Objective Refraction Flashcards
What is objective refraction?
obtaining a refractive prescription that does not require any response from the patient - this is obtained by retinoscopy; for children or adults with learning disability, this may be the sole basis for a spectacle prescription
What is subjective refraction?
relates to fine-tuning the prescription obtained from retinoscopy by asking the patient a number of clear, closed questions whilst avoiding fatigue.
How to measure interpupillary distance
Sit directly in front of them, resting a ruler on the bridge of their nose, ask the patient to look at your left eye and close your right eye. Line up the temporal limbus of the patient’s right eye with the zero marking of your ruler.
Ask them to look at your other eye (now close your left eye and open your right eye), and holding the rule very still, record the position of the nasal limbus of the left eye on the ruler in millimetres.
What is the range of normal IPD
The IPD typically lies between 55 and 75 mm.
How to measure interpupillary distance near
Check you are at the same height as the patient and sitting level with the patient’s reading distance. Ask the patient to look at the bridge of your nose. Close your right eye, and with your left eye, line up the zero of your ruler with the temporal limbus of the patient’s right eye. Keeping your right eye shut, record the position of the nasal limbus of the patient’s left eye on the ruler.
Typically, the IPD for near is 2 to 4 mm less than for distance due to the convergence that occurs with near stimulation.
How to know if IPD is correct
Check that the pupil is easily seen - if it is obscured in the horizontal plane, you will need to re-check your IPD; if it is obscured in the vertical plane, you will need to adjust the nasal rest (if the pupil is too high, lower the central frame bracket to elevate the trial frame)
What is back vertex distance (BVD)
Distance between patient’s cornea to the back of the lens (the surface of the lens nearest the cornea)
What is normal BVD
12 to 14 mm
What is visual acuity
‘Acuity’ is a measure of the resolving power of the eye —the ability to discriminate between two points.
What VA do you need to assess for patients in the exam
*distance acuity unaided (Snellen or LogMAR)
*distance acuity with pinhole
*near acuity unaided (N-series; remember to use a bright lamp).
What does it mean when VA does not improve with pinhole
amblyopia, retinal, nerve, or cerebral pathology (pinhole acuity can be worse than unaided acuity in patients with macular pathology, since it precludes eccentric fixation).
What dioptric power gives 6/12 vision
1D
What dioptric power gives 6/24 to 6/36 vision
2D
What dioptric power gives 6/60 vision
3D
How can you tell if a patient is myopic from VA
If a patient has poor distance vision but good near vision, you know they are myopic. For example, if a presbyope has an unaided Snellen distance acuity of 6/60, yet is N5 at reading distance (on the near vision N-series reading chart), their refraction is probably around -2.00 to -3.00 spherical dioptres.
What does it mean if the patient has poor distance and near vision
They are hypermetropic (or they have amblyopia, ocular pathology, or cerebral visual impairment - this should be clear from your history).
How do you know if neutralisation is being approached in retinoscopy
The reflex will become faster and brighter.
(the entire pupil lights up when the slit enters the pupil)
What characteristics of the retinoscope reflex are important to note
(a) direction
(b) orientation
(c) brightness and speed.
What does a ‘with’ retinoscope reflex mean
as your slit passes across the pupil, a light within the pupil (the reflex) moves in the same direction
How to approach neutralisation when a with reflex is seen
A plus lens must be added to the trial frame to approach neutralisation
What does an against reflex mean
as your slit passes across the pupil, a light within the pupil (the reflex) moves in the opposite direction
How to approach neutralisation when an against reflex is seen
A minus lens must be added to the trial frame to approach neutralisation.
To ensure endpoint has been reached which lens do you add to the trial frame
+0.25D which gives an against reflex
How should the orientation of the reflex be
should be parallel to the pupil reflex.
Treatments for keratoconus
contact lenses, scleral con- tact lenses, or surgical intervention (riboflavin with ultraviolet A/collagen cross- linking, intrastromal implants, deep lamellar or penetrating keratoplasty).
What are some causes of a dull retinoscope reflex
medial opacity (such as with a cataract or vitreous haemorrhage). A dull reflex can also occur as a result of flat retinoscope batteries!
What lens to choose if the reflex is dull and slow
a dull, slow reflex is far from neutralisation and sometimes it pays to begin with a +5 or +10 dioptre spherical lens to start off with
What is the working distance
the distance from the patient’s cornea to your retinoscope
How to correct for working distance in retinoscopy
To convert to the corrected prescription, it is necessary to add a -1.50 sphere to the trial frame (to correct for a66 cm working distance) or a -2.00 sphere (to correct for a 50 cm working distance). Note that the cyl remains unchanged.
What is the corrected refraction if neutralisation occurs with +4.25 /-1.75 x 030 at 66 cm
+2.75 / -1.75 x 030, since +4.25 plus -1.50 = +2.75
What is the corrected refraction if neutralisation occurs with -3.75 / +0.75 x 044 at 50 cm
-5.75 / +0.75 x 044, since -3.75 plus -2.00 = -5.75.
Room light for retinoscopy
DIM
not dark or bright
Key points for retinoscopy
1) Establish a dim room.
2) Fog (or occlude, if necessary) the fellow eye.
3) Scope the patient’s right eye with your right eye/right hand.
4) Scope the patient’s left eye with your left eye/left hand.
5) Keep your scope as close as possible to their visual axis, without interrupting continuous distant fixation.
6) Correct for working distance (add -1.50 sphere if at 66 cm; add -2.00 sphere if at 50 cm).
7) Record in either positive cyl notation for both eyes or negative cyl notation for both eyes (never positive for one eye and negative for the other).
