Clinical Refraction

Retinoscopy, an autorefractor or the patient’s old glasses give a starting point. The subjective refraction turns it into a prescription by asking the patient, one careful question at a time, which of two views is clearer. This chapter follows that routine, then looks at the things that make prescribing more than arithmetic: hidden hyperopia, the link between accommodation and convergence, and what patients will and won’t tolerate.

#The order of a refraction

A monocular subjective refraction usually runs like this:

1. Sphere first, fogged. Starting with a little extra plus, reduce it to the most plus (least minus) sphere that gives the best acuity: maximum plus for maximum visual acuity, or MPMVA. This places the circle of least confusion on the retina with accommodation relaxed.
2. Cylinder axis, then power, with the Jackson cross cylinder. The axis comes first, because the right axis can be found with the wrong power but not the other way round.
3. Sphere again, ending on the most plus that keeps the best acuity, checked with the duochrome (Chapter 6).
4. Binocular balance, so that both eyes end with accommodation equally relaxed.
5. Near add, for presbyopes.

Each eye is refracted with the other covered, except in latent nystagmus, where covering an eye brings the nystagmus out; the other eye is fogged with a plus lens instead. Chapter 7 found the axis with the clock dial. The cross cylinder is the finer tool, and it works without fogging, at the end point where the patient sees best.

#The cross cylinder: refining the axis

A Jackson cross cylinder has equal and opposite powers in its principal meridians, typically +0.25 D and −0.25 D, so its spherical equivalent is zero: it adds astigmatism without moving the circle of least confusion. Its minus cylinder axis is marked with red dots and its plus axis with white dots, and a handle midway between them lets it be flipped to swap the two.

To refine the axis, the handle is held along the trial cylinder’s axis, so that the cross cylinder’s axes straddle it at 45°. In one position the cross cylinder combines with the trial cylinder to give a resultant whose axis lies a little to one side; flipped, to the other side. If the trial axis is wrong, the side nearer the true axis looks clearer. The rule for minus cylinders: turn the axis toward the red dots of the clearer position (for plus cylinders, toward the white dots). Repeat with smaller steps until both positions look the same.

The step size depends on the cylinder. A small cylinder is forgiving and needs big turns, 15° or more, to make a visible difference; a strong one shows a few degrees of error. A cylinder of power C set θ degrees off its true axis leaves a residual astigmatism of

with a spherical equivalent of zero, at an oblique axis roughly 45° from both the needed and the worn axes. At 15° off, half the cylinder is left uncorrected; at 30°, as much as if there were no cylinder at all. The same rule governs a toric intraocular lens that rotates (Chapter 16).

#The cross cylinder: refining the power

To refine the power, the cross cylinder is turned so that its axes lie along and across the trial axis. In one position its minus axis (red) lies along the trial minus cylinder’s axis, adding −0.50 D of cylinder; in the other its plus axis does, taking 0.50 D away. If the red position is clearer, add −0.25 D of cylinder; if the white is clearer, reduce it by 0.25 D.

Changing the cylinder moves the circle of least confusion, so the sphere has to follow: for every −0.50 D of cylinder added, add +0.25 D of sphere, keeping the spherical equivalent constant. The figure does this automatically:

The ±0.25 D cross cylinder suits patients who see well; with poorer acuity a ±0.50 D or ±1.00 D cross cylinder gives a difference the patient can actually notice. Patients sometimes find the choice hard, and repeating the flip, or presenting the positions in a different order, guards against guesses. When the starting point has no cylinder, the cross cylinder can still look for one, presented with its axes at 90 and 180 and then at 45 and 135.

#Refining the sphere

With the cylinder in place, the sphere is checked again. Add +0.25 D: if the acuity doesn’t fall, keep it, and repeat. Too much minus is the commonest error, because a young patient accommodates through it and still reads the chart; the clue is that they report the letters looking smaller and darker, which is minification and extra contrast rather than better focus. The duochrome test (Chapter 6) gives a quarter-dioptre check: red clearer, add minus; green clearer, add plus.

