Refraction and prescribing

4.1.1

The evidence base: When and how to assess lens powers and optical centres

Focimetry (vertometry/lensometry) should be used to assess lens powers and optical centres of glasses that are newly glazed (for checking they comply with the current international standard ISO 21987:2017), of unknown correction and any that patients return because they are dissatisfied with them. Ready-made reading glasses should be measured because they cannot be assumed to provide the powers suggested and the higher-powered glasses often include vertical prism. Note that, although focimeters measure the vertex power, axis direction, prism, and optical centres of ophthalmic lenses, they do not provide information about other features of lenses that could cause patients problems and need to be checked, such as base curve and lens form, segment style, height, size and inset, centre or edge thickness, optical and surface quality, and the presence of a lens tint and/or surface coating (antireflection, antiscratch). Automatic focimeters are available that measure the lens powers, optical centres, and any prism automatically once the lens has been appropriately positioned and provide a printout of the results. These are very simple to use and the measurement procedure will not be explained. Their main disadvantage is that they break down more often than manual focimeters and require regular calibration.

4.1.2

Procedure for focimetry (vertometry/lensometry)

See online video 4.1 and summary in Box 4.1

• 1.

  Explain the test to the patient: “I am going to measure the power of your glasses.”

• 2.

  Set the power to zero and focus the eyepiece (turn it as far anti-clockwise as possible, then slowly turn it clockwise until the target and graticule first come into sharp focus. The graticule is the network of circular and/or straight lines in the focal plane of the eyepiece that allows prism measurement).

• 3.

  Measure the back vertex power by placing the glasses on the focimeter with the back (ocular) surface away from you. Position the middle of the right lens against the lens stop.

• 4.

  Adjust the glasses vertically (using the lens table) and horizontally until the illuminated target is positioned in the middle of the graticule. If the lens is high powered, you may need to turn the power wheel to bring the target into focus before it can be centred.

• 5.

  Fix the lens into position using the lens retainer.

• 6.

  Turn the power wheel to bring the target into focus.

  • (a)

    If the entire target is focused at the same time ( Fig. 4.1 ), the lens is a sphere and there is no cylindrical component. Record the sphere power for the right eye from the power wheel or the internal scale and go to step 8.

    

  • (b)

    If parts of the target are in focus at different powers and to record in the standard negative cylinder format, turn the power wheel until the meridian with the most plus power or least minus power is focused.

  • (c)

    With focimeters using line targets, rotate the axis wheel until the sphere line (see Fig. 4.1 a) is in focus and the line is continuous without breaks. You may need to use the power wheel to gain best focus.

  • (d)

    Record the sphere power from the power wheel or internal scale.

• 7.

  To obtain the power and axis of the cylinder:

  • (a)

    Focus the image in the meridian at 90° from the first meridian by turning the power wheel towards the most minus (or least plus) power.

  • (b)

    Read off the power when this meridian is in focus. With focimeters using line targets, the cylinder lines will be in focus.

  • (c)

    Record the difference between the sphere power from step 6d and the new meridian power as the cylinder power. For example, if first (sphere) power was focused at −5.00 D and the second power at −5.75 D, the cylinder power is −0.75 DC.

  • (d)

    Record the orientation of the second meridian from the eyepiece protractor or the axis wheel as the cylinder axis. With focimeters using line targets, this will be the orientation of the cylinder lines (see Fig. 4.1 a).

• 8.

  Make sure the target is centred in the graticule and dot the right lens using the marking device. This could be just one spot (the lens optical centre) or three dots (the middle is the lens optical centre, the other two indicate the horizontal line).

• 9.

  Release the lens retainer and repeat steps 5 to 8 for the left lens. Do not change the vertical position of the glasses between measurements of the right and left lenses as you need to determine if any vertical prism is incorporated in the glasses.

• 10.

  Move the lens horizontally until the target is in the same vertical plane as the centre of the graticule and dot the left lens using the marking device.

• 11.

  If the target is above or below the centre of the graticule, vertical prism is present and should be recorded to the nearest 0.5 ^Δ using the graticule scale (see Fig. 4.1 ).

• 12.

  Remove the glasses and measure the distance between the right and left optical centres to calculate the centration distance and record it in millimetres.

• 13.

  For front-surface solid multifocal lenses, the reading add must be measured using front vertex power. Turn the lens around so that the ocular surface faces you and reposition the glasses in the focimeter. Measure the front vertex power along one meridian in the distance portion of the glasses. Measure the front vertex power along the same meridian in the near portion of the glasses. The difference between these powers is the reading addition. Repeat the measurement in the left lens. For low-powered lenses, the front vertex power approximately equals the back vertex power, and the back vertex power add can be measured.

• 14.

