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Artificial vision and retinal prostheses
Published in A Peyman MD Gholam, A Meffert MD Stephen, D Conway MD FACS Mandi, Chiasson Trisha, Vitreoretinal Surgical Techniques, 2019
Humayun Mark S, Lakhanpal Rohit R, Weiland James D
The subretinal approach to retinal prosthesis design involves the implantation of a microphotodiode array between the bipolar cell layer and the RPE (Fig. 72.3a). This is accomplished surgically via either an intraocular approach through a retinotomy site (ab interno) or a transscleral approach (ab externo). The subretinal prosthesis, using microphotodiodes (solar cells) alone as a powering mechanism, offers an attractive solution to enhance the vision of patients affected by RP and ARMD. However, several limitations currently hinder its realization. Of primary concern is the inefficiency of current photodiode technology. The illumination levels required to achieve adequate electrical current generation are not realistically attainable, as the solar cells need to generate electrical energy many orders of magnitude greater than that which is currently possible.55–;62 This limitation jeopardizes the passive, all-inclusive nature of a subretinal prosthesis, which more than likely will require active power supplementation from an external source in order to achieve threshold current levels. Because a prosthetic device based solely on microphotodiodes (solar cells) is very unlikely to be realizable, this section will only focus on subretinal prostheses that have enough power to function as such. The subretinal approach was initially championed by Chow and colleagues,55–;62 who claimed that a simple photodiode (solar cell) would be sufficient without any need for other electronics or power or an external camera.
Rheology of Retinal Disorders
Published in Gordon D. O. Lowe, Clinical Blood Rheology, 2019
Gordon D. O. Lowe, W. S. Foulds
The intraocular pressure is normally high compared to other tissue pressures, presumably for maintenance of eyeball shape and normal refraction. This high pressure tends to cause the retinal veins to collapse, which is prevented by their high flow resistance, particularly at the level of the lamina cribrosa, where the central retinal vein egresses from the eye and where the central retinal artery and vein are in close contact.9,10 McGrath et al.11 have pointed out that the high flow resistance and anatomical peculiarities of the retinal venous circulation severely limit its ability to adapt to increased blood viscosity, and have suggested that this may be the principal reason for the frequent involvement of the retinal circulation in blood hyperviscosity states.
Hereditary Malignant Melanoma and the Fammm Syndrome
Published in Henry T. Lynch, Ramon M. Fusaro, Hereditary Malignant Melanoma, 2019
Henry T. Lynch, Ramon M. Fusaro, Lavonne Johnsen, Jane F. Lynch
Intraocular malignant melanoma (IOM) comprises a relatively small proportion of all varieties of malignant melanoma. However, it is the most common malignant neoplasm of the eye. This lesion occurs most frequently after the 5th and 6th decades of life and is extremely rare in children. It seems to have a definite racial predilection, being most frequently found in Caucasians and rarely in Negroes. This racial difference is reflected by the Registry of Ophthalmic Pathology of the Armed Forces Institute of Pathology, where the ratio of ocular melanoma in Caucasians to non-Caucasians is 250:1.27 There is no significant difference in frequency in men and women.
Current Concepts of the Uveitis-Glaucoma-Hyphema (UGH) Syndrome
Published in Current Eye Research, 2023
Meera S. Ramakrishnan, Kenneth J. Wald
The uveitis-glaucoma- hyphema syndrome, “UGH,” is a well-known, but incompletely characterized and understood, iatrogenic ophthalmologic condition. Causation is conventionally believed to be contact between an intraocular lens component and uveal tissue. Aspects of the condition were originally encountered during the early evolution of cataract surgery – prior to the advent of the operating microscope (Figure 1),1 and then formally given the eponym “UGH” during the initial period of intraocular lens implantation.2 Over the ensuing decades, its importance declined largely due to improved surgical techniques and implant design, however the prevalence is again believed to be increasing due to the rising incidence of late intraocular lens dislocations3,4 and the surgical repair procedures to remedy that condition.
Bootstrap Outlier Identification in Clinical Datasets for Lens Power Formula Constant Optimization
Published in Current Eye Research, 2023
Achim Langenbucher, Nóra Szentmáry, Alan Cayless, Jascha Wendelstein, Peter Hoffmann
Lens power calculation formulae based on ocular biometry are widely used to calculate the appropriate power of intraocular lens implants (IOLs) prior to cataract surgery.1 Lens power formulae are aimed to be valid for the general case, and can customized to different lens types or styles, the surgical environment and the surgeon, the biometer used for ocular biometry, and also for the ethnicity and characteristics of the study population, by means of formula constants.2 Such formula constants may interact with the power of the emmetropic lens (e.g. in the old SRK formula;3 with the effective axial lens position (ELP), or other parameters. In most of the lens power formulae we use a single formula constant (such as in the SRKT, HofferQ, or Holladay formula), but other formulae use more than one constant (e.g. the Haigis, the Barrett, or the Castrop formula) to individually tune the formula to the lens specifications.1,4
Classification of Seasonal Hyperacute Panuveitis (SHAPU)
Published in Ocular Immunology and Inflammation, 2022
Ranju Kharel Sitaula, Anadi Khatri, Pratap Karki, Sagun Narayan Joshi, Haramaya Gurung, Eliya Shrestha, Indraman Maharjan, Ananda K Sharma, Madan Prasad Upadhyay
Moderate SHAPU cases have more severe anterior chamber reaction with ≥1+ cell and ≥1+ flare (based on SUN’s Classification11). The fibrinous exudates may be seen in front of the pupil and iris crypts with or without the presence of hypopyon. The hypopyon is mobile, predominantly yellow in color, plano or convex in shape (Figure 2a). The lens is clear but the pupillary light reflex may be altered due to the exudates present in the vitreous leading to subtotal leucokoria in a red eye. But the fundal glow is still appreciable and the visible portion of retina appears apparently normal. The B scan ocular ultrasonography shows hyperechoic shadows in anterior and mid vitreous with attached retina but increased thickness of retinochoriodal complex (Figure 2b). Intraocular pressure can be (i) normal, (ii) lower due ciliary shutdown, or (iii) raised due to the outflow obstruction in the trabecular meshwork by the cells/exudates or pupillary block secondary angle closure glaucoma. Hence, moderate SHAPU cases have features predominantly of anterior and intermediate uveitis, which may start involving the retina if appropriate interventions are not taken at this stage.