#Balancing the two eyes

Accommodation is shared: both eyes always accommodate by the same amount. If one eye has been left with more plus than the other, the two cannot both be clear at once. A binocular balance makes sure the two refractions leave accommodation equally relaxed:

• Fog both eyes with about +0.75 D, so that both see the chart a little blurred (about 6/9 to 6/12, 20/30 to 20/40) and accommodation relaxes.
• Separate their views, usually with prisms: 3 to 4 Δ base up before one eye and the same base down before the other give two charts, one above the other (alternate occlusion, or polarised charts, also work).
• Ask which chart is clearer, and add +0.25 D to the clearer eye until both look equally blurred.
• Remove the prisms and reduce the fog binocularly, to the most plus that gives the best binocular acuity.

A balance only makes sense when the two eyes can reach the same acuity; if they can’t, each eye keeps its own best sphere. If the two charts cannot be made equally clear, the dominant eye is left slightly clearer.

#Cycloplegia and hidden hyperopia

A young hyperope can hide much of their hyperopia by accommodating, and some of it stays hidden even during a careful manifest refraction, held by the tone of the ciliary muscle. Clinicians divide it up:

• Total hyperopia, revealed by cycloplegic refraction, is the sum of the latent and manifest parts.
• Latent hyperopia is held by ciliary tone and cannot be relaxed without cycloplegic drops.
• Manifest hyperopia is the largest plus lens accepted at the end of a manifest refraction. It has two parts: facultative hyperopia, which the patient can overcome by accommodating, and absolute hyperopia, which they cannot. The absolute part is the weakest plus lens that gives clear distance vision.

As accommodation fails with age, facultative hyperopia becomes absolute, which is why a hyperope who saw well for years suddenly needs glasses for distance as well as near in their forties. Cycloplegia is essential for refracting children, especially with esotropia, and whenever results are inconsistent or accommodative spasm is suspected. Cyclopentolate 1% is the usual agent (weaker strengths in infants), with maximal cycloplegia after about 30–60 minutes; atropine, used over several days, is the most complete and lasts one to two weeks; tropicamide dilates well but is a weak cycloplegic. All can have systemic effects, especially in small children: cyclopentolate can cause transient drowsiness, confusion or hallucinations, and atropine flushing, fever and a fast pulse. Pressing on the puncta after the drops reduces absorption.

#Accommodation and convergence

Looking at a near object, the eyes accommodate, converge and constrict their pupils together: the near reflex. Each dioptre of accommodation brings with it a certain amount of convergence, the accommodative convergence, and their ratio, the AC/A ratio, is normally about 3 to 5 prism dioptres per dioptre. It can be measured two ways:

• The heterophoria method compares the deviation at distance and near (eso positive, exo negative), allowing for the convergence that the near target demands anyway, which depends on the interpupillary distance: $\text{AC/A}=\text{IPD (cm)}+({\Delta }_n-{\Delta }_d)/D$, where D is the accommodative demand at near.
• The gradient method measures the near deviation with and without a lens that changes the accommodation: $\text{AC/A}=({\Delta }_{\text{lens}}-{\Delta }_{\text{no lens}})/\Delta A$, where ΔA is the change in accommodative demand the lens causes: +1.00 D for a −1.00 D lens, −1.00 D for a +1.00 D lens. It excludes proximal convergence (the awareness of nearness) and gives lower values.

A high AC/A ratio gives more eso at near than at distance (convergence excess, including high AC/A accommodative esotropia), and plus at near reduces it. A low ratio goes with convergence insufficiency, an exo deviation that is larger at near. In children, office-based vergence and accommodative therapy with home reinforcement works best; pencil push-ups alone did little better than placebo in trials, and so did base-in prism reading glasses, which can help presbyopes. The total convergence a target demands is easily estimated: in prism dioptres, it is the interpupillary distance in centimetres divided by the target distance in metres, so 6 cm at 33 cm asks for 18 Δ.

