Transcript
Visual pigment in Rods : Rhodopsin
- glycoprotein and chromophore
[Opsin] [retinal]
- Peak absorption at 500 nm wavelength [blue]
- Most sensitive in dark adapted eye .
- Hence blues and greens will appear
brighter at nightfall - Purkinje effect
Intracellular disk
Disk membrane
Connectingcilium
outer segmentouter segment
Disk membrane
Intracellularspace
Extracellularspace
Visualpigment
Visualpigment
Extracellularspace
Plasmamembrane
Intracellularspace
Connectingcilium
Rods and Cones
ROD CELL CONE CELLPHOTORECEPTORS IN THE EYE
Physiology of night vision :
Rods are elongated cells [120 million in number ] mainly confined in the periphery of the retina. These are meant for Dim vision in low light .
Rods, are extremely light sensitive and their sensitivity is about 500 times greater than the sensitivity of cones.
Only a small number of photons is required to stimulate a rod to send a signal to the brain. Also more number of pigments than cones .
The key to the rod’s ability to convert light to electric impulses is the Visual pigment Rhodopsin
Through a complex chain of chemical changes activated by light ,Rhodopsin begins the events that activates the phototransduction cascade
Dark adaptation : When a person shifts from bright to dim
environment 2 processes occur
- Pigment regeneration
- Photochemical changes [Phototransduction]
All this happens in a period of 30 minutes.
Nyctalopia : In greek : Nykt – night , alaos – blindness Condition in which inability to see in relative dim light.
Causes :1. Vitamin A deficiency2. Retinitis pigmentosa3. Uncorrected refractive error - Myopia4. Media opacification - cataract5. Following pan retinal photocoagulation6. Congenital stationary night blindness7. Hydroxychloroquine & Phenothiazine8. Advanced glaucoma
Medical history to be sought Operations [intestinal surgeries] Liver diseases Use of medicine :Hydroxychloroquine
Phenothiazine Hearing status( RP, Usher’s Syndrome, Refsum’s)
Mental retardation( BBS,LMS) Renal disease (BBS) Heart disease (KSS, Refsum’s)
Introduction: Progressive Rod/cone dystrophy Hereditary, progressive dystrophies of the
photoreceptors and RPE of Retina Otherwise known as pigmental retinal dystrophy.
The condition is abiotrophic in nature and is genetically determined.
Various inheritance patterns [AD,AR, X linked]
In majority of families it occur as recessive trait., occasionally it shows dominant hereditary and even sex linked inheritance.
Symptoms : Patient presents with symptoms which become
apparent between ages of 10 and 30 yrs.
Night vision problems / prolonged dark adaptation Slow progressive loss of vision [peripheral] Sparing of central vision Ring Scotoma
As the disease develops, people with RP may often bump into chairs and other objects as side vision worsens and they only see in one direction - straight ahead.[Tunnel vision]
Signs : Characteristic fundus changes :
Black bone spicule /Corpuscular retinal pigments
[denser in mid periphery retina ]
Attenuated retinal blood vessels
Waxy type of optic disc atrophy
Cystoid macular edema
Associated ocular problems
Myopia Subcapsular cataract Open angle glaucoma Keratoconus Posterior vitreous detachment
Atypical RP
Retinitis pigmentosa albescens Sector RP Unilteral RP Pericentric RP
VARIANTS
Retinitis pigmentosa sine pigmento: with same symptoms but without visible retinal pigmentation.
Retinitis puncta albescens: is an allied condition with same history and symptoms but here the retina shows hundreds of small white dots distributed uniformly over the whole fundus
Systemic findings
Bassen Kornzweig syndrome Refsum’s syndrome Kearns Sayre syndrome Bardet – Biedl syndrome Usher’s syndrome
Diagnosis
Refraction Direct ophthalmoscopy Indirect slit lamp biomicroscopy Visual field evaluation Dark adaptametry Electroretinography
Treatment Eminently unsatisfactory since, despite many
claims, nothing appears to have a desired influence upon the course of the disease.
but there is research that indicates that vitamin A supplementation and lutein may slow the rate at which the disease progresses.
Antioxidants , omega-3 fatty acid, DHA.
Low phytol and low phytanic acid diet
Systemic carbonic anhydrase inhibitors like
acetazolamide & Intravitreal triamcinolone
Neurotrophic factors
Low vision aids, including telescopic and magnifying lenses, night vision scopes help people maximize the vision that they have remaining.
Ultraviolet protective sunglasses
Parental and supportive care Genetic counselling
ARMD
Degenerative anomaly of macula –exaggeration of normal ageing –Visual threatening disability
This is one of the leading cause of blindness in the world. More common >65 years and in whites and females.
TYPES: 1. Dry or ‘atrophic’ type: 2.Wet or ‘exudative’ type:
PATHOGENESIS Impaired metabolism of RPE
Accumulation of metabolic debris in bruch membrane
Thickening and fragmentation of bruch membrane with damage
Choroidal neovascularization
Hereditary factors, age, nutrition, smoking, hypertension ,high cholestrol and exposure to sunlight are risk factors.
CLINICAL FEATURES
Gradual painless diminution of vision Metamorphopsia - distorted vision. Central scotoma & Paracentral scotoma
OPHTHALMOSCOPY
Dry type - Hallmark is drusen and loss of RPE. Drusen are small yellowish deposits on bruch’s membrane derived from metabolic products of visual receptors and RPE deposited as lipid
Exudative type – Hallmark is CNV elevated area in neuro sensory retina or
pigment epithelium beneath which abnormal blood vessels, fluids or haemorrhage are present.
DRY ARMD
WET ARMD with haemorage
MANAGEMENT
Amsler grid ,preferential hyperacuity tests Provide micronutrients [zinc & Antioxidants] Avoid UV light Life style changes Laser photocoagulation Photodynamic therapy Anti –VEGF Transpupillary thermotherapy
MANAGEMENT
Surgical treatment :
Sub macular surgery
Macular translocation
THANK YOU
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