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Mark S Humayun

Publications and source records attributed to Mark S Humayun.

51 records · Page 3Linked to original sources
PubMed2002

A new 25-gauge instrument system for transconjunctival sutureless vitrectomy surgery.

OBJECTIVES: To introduce and evaluate the infusion and aspiration rates and operative times of the 25-gauge transconjunctival sutureless vitrectomy system (TSV) DESIGN: In vitro experimental and comparative interventional study. PARTICIPANTS AND CONTROLS: Twenty eyes of 20 patients underwent a variety of vitreoretinal procedures using the 25-gauge TSV, including idiopathic epiretinal membrane (n = 10), macular hole (n = 4), rhegmatogenous retinal detachment (n = 3), branch retinal vein occlusion (n = 2), diabetic vitreous hemorrhage (n = 1), and 20 cases similar in diagnosis and severity were matched to provide comparison between duration of individual portions of the surgical procedures with the existing 20-gauge vitrectomy system. METHODS: Description of the 25-gauge TSV is provided; infusion and aspiration rates of the 25-gauge and standard 20-gauge vitrectomy system were measured in vitro using balanced saline solution and porcine vitreous for several levels of aspirating power and bottle height, and operating times of individual portions of surgical procedures were measured for the 25-gauge and 20-gauge vitrectomy system. MAIN OUTCOME MEASURES: Infusion, aspiration rates, and operative times of the 20-gauge and 25-gauge vitrectomy system. RESULTS: Infusion and aspiration rates of the 25-gauge TSV system were reduced by an average of 6.9 and 6.6 times, respectively, compared with the 20-gauge system when balanced saline solution was used. The average flow rate of the Storz 25-gauge cutter (at 500 mmHg, 1500 cuts per minute [cpm]) was 40% greater than that of the 20-gauge pneumatic cutter (at 250 mmHg, 750 cpm) but about 2.3 times less than the 20-gauge high-speed cutter (at 250 mmHg, 1500 cpm). Mean total operative time was significantly greater for the 20-gauge high-speed cutter (26 minutes, 7 seconds) than for the 25-gauge vitrectomy system (17 minutes, 17 seconds) (P = 0.011). CONCLUSIONS: Although the infusion and aspiration rates of the 25-gauge instruments are lower than those for the 20-gauge high-speed vitrectomy system, the use of 25-gauge TVS may effectively reduce operative times of select cases that do not require the full capability of conventional vitrectomy.

Diabetes Complications↗
PubMed2002

Limited macular translocation: current concepts.

Macular translocation is a promising treatment modality that offers patients a chance to improve their vision, potentially to a level that may allow reading and driving. Proper patient selection avoids surgery in eyes with permanently damaged central retina and identifies eyes with the greatest potential for good visual outcomes. Functionally, those patients with recent-onset subfoveal CNV without permanent foveal neurosensory retina damage have the greatest likelihood of good visual outcome. Use of fixation evaluation by means of the SLO appears to greatly optimize patient selection. Anatomically, the best candidates have small minimum desired translocations and healthy RPE-Bruch membrane-choriocapillaris complex beyond the borders of the lesion. The exact role of limited macular translocation for the management of subfoveal CNV in this era of photodynamic therapy remains to be evaluated, and a pilot multicenter, randomized, clinical trial is under way to compare the efficacy of limited macular translocation with photodynamic therapy in eyes with subfoveal CNV secondary to AMD.

Choroidal Neovascularization↗
PubMed2002

Ganglion cell responses to retinal light stimulation in the absence of photoreceptor outer segments from retinal degenerate rodents.

PURPOSE: To determine whether a severely degenerated retina without photoreceptor outer segments and a non-recordable electroretinogram (ERG) can still show retinal ganglion cell (RGC) responses to retinal light stimulation. METHODS: The authors measured ERGs and retinal surface RGC responses from six week old rd mice and three month old homozygous S334ter line3 rats. Animal eyes were also studied by light microscopy, transmission electron microscopy, and immunohistochemistry (rats). RESULTS: The corneal ERGs were non-recordable and no photoreceptor outer segments were found in either retinal degeneration model. A few cell bodies (without outer segments) that were immunoreactive for cone opsin and rhodopsin were found in the outer nuclear layer of the rats. Light-driven ON-RGC responses, however, were recordable from six week old rd mice. In addition, light-driven ON and OFF-RGC responses were recordable from three month old homozygous S334ter line 3 rats. CONCLUSIONS: This study suggests that despite the apparent absence of photoreceptor outer segments and a non-recordable ERG, ganglion cell responses to retinal light stimulation may remain preserved in some severe retinal degenerate transgenic rodents.

Animals↗
PubMed2002

In vitro electrical properties for iridium oxide versus titanium nitride stimulating electrodes.

