Heat Effects on the Retina

Abstract

* BACKGROUND AND OBJECTIVE: To study the heat and power dissipation effect of an intraocular electronic heater on the retina. The determination of thermal parameters that are nonharmful to the retina will aid in the development of an implantable intraocular electronic retinal prosthesis. * MATERIALS AND METHODS: In dogs, five different retinal areas were touched with a custom intraocular heater probe (1.4 × 1.4 × 1.0 mm) for 1 second while the heater dissipated 0 (control), 10, 20, 50, or 100 mW. In a second protocol, the heater was mechanically held in the vitreous cavity while dissipating 500 m W for 2 hours while monitoring intraocular temperature. The animals were observed for 4 weeks with serial fundus photography and electroretinography. The procedure was then repeated in the fellow eye. The dogs were killed and both eyes were enucleated and submitted for histology. * RESULTS: In experiments using protocol 1, heater settings of 50 mW or higher caused an immediate visible whitening of the retinal tissue. Histologically, this damage was evident only if the eyes were immediately enucleated. Permanent damage was caused by heater settings of 100 m W or higher. Under protocol 2, no ophthalmologic, electroretinography, or histologic differences were noted between the groups. Temperature increases of 5°C in the vitreous and 2°C near the retina were noted. * CONCLUSIONS: The liquid environment of the eye acts as a heat sink that is capable of dissipating a significant amount of power. An electronic chip positioned away from the retina can run at considerably higher powers than a chip positioned on the retinal surface. [Ophthalmic Surg Lasers Imaging 2003;34:114-120.]

Investigation of thermal effects of infrared lasers on the rabbit retina: a study in the course of development of an active subretinal prosthesis

“…Our results show no acute thermal damage to the retina from IR power up to 40 mW. Our results are consistent with those of Piyathaisere et al [31], who examined the influence of heat on the retina. They examined retinal damage after direct contact with an intraocular heat probe.…”

“…Their experiments showed that heater settings of 50 mW (255 mW/cm 2 ) or higher caused immediate visible damage to the retinal tissue. Comparison of the results of our examination with those of Piyathaisere et al [31] is very difficult, because in our experiments, the thermal source is in the subretinal space. We therefore hesitate to conclude that no damage would occur with our setup below 60 mW (∼255 mW/cm 2 ).…”

Evaluation of extraocular electrodes for a retinal prosthesis using evoked potentials in cat visual cortex

“…If electronics for neurostimulation are to be implanted wholly within the globe, it can be challenging to ensure hermetic encapsulation of the device. Electronics placed intraocularly also risk damage to the retina by heat, 23 and replacement of damaged devices may be difficult. The use of a per-scleral 15 connection to extraocular electronic devices creates a defect in the integrity of the globe which may be a pathway for infection.…”

Retinal Prosthesis Safety: Alterations in Microglia Morphology due to Thermal Damage and Retinal Implant Contact

“…However, direct temperature in the neural retina was not assessed. More recent studies have assessed the effects of applying heat to the retina directly, finding that 50 mW induced retinal whitening, 9 while 40 mW induced a 4.58C increase in retinal temperature. 39 This second study, by Sailer et al, suggested that a 3.28C increase induced by 15 mW (4.8 mW/mm 2 ) of heat applied by infrared radiation was a tolerable increase for ocular tissues, based on intact gross morphology of the retina.…”

“…Determination of temperatures that cause thermal damage can be used to assist in the design of implant circuitry and stimulation protocols to maximize the performance of visual prostheses while ensuring that patient safety is not compromised. While power dissipations above 50 mW have been reported to cause immediate trauma, inducing retinal whitening, edema, and other lesions, 9 the thermal increases that can be tolerated by retinal tissues remain largely unknown.…”