  For progressive addition lenses (progressives or PALs), you need to locate the appropriate position on each lens to measure the distance and near prescription, optical centres, and any prism ( Fig. 4.2 ). Faint symbols are etched into the nasal and temporal sides of each lens, and these must be found and marked with a non-permanent marker. The symbol may indicate the PAL manufacturer and include the power of the addition. Use the manufacturer’s marking-up card to locate the appropriate distance and near centres and measure the sphero-cylindrical power and any prism at these points as previously described (see Fig. 4.2 ).

  

• 15.

  Compare the centration distance and the patient’s interpupillary distance (PD). If these distances differ, calculate the induced horizontal prism using Prentice’s rule (induced prism = Fc, where F is the power of the lens along the horizontal meridian and c is the difference between the centration distance and PD in centimetres). The direction of the prism also needs to be deduced.

4.1.3

Recording of focimetry results

Record the sphero-cylindrical correction in minus cylinder form for both lenses and the reading addition power if a multifocal. Also record any prism, the type of lens, any tints or coatings, and so forth. Use ‘x’ rather than the word ‘axis.’ Record the spherical and cylindrical power to the nearest 0.25 D, and the cylinder axis to the nearest 2.5°. The axis should be between 2.5° and 180°. Use 180 rather than 0 degrees. Do not use a degree sign as ° can look like a 0 and make an axis of 10° look like 100 degrees.

Examples:

• D28 segment bifocal, CR39, MAR coat

  • RE: –2.00/–1.00 × 35, LE: –2.25 DS Add +2.00 DS

  • NV, CR39. OD: +2.25/–0.75 × 80, OS: +2.50 / –0.50 × 105

4.1.4

Interpreting focimetry results

One of the most common errors is an axis reading incorrect by 90°. Given that the cylindrical axes in the two eyes are often mirror images of each other (e.g., both axes 90° or both axes 180°; 175 with 5°; 20° with 160°; 45° with 135°), if axes are 90° different to this (e.g., 180° with 90°; 175° with 95°; 20° with 50°; both axes ∼45°; both axes ∼135°) then recheck the two cylindrical axes. Reading additions are typically the same in both eyes, so that if they are read as different, they should be rechecked.

Patients are increasingly using ready-made reading glasses and glasses bought online and clinicians should be aware of their limitations. Ready-made reading glasses, particularly if higher powered (+3.00 and +3.50), can often include vertical prism and horizontal prism owing to using a standard 62-mm centration distance. Glasses bought online without provision of a PD typically use an estimated distance centration distance of ∼63 mm. Errors owing to using estimated fitting heights and centration distances with multifocals bought online are particularly a problem ( section 4.15.4 ).

4.1.5

Most common errors in focimetry

• 1.

  Reading one or both of the cylindrical axes incorrectly by 90°.

• 2.

  Not focusing the eyepiece. This can lead to inaccuracies for high-powered lenses.

• 3.

  Ignoring the relative vertical position of the target between the right and left lens, thereby missing vertical prism.

4.2.1

The evidence base: When and how to measure PD

The PD is measured (1) prior to refraction and (2) during dispensing, so that you can place the optical centre of the (1) phoropter/trial frame lenses and (2) new glasses in front of the patient’s visual axes to avoid unwanted prism and aberrations.

The anatomical PD is typically measured as it is a simple, quick measurement that just requires a millimetre ruler. Inaccuracies in anatomical PD can occur owing to parallax error when there is a large difference between your PD and the patient’s PD. However, the error is slight, with an 8 mm difference in the examiner’s and patient’s PDs leading to a 0.5 mm error in the measured patient’s PD. The repeatability of anatomical binocular PDs taken by an experienced clinician is approximately ± 1 to 2 mm, ^, slightly poorer between clinicians at about ± 1.5 to 2 mm, and similar to that for a pupillometer. ^, Pupillometers allow monocular PDs to be measured more accurately than an anatomical measurement. This is beneficial when ordering glasses for high refractive errors or for varifocals/PALs where precise centration of each lens along the patient’s visual axes is necessary. In addition, pupillometers can be performed by a clinical assistant and the examiner does not need to be binocular. The PD measured with a corneal reflection pupillometer will typically be 0.5 to 1 mm smaller than the anatomical PD because it measures the physiological PD and locates the visual axes, whereas the anatomical PD locates the lines of sight or optical axes. ^, Note that many pupillometers use a correction for the parallax error mentioned in the anatomical PD section. Inaccuracies can occur if the pupillometer sits higher or (usually) lower on the bridge than the intended glasses frame and the nose is not straight, so that the monocular PDs can be shifted to one side.

4.2.2

Procedure for PD measurement

• 1.

  Explain the test to the patient: “I am going to measure the distance between the pupils of your eyes so that I can put your lenses in the correct position.”

• 2.

  Face the patient directly at the distance desired for the near PD (usually about 40 cm).

• 3.

  Rest the PD ruler on the bridge of the patient’s nose or on the patient’s forehead and steady your hand with your fingers on the patient’s temple to ensure that the ruler is held firmly in place.

