes so that their effects on retinal function and histological changes could be investigated.

METHODS

After anaesthetised albino rabbits underwent closed vitrectomy, their vitreous cavities were continuously irrigated for 30 minutes at a perfusion pressure of 140 mm Hg. The rabbits were divided into three groups according to their intraocular perfusion temperatures--8 degrees C, 22 degrees C, and 38 degrees C. Electroretinograms were taken before and after irrigation. Glutamate levels in the vitreous were examined after irrigation. Eyes were enucleated on the seventh postoperative day and examined histologically.

RESULTS

On the seventh postoperative day, the recovery rate of a-wave amplitudes was significantly lower in the 38 degrees C group than in the 8 degrees C group, and that of b-wave amplitudes was significantly lower in the 38 degrees C group than in either the 8 degrees C or 22 degrees C group. Retinal damage in the 38 degrees C group revealed more severe histological impairment than in either the 8 degrees C or 22 degrees C group. Oedema of the inner retinal layer was significant in both the 22 degrees C and 38 degrees C groups. Glutamates reached peak values 30 minutes after the end of ischaemia in the 38 degrees C group. However, no significant glutamate increases were detected 15 to 60 minutes after ischaemia in either the 8 degrees C or 22 degrees C group.

CONCLUSION

Local hypothermia during vitrectomy in acute ischaemic eyes appears to decrease retinal damage.

Intraocular pressure decrease after local ocular cooling is underestimated by applanation tonometry

BACKGROUND

The effects of intense systemic or local cooling on aqueous humor dynamics in animals are an increased total outflow facility and a decreased aqueous flow. Few studies suggest that only vasoconstriction of arteriolar segments of the episcleral vasculature may be the cause for a decrease in intraocular pressure after local cooling in humans. Because corneal changes may have influenced such studies, the effect of local cooling was assessed in normal subjects.

METHODS

Intraocular pressure and corneal thickness were measured in 18 healthy human subjects before and after exposure of the right eye to both, an air stream at 20 degrees C, and an air stream at 0 degrees C.

RESULTS

No significant changes in IOP or corneal thickness occurred under 20 degrees C conditions. After local cooling, the mean corneal thickness increased from 0.52 +/- 0.01 mm to 0.57 +/- 0.02 mm (p < 0.001). Mean intraocular pressure decreased from 13.8 +/- 2.9 mmHg to 12.9 +/- 3.1 mmHg (p < 0.026). The observed decrease in IOP correlated significantly but negatively (R = -0.53; p = 0.024) with the increase in corneal thickness, indicating that the cooling effect on IOP may be rather underestimated.

CONCLUSION

The eye is very sensitive to local cooling effects, which may, however, partially be masked by changes in corneal thickness.

The effect of hypothermia and hyperthermia on photodynamic therapy of normal brain

The effect of whole body hyperthermia and hypothermia in conjunction with photodynamic therapy (PDT) was determined on normal rat brain. Hyperthermia animals (Group I, n = 18) were warmed until their core body temperature reached 40 degrees C, (brain temperature, 39.7 +/- 0.5 degree C) and maintained at 40 +/- 1 degree C for 30 minutes prior to and after PDT. Hypothermia (Group II, n = 31) animals were cooled to 30 +/- 1 degree C (brain temperature, 29.3 +/- 0.4 degree C) for 1 hour. PDT treatment was performed, and the body temperature of the animals was maintained at 30 degrees C for 2 hours post-PDT. A population of animals was subjected to PDT under normothermic (Group III, n = 16; body temperature, 37 +/- 1 degree C; brain temperature, 36.7 +/- 0.8 degree C) conditions and treated in a manner identical to that of hyperthermic animals. PDT was performed with 17 J/cm2, 35 J/cm2, or 70 J/cm2 (100 mW/cm2). Photofrin (Quadralogic Technologies Ltd., Vancouver, Canada) (12.5 mg/kg) was injected intraperitoneally 48 hours prior to laser treatment on all three groups. Wet-dry weight measurements were obtained on a separate set of all three groups of animals (n = 27). Cortical lesion depths were measured, and pathological evaluation was made at 24 hours post-PDT. No difference in the wet-dry weight measurements or histopathology was present between the three groups of animals. Lesion depths for Group I animals did not significantly differ from Group III animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical and histologic effects of extreme intraocular hypothermia

Rabbit eyes in which vitrectomy was performed underwent three hours of hypothermic perfusion with either 22 C or 2 C lactated Ringer's solution. Clinical, electroretinographic, fluorescein angiographic, histologic, and ultrastructural examinations were performed. No changes were noted in eyes cooled with 22 C fluid. Reversible lens opacities, equivocal electroretinographic changes, and subclinical retinal detachments were found in the eyes cooled with 2 C fluid. Extreme cooling of eyes is not advisable, although temperatures of 22 C are well tolerated.