Describe astigmatism, accommodation and presbyopia.

Astigmatism

• Astigmatism is the refractive state in which the refraction differs IN different meridia, so that rays are not brought to a point focus on the retina.
• The word means "without a point".
• Regular astigmatism — the two principal meridia are at right angles to each other and the power changes progressively between them, so it is correctable with a cylindrical lens

Accommodation

The ciliary muscle contracts (parasympathetic, via the third nerve and the ciliary ganglion) → The ciliary ring is drawn forward and inward, so the zonules relax → Released from zonular tension, the lens assumes its own more spherical (convex) shape through the elasticity of its capsule → The anterior surface curves more than the posterior — the anterior radius falls from about 10 mm to 6 mm → The lens thickens, the anterior pole moves forward, and the anterior chamber becomes shallower → The dioptric power increases and the near object is focused → This is helmholtz’S theory, which is the accepted account

• The near reflex is a triad, occurring together and mediated centrally: accommodation, convergence and miosis — the last usefully increasing the depth of focus

Presbyopia

• Presbyopia is the physiological, age-related insufficiency of accommodation, in which the near point recedes beyond the customary working distance.
• It is not a refractive error but a normal consequence of ageing, occurring in every eye.
• Why it occurs at 40 to 45 — comfortable near work at 25 to 33 cm requires about 3 to 4 D of accommodation, and no more than half to two-thirds of the amplitude can be used comfortably for sustained work. The amplitude falls below that threshold at 40 to 45 years, and symptoms begin

Types of Regular Astigmatism

Applied Aspects

• Refract every child under cycloplegia; the latent hypermetropia is otherwise missed, and it is the component that causes accommodative squint and amblyopia
• Suspect keratoconus in rapidly changing or oblique astigmatism, particularly in a young person with allergic eye disease who rubs the eyes, and get topography
• Offer a rigid gas-permeable lens for irregular astigmatism; spectacles cannot correct it, and the improvement with a rigid lens is often dramatic
• Explain that presbyopic glasses do not weaken the eyes; the belief is near-universal and leads people to struggle without them
• Do not exceed +3.00 D for near; a stronger add forces the working distance too close and the patient will not use it
• Suspect diabetes in sudden refractive change in an adult; hyperglycaemia causes a reversible myopic shift and is a recognised presenting feature
• Consider "second sight" in an elderly patient reading without glasses again; it signals index myopia from nuclear sclerosis, and cataract is developing
• With-the-rule astigmatism is normal in the young, from lid pressure, and shifts against-the-rule with age as lid tone falls
• Irregular astigmatism needs a rigid lens, which replaces the irregular surface with a regular one; spectacles cannot correct it
• The circle of least diffusion lies midway between the focal lines, and is where the spherical equivalent places the image