4.2.3

Recording of PD

The values are normally recorded as PD: distance/near (in mm). For example, PD: 63/60.

4.2.4

Interpreting PD results

For women the distance PD is commonly in the range of 55 to 65 mm, and for men it is 60 to 70 mm. Young children may have PDs as low as 45 mm. The near PD value is usually 3 or 4 mm less than the distance PD. Glasses bought online (without a PD measurement) and ready-made reading glasses are commonly provided with centration distances of 62 or 63 mm. ^,

4.2.5

Most common errors in PD measurement

• 1.

  Moving the ruler during the measurement.

4.3.1

The evidence base: When to use a phoropter

The use of a manual or digital phoropter is the preferred technique for distance vision refraction of the majority of patients. The main advantages are:

• A quicker refraction: It is much quicker to change lens powers for both retinoscopy and subjective refraction in a phoropter compared with a trial frame.

• Less back strain: particularly with digital phoropters. This can be a problem for eye care clinicians.

• Greater comfort: A trial frame containing several lenses can become uncomfortably heavy, particularly for older patients with thin skin.

• Jackson cross-cylinder (JCC) accuracy: The JCC is automatically aligned with the cylinder axis in a phoropter.

• No lens smear: Trial case lenses can become covered with fingerprints (particularly in university clinics), and require regular cleaning.

• High-tech appearance: Some patients may have more confidence in the results using phoropters, particular the digital versions, over the ancient-looking trial frame.

• Digital phoropters: These include data links to automated focimeter (vertometer/lensmeter) and/or autorefractor and can provide easy comparison of the patient’s current glasses power with the latest subjective refraction result. The refraction results can automatically be included in the patient’s electronic record.

• Risley prisms: These are standard on phoropters and make measurements of fusional reserves and binocular prism-dissociated balance much faster and easier. A disadvantage is that their convenience has led to the widespread use of the unreliable von Graefe prism-dissociation subjective heterophoria measurement ( section 1.1.2 ).

4.3.2

The evidence base: When to use a trial frame

A trial frame ( Fig. 4.5 , online video 4.3 ) is required for refractions during home (domiciliary) visits and is preferred when refracting:

• The near addition: It can be performed more naturally at the patient’s preferred working distance(s) and position.

• Children and patients with binocular vision problems: Children can more easily see their parent/guardian and feel more at ease with them. The trial frame can stimulate less proximal accommodation than a phoropter and provides more repeatable results of oculomotor status. In addition, it is possible to perform the cover test with large aperture lenses in a trial frame, but not with a phoropter.

• Patients with visual impairment (or poor subjective responses): Large dioptric changes in sphere and a high-powered JCC (± 0.75 or ± 1.00 D) are required to enable patients to appreciate a difference in vision and this can easily be done using a trial frame. The trial frame with large aperture lenses also allows unusual head and eye positions that may be necessary for visually impaired patients using eccentric fixation.

• Patients with hearing problems: The phoropter obscures the patient’s view of the examiner and therefore prevents communication with sign language or simple hand signals.

• Patients with high refractive error: The trial frame is fixed to the patient’s head so that the vertex distance does not change with patient head movement and is typically 8 to 12 mm and similar to the vertex distance for glasses. One study reported vertex distances with phoropters to vary from 10 to 34 mm, and vertex distances can be variable during a refraction if the patient moves behind the phoropter. In addition, the back vertex power of a combination of high-powered lenses in a trial frame can be measured in a focimeter rather than simply assume that it is the algebraic sum.

• When over-refracting patients being fitted with multifocal contact lenses: helps to keep the visual environment, binocularity, and pupil size as close to normal as possible ( section 5.9.2 ).

4.4.1

The evidence base: When to use objective refraction and which techniques

An objective measurement of refractive error, usually by retinoscopy or autorefraction, provides an initial estimate of refractive error that can be refined by subjective refraction. It is the only assessment available in patients who are unable to co-operate in a subjective refraction, such as young children and it is heavily relied on when subjective responses are limited or unreliable.

Retinoscopy provides a more accurate result of refractive error in a greater array of patients than autorefraction, although autorefraction is a useful and reliable alternative in many ‘standard’ adult patients and can be particularly accurate at determining astigmatism. Autorefractors should not be used with young children without cycloplegia because of proximal accommodation errors producing significantly more minus results than with cycloplegia, ^, particularly in young hyperopes (e.g., an average 2.50 D more minus for 4-year-old children with individuals varying from 0 up to a nearly −7.00 D difference). These errors remain in children up to 18 years of age. ^, As the measurement of autorefraction is simple, it will not be discussed further.

Retinoscopy also provides a sensitive assessment of the ocular media (e.g., early detection of cataracts, keratoconus), can be used to determine refractive error at distance and near, measure accommodative accuracy ( section 6.8 ), identify accommodative dysfunction, and is portable, less expensive, and less likely to break down. Retinoscopy’s major disadvantage is that it requires several years of training to become proficient.

