Accommodation, Presbyopia & Near Additions

The eye can increase its power to focus on near objects

The ciliary muscle controls the tension on the zonule – this allows the ciliary muscle to contract, causing the zonule fibres to relax, causing the lens to thicken/bulge out & this increases the power of the eye

SEE GRAPHS ON LECTURE

With high minus lenses the eye accommodates a lot, creating an unsteady line on graph (VA becomes unsteady)

When refractive error is higher than the amplitude of accommodation (how much we’re able to accommodate) then VA deteriorates/is weak

The highest peak of graph (best VA) is the most plus/least minus that gives the best VA

If you give any more plus VA deteriorates

  • At birth, our amplitude of accommodation is ~20.00D, it reduces with age from the day we are born – this happens because the lens capsule in the eye loses its elasticity & the number of fibres increases over time.
  • This varies between people but we don’t notice it happening – only when amplitude goes below that needed for reading do we notice it reducing
  • Each individual probably has a linear reduction of amplitude (& hence reduction of being able to focus at near) with age (can lower to as far as 0.50D)
  • If you have 2.00D amplitude of accommodation, you can only see as close as 50cm (1/2 = 0.5m)
  • Presbyopia happens to everyone & is not related to patient’s refractive error (therefore near tests should be done on everyone) – presbyopes may require additional tests
  • Reading glasses, positive spheres make near objects clear (& distant objects blurred)
  • Pre-presbyopes are usually less than 45 y/o & have adequate amounts of accommodation for near vision
  • Presbyopes need near Rx, as their accommodation is no longer adequate for near vision. They are generally over 45 y/o but may depend on: race, occupation, environment (e.g. light levels – higher light levels of sunlight/UVradiation causes lens to go through aging process quicker i.e. in hot country), pupil size, medication etc.
  • Regardless of refractive error, when wearing the distance correction near objects are blurred
  • Presbyopes therefore need different correction for distance & near
  • ALWAYS consider near visual requirements for ALL PATIENTS
  • All tests so far have been at 6m
  • Now consider near vision – usually 33-40cm
  • Adjust trial frame for near PD
  • Adjust lighting levels to match patients’ usual setting (artificial light/bright light makes pupil smaller so acts as a pinhole to make vision clearer)
  • Tests for near vision:
  • Near acuities – monocularly & binocularly
  • Accommodation – monocular & binocular
  • Determination of near addition if presbyopic
  • Convergence
  • Near Acuities:
  • Important to determine & record near acuities & the distance at which it’s measured (ask patient to hold a book at their normal reading distance & measure distance from book to eye – this is after you’ve added the near addition for presbyopes/older patients)
  • Do this for each eye independently
  • If they have poor near acuity may need to determine near addition first – e.g. presbyopes
  • Measurement of the Amplitude of Accommodation (for everybody)
  • Distance correction in place
  • Occlude each eye in turn
  • Use the RAF rule:
  • Bring N5 print closer to the eye until it blurs
  • Move print away from eye until just clears
  • Read off the value (in dioptres)
  • Repeat for other eye
  • Repeat binocularly
  • Values should be:
  • Equal in the two eyes
  • Correct for the age
  • Binocular > monocular (~0.50D)

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Cross Cylinder

We done experiments based on: Gordon’s Glazing Window Company. Convenient & common method for fine-tuning astigmatic element found in retinoscopy. Requires the retinoscopy to be within ~1.00DC. It Cannot be relied upon if remaining cyl to be corrected is >1.00DC. Either 0.25 or 0.50 cyls depending on level of astigmatism that needs correction.

Handle is at 45° to axes. The number written on the handle indicates the astigmatic interval between the two meridians. E.g. a cross cyl marked 1.00D has meridians of power +0.50D & -0.50D

The markings on the lens are at plus & minus 45° to the handle, & indicate the axes meridians, not the power meridians.

Cross cyls are available in ±25, 0.50, 0.75 & 1.00 D powers, which result in astigmatic intervals of twice these amounts.

Choose a cross cyl that has an astigmatic interval equal to or less than value of Px’s estimated cyl

  • For the target, most practitioners use double black rings on a white background as targets. (One concentric circle, say which one to look at)
  • The circles subtend 6/12 & 6/4.5 so most patients can resolve larger circles with their best sphere acuity
  • Sometimes letter “O” from Snellen chart is used – letter size must be just better than the patient’s best sphere acuity for each eye
  • Cross cyl method is divided into two parts:
  • To find & refine the cylinder axis
  • To refine the cylinder power
  • To find cyl axis:
  • Occlude one eye
  • Start with cyl obtained from ret in the trial frame (of other eye)
  • Hold cross cyl handle parallel to axis of cylinder in trial frame in front of other eye & twirl (flip over, still keeping handle in that position)
  • Ask for preference while looking at the rings: “Are the circles blacker, clearer & rounder with lens 1 or lens 2?”
  • Make sure to notice which position you’re holding cross cyl, & whichever “lens” they prefer, (flip back if lens 1) move the axis 10° towards the –ive/minus axis meridian
  • Then if patient tells you to go the other way, only move it by 5°, & when alternating keep decreasing the interval until you get to 2.5°/1°
  • To find cyl power:
  • Turn cross cyl through 45° so that cross cyl axes are parallel with & at right angles to the trial case cyl axis
  • Ask again for preference while looking at rings: “Are the circles blacker, clearer & rounder with lens 3 or lens 4?”
  • Put in the power they prefer
  • If no cyl has been found with ret
  • Hold cross cyl axes at 90 & 180 & twirl (flip)
  • If for e.g. prefers 90 then turn through 45° & twirl (essentially asking would you like minus cyl a bit >90 or a bit <90) – first preference narrowed it down to vertical rather than horizontal, the second preference could be from 45 to 90, or 90 to 135)
  • Then put axis meridians at suitable axis (middle of interval) & ask which power they prefer, plus or minus & give
  • Then refine axis again
  • If there’s no preference on the first twirls with cross cyl to refine axis then either they don’t have any astigmatism or by chance the axes are 45 & 135. Try again with the handle at 90°, if still no preference – no astigmatism. If there is a preference don’t forget that axes must be 45 or 135
  • After retinoscopy check VA & do +1.00 blur test
  • If ~6/9 & blurs to ~6/18 do cross cyl
  • If ~6/9 & does not blur with +1.00, optimise sphere (best sphere) & then do cross cyl
  • If worse than 6/9, optimise sphere
  • Still poor VA? Pinhole? F&B?
  • Patients need to have a small amount of active accommodation ~0.25 for cross cyl i.e. +1.00 only blurs to ~6/12 – R = G or just on green on duochrome. So before doing cross cyl check that VA is on green (can add more -ive spheres but not more than -1.00DS, then take it out after finishing). Distance VA should be R = G or just on red whereas near VA should always be on green.

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