Stimulating electrode materials must be capable of supplying high-density electrical charge to effectively activate neural tissue. Platinum is the most commonly used material for neural stimulation. Two other materials have been considered: iridium oxide and titanium nitride. This study directly compared the electrical characteristics of iridium oxide and titanium nitride by fabricating silicon substrate probes that differed only in the material used to form the electrode. Electrochemical measurements indicated that iridium oxide had lower impedance and a higher charge storage capacity than titanium nitride, suggesting better performance as a stimulating electrode. Direct measurement of the electrode potential in response to a biphasic current pulse confirmed that iridium oxide uses less voltage to transfer the same amount of charge, therefore using less power. The charge injection limit for titanium nitride was 0.87 mC/cm2, contradicting other reports estimating that titanium nitride was capable of injecting 22 mC/cm2. Iridium oxide charge storage was 4 mC/cm2, which is comparable to other published values for iridium oxide. Electrode efficiency will lead to an overall more efficient and effective device.

Coated Materials, Biocompatible↗
PubMed2002

Increased spontaneous retinal ganglion cell activity in rd mice after neural retinal transplantation.

PURPOSE: To study the functional success of neural retinal transplantation by means of retinal surface ganglion cell recordings. METHODS: Eight-week-old C3H/HeJ (rd/rd) retinal degeneration mice received transplants (subretinal) in one eye only of neural retinal tissue isolated from newborn normal C57/BL6J mice. Four weeks after transplantation, ganglion cell responses were recorded directly from the retinal surface over the transplant, with a differential bipolar surface electrode. Measurements were performed, both with and without light stimulation. Similar recordings were performed in nontransplant areas of the transplant-recipient eyes, and in age-matched sham-treated and untreated control eyes. After the recordings, the eyes were processed for light and transmission electron microscopy. RESULTS: Histologic examination showed that in some areas, transplanted cells were organized into small sheets and differentiated into photoreceptors with outer segments in intimate contact with the host RPE. No light-driven ganglion cell responses were recordable from the transplant-recipient or control eyes. However, the spontaneous ganglion cell activity was higher in the transplant areas (mean: 10.8 +/-12.0 spikes/1.6 sec) compared with nontransplant areas of these recipient eyes (mean: 2.4 +/- 5.7spikes/1.6 sec; P < 0.001), sham-treated eyes (mean: 2.5 +/- 4.8 spikes/1.6 sec; P < 0.001), and the untreated control eyes (mean: 2.2 +/- 4.4 spikes/1.6 sec; P < 0.001). CONCLUSIONS: Subretinal transplantation of neural retinal tissue results in a local increase of spontaneous ganglion cell activity. The increased activity may be due to the release of neurochemically active substances as a result of the presence of the transplant. Although light responses were not recordable, the technique of retinal surface ganglion cell recording may be useful for assessing the functional success of transplantation.

Action Potentials↗
PubMedDate not supplied

Comparison of electrical stimulation thresholds in normal and retinal degenerated mouse retina.

PURPOSE: To compare the threshold for electrically elicited action potentials of retinal ganglion cells in normal mouse retina and photoreceptor degenerated (rd) mouse retina. METHODS: Microelectrode recordings were made from retinal ganglion cells of normal and rd mice. Mice with a genetically based retinal degeneration (rd mice) were grown to the age of 16 weeks, when light-evoked responses could no longer be recorded. A bare wire was placed in the vitreous to stimulate the retina with charge-balanced current pulses. The following pulse shapes were investigated: single, square biphasic pulse, single sine wave, and biphasic pulse trains. RESULTS: Normal mice had significantly lower stimulus thresholds than rd mice for all pulse shapes. In normal and rd mice, short pulses were more efficient with respect to total charge used, but required a higher current. In normal mice, sine wave stimulation was significantly more efficient than a biphasic pulse of the same duration. No difference was noted between sine wave and square wave stimulation in rd mice. Pulse trains offered little benefit over single pulses. CONCLUSION: The amount of electrical charge required to elicit an action potential is dependent on the condition of the retina and the shape of the stimulus pulse used to deliver the charge.

Action Potentials↗
PubMedDate not supplied

Retinal prosthesis for the blind.

Most of current concepts for a visual prosthesis are based on neuronal electrical stimulation at different locations along the visual pathways within the central nervous system. The different designs of visual prostheses are named according to their locations (i.e., cortical, optic nerve, subretinal, and epiretinal). Visual loss caused by outer retinal degeneration in diseases such as retinitis pigmentosa or age-related macular degeneration can be reversed by electrical stimulation of the retina or the optic nerve (retinal or optic nerve prostheses, respectively). On the other hand, visual loss caused by inner or whole thickness retinal diseases, eye loss, optic nerve diseases (tumors, ischemia, inflammatory processes etc.), or diseases of the central nervous system (not including diseases of the primary and secondary visual cortices) can be reversed by a cortical visual prosthesis. The intent of this article is to provide an overview of current and future concepts of retinal and optic nerve prostheses. This article will begin with general considerations that are related to all or most of visual prostheses and then concentrate on the retinal and optic nerve designs. The authors believe that the field has grown beyond the scope of a single article so cortical prostheses will be described only because of their direct effect on the concept and technical development of the other prostheses, and this will be done in a more general and historic perspective.

Blindness↗
PubMedDate not supplied

Transvitreal limited arteriovenous-crossing manipulation without vitrectomy for complicated branch retinal vein occlusion using 25-gauge instrumentation.