There appears to be no research literature that compares the accuracy of streak or spot retinoscopes or refractions using negative or positive cylinders. The procedure will be described for streak retinoscopy, but spot retinoscopy is an acceptable alternative. Positive cylinders have the advantage of making retinoscopy easier to learn, as ‘with’ movement is typically easier to see than ‘against’ movement. However, negative cylinders are preferred as they are standard in most phoropters. In addition, there is the possibility of stimulating accommodation during subjective refraction when removing a plus cylinder from a trial frame to replace it with one of another power. For these reasons the procedure will be described using negative cylinders.

4.4.2

Procedure for retinoscopy

See online 4.4, 4.5, 4.6, 4.7 and summary in Box 4.2 . See section 4.5.3 for adaptations to this standard technique.

• 1.

  Prior to retinoscopy, it is useful to estimate the refractive correction from relevant case history (e.g., ability to read without glasses in an older patient suggests a low myope), the current glasses, and visual acuity (VA) information. (e.g., For low myopes and older hyperopes without accommodation, a one line loss of logMAR VA corresponds to approximately −0.25 DS of refractive error. )

• 2.

  Set the retinoscope mirror to the plano position (maximal divergence, with the retinoscope collar at the bottom of its range) and use the smaller sight hole if this option is available ( Fig. 4.6 ; section 4.4.3 , point 9c). Hold the retinoscope with your right hand.

  

• 3.

  Determine a comfortable working distance from the patient so that you can change lenses easily. Typically used distances are 67 cm or 50 cm (+1.50 or 2.00 DS dioptric values, respectively) and depend on arm length.

• 4.

  Switch on the duochrome (bichromatic), spotlight, or a similar distance fixation target that is easy to see when blurred and does not provide a stimulus to accommodation.

• 5.

  Clean the parts of the phoropter or trial frame that contact the patient using alcohol wipes or similar. Fit the trial frame or phoropter so that the patient is comfortable and adjust it to the patient’s distance PD.

• 6.

  Explain the test to the patient: “I’m going to shine a light in your eye and get an indication of the power of the glasses you may need. Please look at the red and green target and let me know if my head blocks your view. Don’t worry if the target is blurred.” Ensure that your head does not block the patient’s view at any time, otherwise the patient is likely to accommodate to it.

• 7.

  Either:

  • (a)

    Dial in the +1.50 DS (or if available and preferred +2.00 DS) retinoscope lens into the phoropter or place +1.50 DS or +2.00 DS working distance lenses in the back cells of the trial frame. This technique has the advantages that all ‘with’ movements indicate hyperopia and all ‘against’ movements indicate myopia, it avoids any calculations and provides ‘fogging’ lenses that will relax accommodation in a low hyperope. The disadvantage is that the working distance lens introduces two reflections, which can make retinoscopy more difficult when you are first learning the technique.

  • OR

  • (b)

    Do not add a working distance lens. The working distance power (+1.50 D or +2.00 D) must later be subtracted from your final retinoscope result.

• 8.

  Position yourself vertically and horizontally ( Figs. 4.7a and 4.8 ) to align your right eye and retinoscope sight hole with the visual axis of the patient’s right eye (the patient’s left eye is fixating the distance target; see Fig. 4.7 a), otherwise you will obtain off-axis errors. If the patient is looking slightly upwards to view the target, you will need to be positioned slightly higher than the patient’s eye (see Fig. 4.8 ).

  
  

• 9.

  Dim the room lights to provide a higher contrast view of the pupillary reflex, while providing enough light to allow easy viewing of the phoropter/trial case. Explain this to the patient: “Dimming the lights helps me to get an accurate result.”

• 10.

  Position the streak so that it is vertical. Look through the aperture of the retinoscope and direct the light at the patient’s pupil and you should see the red retinoscope reflex. Sweep the retinoscope streak across the patient’s pupil horizontally and compare the movement of the reflex in the pupil with the movement of the retinoscope. If the reflex moves in the same direction as the movement of the retinoscope streak, this is known as ‘with’ movement. If the reflex moves in the opposite direction to the movement of the retinoscope streak, this is known as ‘against’ movement.

• 11.

  If a patient is pre-presbyopic, ensure that the patient will not accommodate while looking at the target by quickly scoping the left eye (you do not need to be on the left eye visual axis as this does not need to be accurate) and if ‘with’ movement is observed, add positive lenses until ‘against’ movement is obtained. This will ensure that the left eye (which is viewing the target; Fig. 4.7 a) is blurred by at least +1.50 D.

• 12.

  Sweep the vertical retinoscope streak across the patient’s right pupil and compare the movement of the reflex with the movement of the retinoscope. Mentally note the direction of movement plus the reflex brightness, speed, and width. Then rotate the retinoscope streak so that it is horizontal and sweep across the pupil vertically and repeat the process for the two oblique meridians (45° and 135°). For all four streak positions, mentally note the direction of the reflex movement and its relative brightness, speed, and width.

