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Biomedical subjects

A Y Chow

Publications and source records attributed to A Y Chow.

At least 19 recordsLinked to original sources
PubMed2001

Immunohistochemical studies of the retina following long-term implantation with subretinal microphotodiode arrays.

This study evaluates the feline retina following surgical placement of a semiconductor-based microphotodiode array (MPA) into the subretinal space. Post-operative evaluations of implant durability and clinical biocompatibility have been carried out in these animals. Here, we examine the integrity of the implanted retina using anatomical techniques and immunocytochemical metabolic indicators. After appropriate fixation, the retina was divided into strips to compare areas directly over the implant versus those adjacent to the implant or in the opposite, unimplanted eye. In addition to histological analyses, the distribution of glial fibrillary acidic protein (GFAP), Na, K-ATPase, and the neurotransmitters (glutamate, glycine, and GABA) was examined using immunohistochemistry. Directly above the implant there was a near-complete loss of photoreceptor outer and inner segments and the outer nuclear layer. In comparison, the retina immediately adjacent to the implant appeared normal. In the inner nuclear layer overlying the implant, some cellular disorganization was present, however, the content was not significantly reduced. Also GFAP was up-regulated in the Müller cells directly overlying the MPA, but the retina adjacent to the implant showed a normal distribution of GFAP in the astrocytes located in the ganglion cell layer. The distributions of Na, K-ATPase adjacent to and overlying the implant were not different. Glutamate showed a decrease in overall labeling, but no change in the inner retinal layers. Glycine was found to be up-regulated in the inner nuclear layer immediately overlying the implant, while GABA showed decreased labeling over the MPA. Since photoreceptors overlying the implant degenerate, we compared the changes observed in the implanted retina to those in the Abyssinian cat model of photoreceptor degeneration. Generally, the retinal changes observed over the implant were similar to those seen in the Abyssinian cat, indicating that they may be associated with photoreceptor degeneration. Future studies will concentrate on MPAs designed to improve circulation to the outer retina which may decrease cell loss.

Animals↗
PubMed2001

Visual evoked potentials to infrared stimulation in normal cats and rats.

The absence of effective treatments for retinal degenerative diseases has inspired several laboratories to pursue the development of a retinal prosthetic. In our laboratory, we have focused on the subretinal approach, using an array of photodiodes housed within a silicon chip. These photodiodes generate electrical current in response to wavelengths ranging from 500-1100 nm. Because the native retina is traditionally thought to be insensitive to wavelengths beyond approximately 750 nm, we and others have attempted to isolate implant-mediated electrophysiological responses from those of the native retina by using longer wavelength stimuli in the near infrared range. Evoked potentials recorded over the visual cortex in response to infrared stimuli have been reported as evidence of a functional subretinal implant due to the typical physiological characteristics of the waveform: a direct relationship between amplitude and intensity, increased amplitude over the visual cortex, and repeatability of the response. However, these results should be interpreted with caution since here we report an unappreciated sensitivity of the native retina to infrared light under dark-adapted conditions.

Animals↗
PubMed2001

Implantation of silicon chip microphotodiode arrays into the cat subretinal space.

There are currently no therapies to restore vision to patients blinded by photoreceptor degeneration. This project concerns an experimental approach toward a semiconductor-based subretinal prosthetic designed to electrically stimulate the retina. The present study describes surgical techniques for implanting a silicon microphotodiode array in the cat subretinal space and subsequent studies of implant biocompatibility and durability. Using a single-port vitreoretinal approach, implants were placed into the subretinal space of the right eye of normal cats. Implanted retinas were evaluated post-operatively over a 10 to 27 month period using indirect ophthalmoscopy, fundus photography, electroretinography, and histology. Infrared stimulation was used to isolate the electrical response of the implant from that of the normal retina. Although implants continued to generate electrical current in response to light, the amplitude of the implant response decreased gradually due to dissolution of the implant's gold electrode. Electroretinograms recorded from implanted eyes had normal waveforms but were typically 10-15% smaller in amplitude than those in unimplanted left eyes. The nonpermeable silicon disks blocked choroidal nourishment to the retina, producing degeneration of the photoreceptors. The laminar structure of the inner retinal layers was preserved. Retinal areas located away from the implantation site appeared normal in all respects. These results demonstrate that silicon-chip microphotodiode-based implants can be successfully placed into the subretinal space. Gold electrode-based subretinal implants, however, appear to be unsuitable for long-term use due to electrode dissolution and subsequent decreased electrical activity.

Animals↗
PubMed2000

Scar remodeling after strabismus surgery.

PURPOSE: We sought to investigate abnormal scar lengthening after strabismus surgery. METHODS: Patients with overcorrection after strabismus surgery or undercorrection after extraocular muscle resection underwent exploration of previously operated muscles. Abnormal findings were documented by inspection and photography, and repair was undertaken at first with absorbable sutures and later with nonabsorbable sutures. RESULTS: Lengthened scars, consisting of amorphous connective tissue, were repaired on 198 muscles in 134 procedures by excision of the scar and reattachment of the muscle to sclera; absorbable sutures were used in 64 procedures, and nonabsorbable sutures were used in 70 procedures. Thirty-one procedures were followed by partial recurrence of the original overcorrection; 7 of these had documented restretching. The use of nonabsorbable sutures decreased the recurrence of strabismus from 42% to 6%. Factors that distinguished patients with stretched scars from patients with classic slipped muscles included minimal or no limitation of versions, less separation of the tendons from sclera, and thicker appearance of the scar segments. CONCLUSIONS: A lengthened or stretched remodeled scar between an operated muscle tendon and sclera may contribute to variability of outcome after strabismus repair, even years later. Definitive repair requires firm reattachment of tendon to sclera with nonabsorbable suture support.

