Duplex Retina.pdf

UV damages the cornea and anterior structures Poly. carb. absorbs most UV Visible light: 400-700 nm Our retina has 2 types of photoreceptors: cones and rods We have 2 systems: One adapted for day vision: photopic One adapted for night vision: scotopic Rods Absence of color discrimin...

UV damages the cornea and anterior structures Poly. carb. absorbs most UV Visible light: 400-700 nm Our retina has 2 types of photoreceptors: cones and rods We have 2 systems: One adapted for day vision: photopic One adapted for night vision: scotopic Rods Absence of color discrimination Poor acuity: ~20/200 Best discrimination about 30 minutes after darkness Predominate in peripheral vision ○ Peripheral vision is more sensitive in dim conditions Motion better detected in peripheral vision Discs are shed and recycled/engulfed by RPE Retinitis Pigmentosa Bony spicules are the dark areas that may present in RP. Occurs due to the photoreceptor cell death by apoptosis All disc waste remains in the eye and damages the retina Some RP patients may have healthy looking retinas but have trouble seeing; need ERG Not all RP are in genetics Rod photopigment Rhodopsin: found in rod’s discs Each eye has 120 million rods Each rod has 1,000 discs Each disc has 10,000 molecules of rhodopsin There are ~10^15 molecules of rhodopsin per eye Absorption of one quantum photon enough to activate a rod The large number allows the eye to capture light and increases sensitivity at nighttime Weber’s fraction= Delta I/I = 0.14 ○ How much change in stimuli (or intensity) needed to change vision Radiometer measures amount of transmitted light as a function of wavelength %Absorbed light = 100 - %Transmitted Light If 30% is transmitted then 70% is absorbed Rhodopsin molecules If you flash a very bright light, you bleach 100% of the rod photopigment (rhodopsin) ○ Ex: a bright sunny day. You enter a dark movie theater. 50% of rhodopsin recovers in 5 minutes If you bleach 100 molecules of rhodopsin then… One quantum (photon) may activate a rod 10 rods activate a ganglion cell But… Many quanta are reflected or absorbed by tissue inner to photoreceptors Less than 20% of quanta incident on the retina are absorbed by rhodopsin 80% of absorption through eye structure & retina before rhodopsin The # of rods in the central fovea decreases with age There is no significant rod cell loss in the aging peripheral retina outside the fovea Cones About 6-7 million per eye Remain constant with age Color sensitive Good acuity: 20/20 Adapt rapidly Weber’s fraction = Delta I/I = 0.015 (better contrast sensitivity) which is less than 0.14 for rods [needs less for change in sensitivity] Cone photopigments 3 photopigments in the eye Erythrolabe (“red”) ○ Also called long-wavelength sensitive cones (LWS) ○ Peaks around 557 nm  ○ About 64% of all cones most present ○ However, variations exists: L/M = 1/1 to 16/1 is normal ○ Concentrated in fovea centralis Chlorolabe (“green”) ○ Also called middle-wavelength sensitive cones (MWS) ○ Peaks around 530 nm ○ About 32% of all cones ○ Concentrated in fovea centralis Cyanolabe (“blue”) ○ Also called short-wavelength sensitive cones (SWS) ○ Peaks around 426 nm ○ Around 5-10% of all cones, peaks at 0.5 degrees from fovea ○ Concentrated outside fovea centralis (*not inside fovea) Cone photopigments recover faster from bleaching 50% of the cone photopigment recovers in 1.5 minutes If you bleach 100 molecules of cone photopigment The eye’s sharpest and most brilliantly colored vision occurs when light is focused on the tiny dimple on the retina called the fovea centralis or macula. This region has exclusively cones and they are smaller and more closely packed than elsewhere on the retina. Macula ~2.5-3 mm, fovea = 0.3 mm. Full moon is about 0.2 mm. Cones are thinner and more densely packed at fovea centralis. Combined cone sensitivity: 555 nm Rod sensitivity: 507 nm Photochromatic interval: Difference in sensitivity between the scotopic and photopic systems for a given wavelength Scotopic system more sensitive than the photopic system at all wavelength except long-wavelength (red) region of spectrum It is nearly zero (rods and cones almost equally sensitive) at 650 nm Beyond 650 nm photopic system slightly more sensitive than the scotopic system Photochromatic interval = 0 at 650 nm Changes depending on wavelength Increases to peak at 500 & then decreases Once it hits 650, it flips to being photopic Scotopic is better until 650 Over 650 = photopic better Purkinje Shift Relative increase in the brightness of longer wavelength stimuli as lighting conditions change from scotopic to photopic Scotopic -> less brightness with long wavelength ○ In dark/dim = more sensitive to blue Photopic -> better/more brightness with long wavelength ○ Bright light = sensitive to red Dark adaptation (10 degrees @ 420 nm) Cone and rod thresholds over time 10 degrees @ 420 nm Dark adaptation curve Sensitivity increases by 5 log units in about 35 minutes (100,000 more sensitive) First arm of curve (cone section) plateaus after 10 minutes First arm of curve breaks after 12 minutes (rod-cone break) The break represents time when rods become more sensitive than cones Second arm of curve plateaus after 35 minutes Represents scotopic threshold for the specific wavelength Difference between the scotopic and photopic plateaus Represent the photochromatic interval for the specific wavelength Stimulus at 650 nm Due to 650mnm &/or small stimuli to fovea only (cones only)**** If light is small and only presents to fovea (only cones and no rods); no rod-cone break - graph will look like this  Since the scotopic system is as