• 13.

  Determine if the refractive error is spherical (the observed reflex has the same direction, speed, brightness, and thickness in all meridians) or astigmatic (the reflex differs in different meridians).

• 14.

  If the reflex seems dim and movement slow, so that any difference between the reflex speed, brightness, and thickness in different meridians is difficult to determine, the ametropia is likely high, so place an appropriate spherical lens in the phoropter/trial frame and then repeat retinoscopy along the four meridians.

• 15.

  If astigmatic, determine the principal meridians by rotating the streak axis until the angle of the reflex movement coincides with the angle of the streak in two meridians; one perpendicular to the other ( Fig. 4.9 ).

  

• 16.

  Determine the spherical component by ‘neutralising’ (adding plus lenses to ‘with’ movement and minus lenses to ‘against’ movement until the reflex fills the entire pupil and all perceived movement stops at the ‘neutral’ point) the most plus/least minus meridian first (the meridian with the slowest, dullest ‘with’ or fastest, brightest ‘against’ movement). The most efficient process is to make relatively large lens changes based on estimates of the sphere change needed using the reflex’s brightness and speed (see online videos) and bracketing to the final power. For example, a useful approach would be: (1) slow, dull ‘with’, try +4.00 DS; (2) faster, brighter ‘with’, try +6.00 DS; (3) fast, bright ‘against’, try +5.00 DS; (4) very fast and bright ‘with’, try +5.50 DS; (5) neutral. Result is +5.50 DS. This process will improve as you become better at judging the lens changes needed based on the speed, brightness, and thickness of the reflex.

• 17.

  Check the neutral point by moving forward slightly and observing the movement of the reflex. A bright, fast ‘with’ movement should be seen. If you move backward slightly from your normal working distance, a bright, fast ‘against’ movement should be seen. If not, recheck your result.

• 18.

  Set the minus cylinder axis parallel with the streak orientation of the least plus/most minus meridian. Move the retinoscope with the streak in this position and you should observe ‘against’ movement. Add minus cylinder until you achieve neutrality. As ‘with’ movement can be easier to see than ‘against’ movement, you may wish to add minus cylinder until ‘with’ movement is just seen and then reduce the cylinder by 0.25 D.

• 19.

  Briefly, recheck the sphere and cylinder components for neutrality.

• 20.

  Move over to the left side and align yourself with the patient’s left visual axis. Repeat steps 12 to 19 on the patient’s left eye.

• 21.

  Remove the working distance lenses (or subtract 1.50 or 2.00 D from your final result).

• 22.

  Measure the patient’s VAs with the net retinoscopy result.

4.4.3

Adaptations to the standard procedure

• 1.

  Determining cylinder axis using ‘with’ movements

  • Some clinicians prefer to determine cylinder axis by rotating the retinoscope steak until it aligns with a ‘with’ streak (see Fig. 4.9 a). The thickness of the ‘with’ reflex can also be altered by slightly adjusting the sleeve position. ‘With’ movements are gained by moving the retinoscope collar to the top of the sleeve (concave-mirror position) as this changes all ‘against’ movements to ‘with’ movements and vice-versa.

• 2.

  Large pupils: concentrate on the centre

  • Spherical aberration can provide a relative ‘against’ movement in the periphery of the pupil and a common error is to miss slight ‘with’ movement in the pupil centre by averaging reflex movements across the pupil ( see online video 4.6 ). Concentrate on the central reflex and ignore the reflex at the edges of the pupil.

• 3.

  Coping with a ‘scissors’ reflex

  • This reflex moves like the action of a pair of scissors, moving simultaneously in opposite directions from the centre of the pupil, and accurate neutralisation can be very difficult. The reflex can be caused by optical aberrations, particularly coma in a normal eye or more rarely by abnormalities in the media, such as keratoconus or corneal scarring. Increasing the room light level can help because it reduces the patient’s pupil size and cuts down the peripheral aberrations. Use larger lens steps than 0.25 DS and try to bracket the neutral point.

• 4.

  Dim reflexes in young patients: high ametropia

  • If the reflex in a young patient is very dim or hard to interpret, the patient is likely highly ametropic. If the patient is a high myope, moving increasingly closer to the patient’s eye will move the retinoscope closer to the patient’s far point and the reflex will become increasingly bright and fast. A neutral point would occur at the myope’s far point and could be used as an initial estimate. For example, using a working distance retinoscopy lens, if the neutral point is found at 20 cm this suggests that the patient is a −5.00 DS myope. If the reflex does not become brighter as you move closer to the patient, the ametropia is hyperopia, so add a medium to high plus lens.

• 5.

