vision disorders

Cause: Eyeball is too long or cornea too curved → image focuses in front of the retina.
Effect: Distant objects appear blurry; close objects are clear.
Correction: Concave (diverging) lenses move the focal point back onto the retina.
Everyday examples: Difficulty reading road signs or seeing the board in class, but can read a book easily.

Cause: Eyeball is too short or cornea not curved enough → image focuses behind the retina.
Effect: Near objects appear blurry; distant vision is clearer.
Correction: Convex (converging) lenses move the focal point forward onto the retina.
Everyday examples: Struggling with reading or phone use but good distance vision.

Cause: Irregular curvature of the cornea or lens → light focuses unevenly on the retina.
Effect: Distorted or blurred vision at all distances; vertical or horizontal lines may appear slanted.
Correction: Cylindrical lenses that counteract uneven curvature.
Everyday examples: Words or shapes appear “stretched” or uneven, particularly under bright lights.

Cause: Age-related stiffening of the lens → reduced ability to accommodate for near focus.
Effect: Difficulty focusing on close objects (reading, sewing).
Typical onset: Around age 40+.
Correction: Reading glasses or bifocal/multifocal lenses.
Mechanism link: Similar to hyperopia, but due to lens elasticity, not eyeball length.

Amblyopia (“lazy eye”):
- Cause: Brain suppresses input from one eye during development (often due to strabismus or unequal refractive errors).
- Effect: Reduced visual acuity in one eye even with corrective lenses.
Strabismus (eye misalignment):
- Cause: Extraocular muscles not coordinated properly → eyes don’t point at same target.
- Effect: Double vision or cortical suppression of one eye’s input to prevent confusion.
Intervention: Early patching of stronger eye or vision therapy to promote equal cortical input.

Cause: Degeneration of the macula (central retina, rich in cones).
Effect: Loss of central vision (central scotoma), but peripheral vision intact.
Types:
- Dry (atrophic): Gradual breakdown of photoreceptors.
- Wet (neovascular): Abnormal blood vessel growth causing leakage and scarring.
Impact: Difficulty reading, recognising faces, or fine-detail work.
No cure, but vision aids and lifestyle management help.

Cause: Genetic cone photopigment deficiency, most commonly X-linked (males ≫ females).
Types:
- Protanopia / Protanomaly: Red (L cone) deficiency.
- Deuteranopia / Deuteranomaly: Green (M cone) deficiency.
- Tritanopia: Blue (S cone) deficiency (rare, not X-linked).
Effect: Difficulty distinguishing specific colour pairs, e.g. red–green.
Impact: Can affect fashion choices, driving, team sports, and certain jobs (e.g. electrician, pilot).
Management: Colour filters or assistive apps (e.g. EnChroma lenses, phone accessibility tools).

Cause: Lesions in dorsal or ventral visual processing streams (occipital → parietal or temporal).
Types:
- Optic ataxia: Dorsal stream lesion → inability to accurately reach or interact with objects visually (“where” pathway).
- Visual agnosia: Ventral stream lesion → inability to recognise objects or faces despite normal visual acuity (“what” pathway).
Key feature: Optics are normal — the problem lies in visual processing in the brain.