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.