  Fast pupil and reflex changes = accommodative fluctuations

  • During accommodative fluctuations, the pupil will be seen to vary in size and the reflex movement and brightness will rapidly change. This can be seen with young children who change fixation (typically to look at the retinoscope light or their parent/guardian) and the patient needs to be reminded to keep looking at the distance target. If these changes do not appear related to changes in fixation, then accommodative fluctuations that could be caused by latent hyperopia or pseudomyopia should be suspected and a cycloplegic refraction ( section 4.12 ) and assessments of accommodation ( section 6.8 ) should be performed.

• 6.

  Adapt retinoscopy for children

  • See cycloplegic refraction, section 4.12 . Retinoscopy needs to be particularly quick because the patient’s attention span can be brief.

• 7.

  Adapt retinoscopy for a patient with strabismus

  • A strabismic eye is typically unable to fixate the target, so that retinoscopy on the ‘good’ eye must be performed slightly off-axis (see Fig. 4.7 b) to allow the ‘good’ eye to fixate. This will lead to errors, so minimise the off-axis extent as much as possible. Indeed, it is likely that autorefractors will be able to provide a more accurate indication of refractive error in the ‘good’ eye in strabismics as off-axis fixation is not required. For the strabismic eye, it can be easier to change the fixation point for the ‘good’ eye, so that retinoscopy along the visual axis of the strabismic eye is easier.

• 8.

  Are you a retinoscopist with poor vision in one eye?

  • If you are unable to obtain accurate retinoscopy results in your poorer eye, you can use your better eye on both sides, but you will have to scope off-axis on one side (see Fig. 4.7 b), which will provide slightly incorrect results. An alternative is Barrett’s method in which you perform retinoscopy of both the patient’s eyes while the patient fixates the retinoscope and then checks the spherical component of this initial result with the patient fixating in the distance using your good eye. For example, retinoscopy at near gives: RE: −1.50/−1.00 × 10; LE: −2.00/−0.50 × 170. Retinoscopy in the distance for the right eye gives −2.50/−1.00 × 10, an extra −1.00 DS. Apply this difference to the left eye so that the final retinoscopy result is: RE: −2.50/−1.00 × 10; LE: −3.00/−0.50 × 170.

• 9.

  Adaptations to help cope with a dim reflex in older patients

  • A dim reflex in older patients is common owing to small pupils and some media opacity/cataract so that a reduced amount of light reaches the retina and even less returns to your retinoscope. Increasing the retinoscope light intensity may just reduce the pupil size further and a medium intensity is usually best. An autorefractor result may not be obtainable with these patients, but retinoscopy can provide a useful result if used with the following adaptations:

    • a)

      Perform retinoscopy at a closer distance such as 25 cm or 33 cm because this can provide a brighter reflex. You will have to subtract a larger value from your retinoscopy result to compensate for the reduced working distance (4.00 or 3.00 D, respectively, for 25 cm or 33 cm) and understand that there is a greater chance of dioptric error. For example, a 5 cm error when using a 67 cm working distance (i.e., actually at 62 cm) causes a 0.10 D error, but the same 5 cm error when using a 25 cm working distance (i.e., actually at 20 cm) causes a 1.00 D error. There should be no error for astigmatism as long as your working distance remains constant.

    • (b)

      Use the least number of lenses in the trial frame/phoropter as you lose 8% of the reflex for each lens used owing to reflections. Do not use a working distance lens and refract each meridian using a sphere only and convert to a sphero-cylindrical combination for the subjective refraction.

    • (c)

      Use the large aperture sight hole when available (see Fig. 4.6 ) as this will allow more of the returning light to reach your eye.

4.4.4

Recording of retinoscopy results

Record your retinoscopy results as the sphero-cylindrical correction that neutralised the patient’s refractive error after removing your working distance lenses. Do not use a degree sign as ° can look like a 0 and make an axis of 15° look like 150 degrees. Use ‘x’ rather than the word ‘axis’. Most clinicians record the spherical and cylindrical power to the nearest 0.25 D, and the cylinder axis to the nearest 2.5° (i.e., half the step size of 5° on the axis scale). Use 180 rather than 0 degrees. Some clinicians, typically those who use autorefractors and computerised phoropters that provide refractive correction results in smaller step sizes, record to the nearest 0.125 D and 1°. Also record the monocular VA with the retinoscopy result.

Examples (with VAs in visual acuity rating (VAR), decimal, metric, and imperial Snellen)

4.4.5

Interpreting retinoscopy results

On average, retinoscopy provides a refractive result slightly more positive than subjective refraction in young patients. This decreases with age, so that retinoscopy and subjective results are similar in presbyopic patients. As the stimulus to accommodation is greater in subjective refraction (the target is a line of small letters) than in retinoscopy (the target is a blurred duochrome and the eye is fogged by 1.50 to 2.00 DS), the retinoscopy result in young hyperopes can be much more positive than accepted in subjective refraction. Errors can occur in retinoscopy if it is performed off-axis (see Fig. 4.7 b), which will induce spherical and astigmatic errors, or if it is performed at an incorrect working distance, which will induce a spherical error. Note that cylinder axes in the two eyes are often mirror images of each other. ^, For example, 175° with 5°; 20° with 160°; 45° with 135°.