sensitive as the photopic system at 650 nm, there is no rod-cone break. The photochromatic interval is zero. 2 ways to find the photochromatic interval Use the scotopic and photopic spectral sensitivity functions and find the difference between the two for the wavelength ○ Use the dark adaptation curve for the specific wavelength and find the difference between the photopic and scotopic plateaus  ○ ○ Location of stimuli: small stimulus (0.5 degrees) confined to fovea (absence of rod-cone break) Dark adaptometry Haag-Streit Dark Adaptometer Goldmann-Weekers scotopic sensitivity SSST-1 Scotopic Sensitivity Tester  Retinitis Pigmentosa Most common inherited retinal dystrophy Affects photoreceptors 50% of cases are isolated with no family history 50% are inherited ○ Autosomal recessive ○ Autosomal dominant: diffuse loss of rod function with relative preservation of cone function ○ X-linked: affects first decade. Partial or complete blindness by third or fourth decade of life. Breakdown of rods first (loss of night vision), then cones Initial loss of peripheral vision Decreased color perception, night blindness and loss of peripheral vision Congenital Stationary Night Blindness Mostly X-linked recessive, more frequent in males Primarily rod dystrophies Defective photoreceptor to bipolar cell signals Transmission of rods to bipolar cells affected  CNSB in horses ○ Associated with leopard complex ○ Interspersed white hair ○ Found in present and pre-historic horses ○ Linked to Lp gene- assigned to equine chromosome ○ Strabismus and nystagmus may be present Early symptoms (similar to RP) ○ Night blindness, spared central vision ○ No rod branch of dark adaptation curve Late symptoms (as cones are affected) ○ Reduced VA ○ High myopia ○ Nystagmus ○ Strabismus Traumatic Retinopathy From traumatic injuries Mimics RP Ask patient about injuries or car accidents Syphilis Rubella Mother transmit to babies Can cause abortions Eruptions in skin Can be mistaken with RP Thioridazine Retinopathy Anti-psychotic; forms crystalline deposits in retina (Confused w/ bone spicules of RP) May cause nyctalopia (inability to see in dim or night conditions) Retinitis Pigmentosa: Argus II: Bionic Eye Gives black and white detection & contrast; not full vision, outlines, borderlines, and patterns Artificial retina; helps RP patients Sheet of electrodes in eye; camera captures image and sends to box that processes and sends image to brain Medications Acne medication ○ Generic: Isotretinoin ○ Brand names: Accutane, Amnesteem, Claravis, Sotret ○ Isotretinoin treats severe acne that has not responded to other treatments. Vision symptoms may include dry eyes and a sudden decrease in night vision. Antihistamines ○ Antihistamines relieve hay fever and other allergy symptoms ○ They work by blocking histamines produced by the body that cause runny nose, watery eyes, sneezing, and itching. ○ In some people, these drugs can cause angle-closure glaucoma. Symptoms include headache, severe eye pain, nausea and vomitng, blurred vision, and seeing halos around lights. ○ Patients with narrow angles should avoid anticholinergics, or antispasmodic medication. These may include scopolamine patches for sea sickness and drugs that calm diarrhea and muscle spasms in people with GI problems. Antimalarial drugs ○ Generic and brand names: Hydroxychloroquine (Plaquenil, Quineprox), Chloroquine (Aralen) ○ These medications treat rheumatoid arthritis and lupus; they can prevent or treat malaria ○ While rare, some people who take these medications over time can develop retinal toxicity, leading to permanent vision loss ○ Plaquenil toxicity of the retina is typically dose related ○ Plaquenil: bull’s eye maculopathy Corticosteroids ○ Generic name: Prednisone ○ Brand names include: Deltasone, Meticorten, Orasone, Prednicot, Sterapred ○ Prednisone is a type of corticosteroid used to treat condition such as arthritis, lupus, psoriasis, severe allergies, ulcerative colitis, and breathing disorder. ○ They increase the patient’s risk of developing cataract or glaucoma (steroid responders). ○ Can cause elevation in eye pressure, which is a major risk factor for developing glaucoma Erectile dysfunction drugs ○ Generic and brand names: Sildenafil (Viagra), Tadalafil (Cialis) and Vardenafil (Levitra)  ○ Men taking these drugs for erectile dysfunction (ED) have reported blurred vision, sensitivity to light, and seeing a temporary blue tinge to objects ○ May be associated with nonarteritic anterior ischemic optic neuropathy ○ It is more common in older people who may have poor circulation and ED Ethambutol ○ Brand name: Myambutol ○ A type of antibiotic used to treat TB ○ Associated with ON problems ○ Can cause a loss of VA and color vision deficits ○ Another TB drug, Isoniazid (Tubizid) also may cause eye problems Tamoxifen ○ Brand names: Soltamox, Nolvadex ○ An antiestrogen drug used to treat breast cancer ○ Can lead to crystalline retinopathy, where small, crystal-like deposits (oxalate) form in the retina ○ SE include blurred vision, corneal changes, and an increased risk for cataracts Topiramate ○ Brand names: Topamax, Topiragen ○ An anticonvulsant used to treat seizure disorders such as epilepsy and to prevent migraine headaches ○ Associated with angle-closure Vitamin A deficiency ○ Required for synthesis of rhodopsin ○ May occur by: Dietary restrictions (3rd world areas with poor nutrition) Diseases Celiac disease (disease of small intestine) Chron’s disease (inflammation disease of all GI) Liver disease and cirrhosis ○ Other signs: Xerophthalmia (dry eye) Keratomalacia (keratinization of cornea) ○ Dietary sources ○ Tx: Vitamin A supplements