#Prescribing cylinders

A new or changed cylinder does more than sharpen the image: it magnifies the world a little more in one meridian than the other. With oblique axes, vertical lines tilt slightly, and if the two eyes’ cylinders are mirror images, as they often are, the lines tilt opposite ways in the two eyes. The brain reads that difference between the eyes as a tilt in depth, as if vertical planes were tipped about a horizontal axis: floors seem to slope, walls lean and tabletops tilt. Change the axis below and watch the doorway:

Children adapt quickly and should have their full astigmatic correction. Adults, especially those who have never worn a cylinder or whose axis has changed, may not. Common guidelines are to try the full correction in a trial frame first and walk the patient around; if it is not tolerated, rotate the axis toward 90 or 180 (or toward the old axis) and reduce the cylinder, adjusting the sphere to keep the spherical equivalent. Minus-cylinder (back-surface toric) lenses and a short vertex distance also reduce the distortion.

#Prescribing for children

In young children the aim is clear, equal retinal images while the visual system develops. Significant hyperopia, astigmatism and, above all, anisometropia carry a risk of amblyopia and are corrected; the American Academy of Ophthalmology’s Preferred Practice Pattern gives consensus thresholds by age, which are lower when strabismus is present. Small, symmetrical refractive errors in a child with straight eyes and no symptoms often need no glasses: much infantile hyperopia and astigmatism disappears with emmetropisation. When a hyperopic child with straight eyes does need glasses, the correction is often reduced symmetrically from the cycloplegic finding, whereas accommodative esotropia needs the full correction; in intermittent exotropia, full plus can weaken control by relaxing convergence. Myopic children are given their full correction; deliberate under-correction does not slow myopia and may speed it.

#Slowing myopia

Myopia that starts young progresses most, and high myopia brings a lifelong risk of retinal detachment, myopic maculopathy and glaucoma. Several treatments slow progression in children: low-dose atropine eye drops (0.01% to 0.05%, the higher concentrations more effective in trials), orthokeratology lenses worn overnight (Chapter 15), and spectacle or soft contact lenses that add myopic defocus to the peripheral retina while correcting the centre. In trials the most effective of these roughly halve the rate of progression. More time outdoors in childhood reduces the onset of myopia.

#The near add

The add for a presbyope can be estimated from age, or from the amplitude of accommodation, leaving half of it in reserve as in Chapter 6:

Clinicians refine it with the fused cross cylinder, or by balancing the negative and positive relative accommodation (the amounts of plus and minus that can be added at near before the print blurs). Whatever the method, the add is checked at the patient’s own working distance: too strong an add pulls the range of clear vision in too close, a common complaint with new reading glasses.

#Recap

• Sphere (maximum plus for maximum acuity), cylinder axis then power with the cross cylinder, sphere again, binocular balance, add.
• Cross cylinder: for axis, straddle the trial axis and turn the minus cylinder toward the red dots of the clearer position; for power, red along the axis clearer means more minus cylinder. Keep the spherical equivalent: +0.25 D sphere for every −0.50 D of cylinder added.
• A cylinder θ off axis leaves $2C\sin \theta$ of astigmatism: half at 15°, all of it at 30°.
• Balance with the eyes fogged and dissociated by vertical prisms, adding plus to the clearer eye.
• Total hyperopia = latent (needs cycloplegia) + manifest; manifest = facultative + absolute (the least plus for clear distance vision).
• AC/A ≈ 3–5 Δ/D. Heterophoria method: IPD (cm) + $({\Delta }_n-{\Delta }_d)/D$; gradient method: change in deviation per dioptre of accommodation the lens stimulates.
• Adults may not tolerate a full new oblique cylinder: rotate the axis toward 90 or 180, reduce it and keep the spherical equivalent.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, chapter on clinical refraction and the appendix on prescribing cylinders.
• W. J. Benjamin (ed.), Borish’s Clinical Refraction. Butterworth-Heinemann.
• American Academy of Ophthalmology Preferred Practice Pattern Pediatric Ophthalmology/Strabismus Panel, Pediatric Eye Evaluations and Esotropia and Exotropia Preferred Practice Patterns.
• J. C. Yam et al., Low-Concentration Atropine for Myopia Progression (LAMP) study. Ophthalmology 126 (2019) 113–124.