PURPOSE: To evaluate a new technique, 25-gauge transvitreal limited arteriovenous-crossing manipulation without vitrectomy (LAM), for the treatment of branch retinal vein occlusion (BRVO) complicated by macular hemorrhage and/or macular edema recalcitrant to grid laser photocoagulation. METHODS: Twelve eyes of 12 patients underwent LAM for BRVO performed by a single surgeon (M.S.H.) using the 25-gauge nitinol flexible-extendable blunt pick. The presence or absence of intraoperative reperfusion visualization, pre- and postoperative visual acuity, macular thickness as measured by optical coherence tomography, intraocular pressure, and lens status were evaluated. RESULTS: Restoration of blood flow was noted in all patients and was based on intraoperative reestablishment of a red column of erythrocytes through the previously closed vessel. Mean visual acuity improved from 20/200 (logarithm of the minimal angle of resolution [LogMAR] +/- SD, 1.00 +/- 0.32) preoperatively to 20/70 (LogMAR +/- SD, 0.56 +/- 0.28) (P = 0.0003) at the final visit. Eleven (92%) of 12 eyes had >or=2 lines of visual improvement. Five eyes (45%) had final visual acuity of 20/50 or better. Mean macular thickness +/- SD improved from 401.0 +/- 73.2 to 178.7 +/- 19.6 microm (P < 0.0001) at the final visit. No statistically significant difference was noted in cataract progression or intraocular pressure. Mean follow-up +/- SD was 49.9 +/- 19.6 weeks. All patients were observed for at least 12 weeks. CONCLUSION: LAM may achieve outcomes comparable with those of arteriovenous adventitial sheathotomy for complicated BRVO.

Adult↗
PubMedDate not supplied

Enucleated eye model for intraocular retinal prosthesis implantation.

An enucleated porcine eye model was developed to assess intraocular retinal prosthesis implantation surgery. The surgical technique consists of corneal and crystalline lens removal, keratoprosthesis replacement, and vitrectomy. To test the eye model, the scleral incision was increased to 5 mm and a 10-mm wide retinal prosthesis folded and inserted. One retinal tack was used to fix the prosthesis to the retina. A retinal prosthesis array was inserted without significant damage to the array and conformed to the curvature of the eye. Fundus photography and optical coherence tomography were performed at the end of surgery.

Animals↗
PubMedDate not supplied

Improvements after sheathotomy for branch retinal vein occlusion documented by optical coherence tomography and scanning laser ophthalmoscope.

A 55-year-old man presented with decreased visual acuity of 20/400 and central foveal thickness of 450 microm as measured using optical coherence tomography due to a superior temporal branch retinal vein occlusion. Adventitial sheathotomy was performed with a 25-gauge sutureless transconjunctival vitrectomy system. At 1 day, 1 month, and 6 months postoperatively, visual acuity improved to 20/80, 20/40, and 20/30 OS, respectively; central foveal thickness decreased to 228, 195, and 161 microm, respectively; and the scanning laser ophthalmoscope microperimetry showed improved retinal sensitivity and fixation pattern with more stable and central fixation at both postoperative visits. This case indicates optical coherence tomography can detect an early positive effect of sheathotomy surgery on macular edema, and scanning laser ophthalmoscope can document associated improvement in fixation stability.

Decompression, Surgical↗
PubMedDate not supplied

Heat effects on the retina.

BACKGROUND AND OBJECTIVE: To study the heat and power dissipation effect of anintraocular 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 x 1.4 x 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 mW 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 eyeswere immediately enucleated. Permanent damage was caused by heater settings of 100 mW or higher. Under protocol 2, no ophthalmologic, electroretinography, or histologic differences were noted between the groups. Temperature increases of 5 degrees C in the vitreous and 2 degrees 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.

Animals↗
PubMedDate not supplied

Retinal thickness in normal and RCD1 dogs using optical coherence tomography.

BACKGROUND AND OBJECTIVE: To compare retinal thickness and retinal nerve fiber layer (RNFL) thickness values obtained by optical coherence tomography (OCT) in normal dogs and dogs with rod-cone dysplasia type 1 (RCD1). MATERIALS AND METHODS: Eight eyes of 6 normal hound-bred dogs and 12 eyes of 6 dogs with RCD1, 2 to 5 years of age, were examined using the Fast RNFL Thickness, Fast Macular Thickness, and line scan protocols of OCT. RESULTS: Retinal thickness was significantly higher in the tapetal fundus than in the non-tapetal fundus, in both normal (P = .0036) and RCD1 (P < .0001) dogs. Superotemporal, superonasal, and inferior retinal thickness values were significantly higher in normal dogs (P < .0001). Area centralis thickness was 183.5 +/- 10.66 microm in normal dogs and 136.16 +/- 13.12 microm in RCD1 dogs (P < .0001). There was no difference in RNFL thickness between normal and RCD1 dogs (P > .05). CONCLUSION: OCT scanning in dogs is considered to be a useful method of evaluation in future retinal studies in this animal model.

Animals↗