4.4.6

Most common errors in retinoscopy

• 1.

  Using lenses smudged with fingerprints when performing retinoscopy with trial case lenses. Student trial case lenses are notoriously smudged so try to get into the habit of cleaning lenses before using them.

• 2.

  Performing retinoscopy at an incorrect working distance, often too close as you move slightly forward to see the reflex more clearly (e.g., working at about 50 cm, while using a 1.50 D working distance lens).

• 3.

  Not concentrating on the movement in the centre of the pupil in a patient with large pupils.

• 4.

  Performing retinoscopy off-axis.

• 5.

  Blocking the patient’s view of the distance chart and stimulating the patient’s accommodation.

4.5.1

Procedure for monocular refraction

See online video 4.8 .

• 1.

  Clean the parts of the phoropter or trial frame that contact the patient using alcohol wipes or similar. Fit the trial frame or phoropter so that the patient is comfortable and adjust them to the patient’s distance (PD).

• 2.

  Explain the procedure to the patient and reduce the patient’s concerns about providing wrong answers : “During this test, I will place various lenses in front of your eye to find the lenses that give you the best vision. Don’t worry about giving a wrong answer because we already have a very good indication of the glasses power you need from your (old glasses/the moving light test/the autorefractor machine) and everything is double checked.”

• 3.

  Sit or stand where you can comfortably manipulate the phoropter or trail frame and trial case lenses.

• 4.

  Begin with the net retinoscopy sphere and cylinder before each eye.

• 5.

  Occlude the left eye.

• 6.

  Determine the best vision sphere (BVS) ( section 4.6 for phoropter-based refractions and section 4.7 for trial frame-based refractions).

• 7.

  Check that the circle of least confusion is on the retina (or behind the retina and can be brought onto the retina using accommodation) prior to the use of the JCC using the duochrome test ( section 4.8 ).

• 8.

  Determine the cylinder axis using the JCC ( section 4.9.3 ).

• 9.

  Determine the cylinder power using the JCC ( section 4.9.3 ).

• 10.

  If you have changed the cylinder power or axis significantly, repeat the BVS assessment (step 6).

• 11.

  Check the final spherical endpoint using the duochrome test ( section 4.8 ) or the +1.00 blur test as appropriate.

• 12.

  Measure VA.

• 13.

  Repeat steps 5–10 for the other eye.

• 14.

  Compare the monocular VAs with your subjective refraction result with the patient’s vision or habitual VAs (as appropriate). If the VA is better with the patient’s glasses, then it is likely that your subjective result is incorrect.

• 15.

  Compare the VA with the present subjective refraction with age-matched normal data ( Table 4.1 ). If the VA is worse than expected or worse in one eye compared with the other, remeasure the VA with a pinhole aperture. If the VA improves with the pinhole, either the subjective refraction is not optimal and should be repeated or the patient has media opacity, typically cataract, which is being bypassed by the pinhole.

  Table 4.1

  Average visual acuity data for normal, healthy eyes as a function of age ^a

  From Elliott DB, Yang KC, Whitaker D. Visual acuity changes throughout adulthood in normal, healthy eyes: seeing beyond 6/6. Optom Vision Sci. 1995;72:186–91.

  Age (years)	VAR	Snellen (metric)	Snellen (decimal)	Snellen (imperial)	LogMAR
  20–49	107 (101 to 113)	6/4.5 +1 (6/6 +1 to 6/3 −2 )	1.25 +2 (1.0 +1 to 2.0 −2 )	20/15 +1 (20/20 +1 to 20/10 −2 )	−0.14 (−0.02 to −0.26)
  50–59	105 (100 to 110)	6/5 +1 (6/6 to 6/3.8)	1.25 (1.0 to 1.6)	20/15 −1 (20/20 to 20/13)	−0.10 (0.00 to −0.20)
  60–69	103 (98 to 108)	6/5 −1 (6/6 −2 to 6/4 −1 )	1.25 −2 (1.0 −2 to 1.25 +3 )	20/15 −2 (20/20 −2 to 20/13 −1 )	−0.06 (0.04 to −0.16)
  70+	∼100 (96 to 106)	6/6 (6/7.5 −1 to 6/4.5)	1.0 (0.8 −1 to 1.25 +1 )	20/20 (20/25 −1 to 20/15)	∼0.00 (0.08 to −0.12)

  logMAR, Log of the minimum angle of resolution; VAR, visual acuity rating.

  a The 95% confidence limits are shown in parentheses.

• 16.

