Lens Flashcards
Etiological classification of cataract
Congenital
Aquired (age ,trauma, complicated cataract, systemic diseases, electric cataract, radiational , toxic)
Morphological classification of cataract
Capsular (ant., Post.)
Subcapsular (ant. , Post.)
Cortical
Nuclear
Polar (ant. , Post.)
Cataract of systemic diseases
Metabolic
Skin diseases
Myotonic dystrophy
Neurofibromatosis type 2
Osseous disease
Down’s syndrome
Teacher collins syndrome
Lens Anatomy
Transparent
Biconvex
Crystalline
Diameter (9-10mm)
Thickness (3.5-5mm)
Weight (135-255mg)
Surfaces
Anterior (less convex)
Posterior
Total dioptic power (16-18)
Accomodation
Birth (16-18D)
25yrs (7-8D)
50yrs (1-2D)
Structure of lens
Capsule
Thin (thinnest at post. Pole)
Hyaline membrane
Thickest at pre-equator regions
THICKEST BASEMENT MEMBRANE (in body)
Type IV collagen
Epithelium
Cuboidal cells
Columnar cells (in equatorial region)
Substance (fibres)
Mature cells (lack nuclei)
Different zones of lens nucleus
Adult
After puberty
Embryonic
Upto 3 months of gestation
Consists primary lens fibres
Foetal
From 3rd gestational month to birth
Fibres meet around sutures (opp Y shaped)
Infantile
Birth to puberty
Cortex
Consists youngest lens fibres
Suspensory ligament of lens /zonules of zinn /ciliary zonules
Parts (fibres)
Pars orbicularis (lines pars plana)
Zonular plexuses (attached to ciliary processes in pars plicata region)
Zonular fork (pars plicata to lens)
Functions of lens
Transmits light
Refracts light
Contribute 35% of refractive power
Absorbs UV rays (prevents retinal damage)
Accomodation (for near objects)
Lens transparency
Avascularity
Epithelium (single layered)
Capsule (semipermeable)
Characteristics of fibres
Tightly packed nature
Narrow lens fibre membranes
Loss of organelles
Lens proteins (aquaporins)
Pump mechanism
H2O + electrolyte balance
Maintain rrlative dehydration
Antioxidant