Why is it important to fog the eye of patients who are not presbyopic (especially myopic patients)
The reason why the fellow eye should be fogged is to reduce accommodation, which would give a false result when examining the fellow eye with the retinoscope.
Why is fogging not needed in cycloplegic refraction
accommodative component is removed by the cycloplegia.
Why is fogging better than occlusion
This fogging induces less accommodation than simple occlusion with a black occluder —hence, the effort made to fog rather than simply occlude.
When is occlusion preferred over fogging
1) when the eye being tested is densely amblyopic (since the eye not being tested must have a poorer acuity to help avoid accommodation and a +2.00 lens will probably be insufficient to achieve this)
2) if the patient markedly objects to fogging due to diplopia or asthenopia
3) if you are unable to estimate acuity and provide an adequate fog lens.
Which hand scopes which eye
Use your right hand and right eye to scope their right eye. Scope first with a vertical, then a horizontal, and finally a diagonal slit to locate the principal meridians.
What to do if a dull and slow reflex is seen
try using a +/-5 or even a +/- 10 dioptre lens.Then proceed by refracting in plus or minus cyls or spheres alone
Why is using negative cyls preferred over positive cyls
it induces less accommodation, since the spherical component is more positive.
When using plus cyl, which meridian is neutralised with spheres
The principal meridian that has an against reflex - or, if both reflexes are with, it will be the least with reflex (which is fastest and brightest, as it is nearest neutralisation)
After the first meridian is neutralised with spheres what do you do next?
This will result in the other principal meridian giving a with reflex, which is then neutralised with positive cyls (the axis on the lens in the same orientation as the scope slit and pupillary reflex)
Once your refraction is complete, what do you do to obtain the final prescription
correct for working distance
What does it mean if when rotating from first to your second streak you see against instead of with motion
It means the first meridian you found was not the sphere meridian.
If you find against reflex with scope slit at 135 and a with reflex at 045, how do you neutralise this ( work in plus cyl)
Add minus spheres until the against reflex at 135 is neutralised (say, ~3.00 causes neutralisation). Then add plus cyls (with the axis in the same orientation as the scope slit at 045) to neutralise the with reflex (say, +1.50 at 045 causes neutralisation). The axis line on the cyl lens should be parallel to the scope slit and light reflex (perpendicular to its power)
what do each of the arrowed arms of a power cross represent
represents the direction of movement of the retinoscope sweep.
When sweeping horizontally with the scope slit orientated vertically, which plane is the power observed in
power in the horizontal plane (180) is examined. If a sphere with power +3.50 dioptres neutralises a horizontal sweep, this implies the power in the horizontal direction is +3.50 dioptres
How to obtain the prescription from the power cross in plus cyl notation
1) record the least positive meridian as the sphere
2) record the cyl as the difference between the two meridians
3) record the axis as the same axis of the most positive meridian (remembering
that the axis is perpendicular to the direction of action of the power arrow)
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when the sweeps are neutralised in all meridians
The patient has a ~1.50 ds refractive error.
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get a dull, slow reflex that is difficult to interpret
Provided your battery has not been exhausted from all your enthusiastic work, the patient has a high degree of ametropia, so try interposing a + / - 5 or 10 Jens. Remember, aphakia is a common cause of high hypermetropia, so don’t mistake the initial dull, slow reflex for neutralisation!
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get an against reflex in all meridians which is equally fast and bright.
The patient is more myopic than -1.50 ds, and there is no significant astigmatism (neutralise with minus spheres).
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get a with reflex in all meridians which is equally fast and bright
The patient is more plus than -1.50 ds, and there is no significant astigmatism (neutralise with plus spheres).
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get an against reflex in one meridian, but more against (slower and duller) in another.
The patient has compound myopic astigmatism. Add minus spheres until the most against cyl is neutralised, leaving a perpendicular with reflex that can be neutralised with plus cyls.
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get a with reflex in one meridian but more with (slower and duller) in another.
The patient has compound hypermetropic (or rather more plus than -1.50 ds) astigmatism. Add plus spheres until the least with cyl (faster and brighter reflex) is neutralised leaving a perpendicular with reflex that can be neutralised with plus cyls.
Assuming you are working at 66 cm and have decided to work in plus cyls format, what does it mean when you get a with reflex in one meridian and an against reflex in the perpendicular meridian.
The patient has mixed astigmatism. Add minus spheres to neutralise the against reflex, then add plus cyls to neutralise the with reflex.
For the simulated retinoscopy station with no trial frames, how do you go about refracting?
the practitioner needs to take care in judging their working distance and awareness of the cylindrical axis. This is a similar situation to performing retinoscopy on children (who are averse to trial frames)
How to reduce accommodation in non cycloplegic retinoscopy
- fog fellow eye
- ensure the patient maintains distant fixation
- avoid prolonged retinoscopy bursts
*Failure to reduce accommodation gives a spuriously myopic result.
What might it mean if the prescription is too minus
1) check that the patient is not accommodating, either because they are not looking at the distant target (patients need constant reminders to do this) or because you have occluded rather than fogged the fellow eye.
2) Occlude the fellow eye when checking visual acuity, but when using your retinoscope and for subjective refraction fog the fellow eye (with a +2 to + 4 add on your estimated prescription to reduce accommodation).
3) If the patient is amblyopic or diplopic, avoid fogging and simply occlude the fellow eye for retinoscopy and subjective refraction. If accommodation is an issue (as it is with all children), cycloplegic refraction is required.
What might it mean if the prescription is too plus
remember to subtract the working distance correction factor.