Adolescent↗
PubMed1999

Subretinal implantation of semiconductor-based photodiodes: progress and challenges.

Retinal diseases that result in photoreceptor degeneration may spare the inner retinal layers. This review concerns a prosthetic approach to restoring visual function through the use of a semiconductor-based microphotodiode array implant, designed to be placed under the neural retina in the subretinal space. The fundamental idea is that current generated by the device in response to light stimulation will alter the membrane potential of overlying neurons and thereby activate the visual system. Initial acute studies indicated that the implant will function in the subretinal space in the absence of an external power supply. More recent and ongoing studies involve chronic subretinal implantations in normal animals. Post-operative studies have demonstrated that implant function will persist for many months. These chronic studies have also assessed the biocompatibility of the implant. Photoreceptors are lost directly overlying the implant, due to the blockade of choroidal circulation to the outer retina by the solid disk device. In comparison, the inner retina maintains its characteristic lamellar structure. Away from the implant site, the retina retains normal anatomy and function. Future studies are needed to determine whether the implant can establish a functional connection to the inner retina and to determine the quality of this connection.

Animals↗
PubMed1998

Subretinal semiconductor microphotodiode array.

BACKGROUND AND OBJECTIVE: To examine the function of a semiconductor microphotodiode array (SMA) surgically implanted in the subretinal space. MATERIALS AND METHODS: Positive-intrinsic layer-negative (PiN) or negative-intrinsic layer-positive (NiP) SMAs were surgically placed into the subretinal space of rabbits through a pars plana incision and a posterior retinotomy. The implants required no external connections for power and were sensitive to light over the visible and infrared (IR) spectrum; IR stimuli were used to isolate implant-mediated responses from the activity of native photoreceptors. A stimulator ophthalmoscope was used to superimpose IR stimuli on the implant and adjacent retinal areas, and responses were recorded during the postoperative recovery period. SMA responses were also evaluated in vitro. The animals were given lethal anesthetic overdoses, and the retinas were examined histologically. RESULTS: The in vitro implant response consisted of an electrical spike, followed by a small-amplitude DC offset that followed the time course of the IR stimulation, and an overshoot at the stimulus offset. The SMAs placed in the subretinal space retained a stable position and continued to function throughout the postoperative period. The SMA responses recorded in vivo included additional slow-wave components that were absent from the in vitro recordings. These responses reverted to the in vitro configuration following the death of the animal. There was a significant loss of retinal cells in areas overlying the implant, and the retina appeared normal away from the implant and surgical site. CONCLUSION: SMAs can be successfully implanted into the subretinal space and will generate current in response to light stimulation during an extended period of time.

Animals↗
PubMed1997

Subretinal electrical stimulation of the rabbit retina.

A number of disorders results in photoreceptor degeneration, yet spare the inner retinal layers. We are investigating the possibility that retinal function may be restored in such a situation by electric current applied from the subretinal space. In the present study, bipolar strip electrodes receiving electric current from external photodiodes were implanted into the subretinal space of adult rabbits. Recordings were made from the scalp overlying the visual cortex in response to photic flash stimulation of one eye before surgery. This was compared to the visual cortex response caused by subretinal electrical stimulation of the same eye from an implanted strip electrode. Electric current to the strip electrode was provided by an externally connected photodiode that was stimulated at a remote location with a photoflash. The electrical stimulus was recordable as a brief electrical implant spike during stimulation. In addition, after the implant spike, cortical responses were obtained in response to subretinal electrical stimulation that resembled closely the normal light induced visual evoked potential produced by the pre-implanted eye. These results indicate that the visual system can be activated by electrical stimulation from the subretinal space and indicate that this approach may provide a means to restore vision to eyes blinded by outer retinal disease.

Animals↗
PubMed1984

Evulsion of the optic nerve in association with basketball injuries.

A case of optic nerve evulsion resulted from a finger poke injury to the globe during a basketball game. Significantly, almost no external damage to the eye was evident. The damage may be caused by sudden forceful anterior-posterior displacement and rotation of the globe, inducing a tear of the optic nerve at the papilla. The seriousness and permanence of this injury stress the importance of protective eye wear in contact sports.

Adolescent↗
PubMed1978

Percutaneous absorption of hexachlorophene in rats, guinea pigs and pigs.

A comparative study of the percutaneous absorption of hexachlorophene (HCP) was undertaken in rats, guinea pigs and pigs. [14C]Hexachlorophene ([14C]HCP) was applied evenly over the shaved back of the animals at a dose of 40 microgram/cm2 skin surface. Urine and feces were collected at 24-h intervals for 5 days from animals kept in metabolism cages. Different methods were used for quantitating the percutaneous absorption of HCP. This study showed that skin permeability to HCP decreased in the following order: rat, guinea pig and pig. The permeability characteristics of the pig skin to topically applied HCP were comparable to the published human data. We suggest that pig may be a suitable animal model for studying the percutaneous absorption of antimicrobial drugs.

Animals↗