  If the final refractive correction in either eye is above 5.00 D mean sphere equivalent (MSE, the sphere plus half the cylinder; e.g., −4.75/−1.50 × 180 has an MSE of −5.50 D, +5.50/−2.00 × 90 has a MSE of +4.50 D), then measure the back vertex distance. This is the distance from the back surface of the lens nearest the eye to the apex of the cornea ( Fig. 4.10 ).

  

4.5.2

Recording of refraction

Record the refractive correction using the same format described for retinoscopy (section 4.5.5). Record the monocular VAs. If pinhole VA is measured and reveals no improvement in VA, record PHNI (‘pinhole no improvement’); otherwise record the VA with the pinhole. For refractive corrections above 5.00 D equivalent sphere, record the vertex distance. Make sure that the prescription details that you provide to patients are clearly legible. Illegible prescription forms have been reported as a surprisingly common error in optometric practice.

Examples of recording (with VAs provided in VAR, decimal, metric and imperial Snellen):

4.5.3

Interpreting refraction results

The subjective results should be compatible with the retinoscopy results in most cases, although young patients may provide a more positive (less minus) correction than retinoscopy. A subjective result that is significantly less positive (more negative) than the retinoscopy result could indicate latent hyperopia or pseudomyopia and a cycloplegic refraction may be required ( section 4.12 ). The difference between the patient’s own glasses and the subjective refraction should be compatible with the difference between the habitual VA (i.e., with their own glasses) and optimal VA ( section 4.12.6 ). A patient with reduced VA (typically in both eyes) and a retinoscope result that indicates emmetropia or slight hyperopia may have nonorganic visual loss ( section 4.12.6 ).

4.6.1

Procedure for MPMVA

• 1.

  Occlude the left eye.

• 2.

  Determine the VA of the right eye.

• 3.

  Add +1.00 DS to the spherical lens determined in retinoscopy and check the VA. The VA should be reduced by about four lines. If the VA only worsens by one or two lines (or gets better), add additional positive power to the sphere until four lines of acuity are lost to ensure the eye is ‘fogged.’ Experienced clinicians often use a smaller fogging lens such as +0.50 DS.

• 4.

  Ask the patient: “Are the letters clearer with lens 1...” wait while you give the patient an appropriate period of time to appreciate the clarity of the letters, then reduce the amount of fog by 0.25 DS as you ask “or lens 2?” Ask the patient to read the lowest line of letters he or she can see to check that VA improves with the preferred lens.

• 5.

  Continue to reduce the amount of fog in 0.25 DS steps and stop when there is no improvement in VA.

• 6.

  It can be useful to randomise the letters on computer-based systems when the final decisions are being made to avoid problems owing to letter memorisation.

• 7.

  Remember that the average acuity of a 20-year-old is ∼107 VAR (6/4.5, 1.40, 20/15; Table 4.1 ), so that most younger patients can read beyond 100 VAR (6/6, 1.0 or 20/20).

4.6.2

Adaptation for older patients

Processing speed slows significantly with age, so provide longer presentation times and repeat the presentations for older patients. Note that you are more likely to over-plus than over-minus older patients ( section 4.7.3 ).

4.6.3

Interpreting MPMVA results

The MPMVA approach is designed to take advantage of a patient’s depth of focus to provide the maximum range of clear vision. For example, after refraction, the retinal image should be conjugate with the distance VA chart at 6 m (20 ft). However, this does not take advantage of the depth of focus. For example, if the depth of focus was +0.50 D and the retinal image was conjugate with the distance VA chart so that 0.25 D of the depth of field was in front of the VA chart at 6 m (20 ft) and 0.25 D behind it, the chart would be clear from 2.4 m (8 ft) to ‘beyond’ infinity. Using the MPMVA technique places the distal edge of the depth of focus conjugate with the VA chart, so that with a depth of focus of +0.50 D, the range of clear vision is from 1.5 m to 6 m (5 to 20 ft). However, using this technique does mean that patients are slightly over-plussed by 0.16 D as the distance VA chart is at 6 m (20 ft) and not infinity. This can be offset in young patients owing to a lead of accommodation (+0.25 DS) during distance refraction, but this does not occur in older patients who have lost accommodation. Over-plussed/under-minused refractive corrections are common causes of unhappiness with glasses in older patients. Be particularly aware of over-plussing with older patients with a large depth of focus owing to small pupils, and do not use a truncated VA chart or reduce plus to just the 100, 6/6, 1.0, 20/20 line because this will aggravate the effect. An indication of over-plussing is that the measured addition is lower than expected.

4.6.4

Recording MPMVA

The results of MPMVA are not recorded because the technique is just part of the subjective refraction.

4.6.5

Most common error in MPMVA

• 1.

  Only unfogging VA to 100 (20/20, 6/6, 1.0). Given that most younger and many older patients can read 105 (20/15, 6/4.5, 1.2; Table 4.1 ), the patient would be slightly over-plussed/under-minused. Using the JCC when the circle of least confusion is in front of the retina, as it would be in this case, can lead to an incorrect determination of astigmatism.