Chapter 16: Practical clinical refraction Flashcards
What should you do when testing visual acuity in a patient with nystagmus?
fog the fellow eye with a high plus lens, as complete occlusion makes nystagmus worse and lowers uniocular acuity
Why should detailed scrutiny of the fundi be left until after refraction?
to avoid photostress-induced reduction of acuity
When is it not possile to test the muscle balance using the Maddox rod?
it patient has manifest squint without diplopia i.e. binocular vision is lacking (Maddox rod + wing tests depend on binocular vision)
What should the patient be focusing on for objective refraction (i.e. retinoscopy)?
gazing at distant object e.g. top letter of test type
Which eye should the examiner use for patient’s L eye and vice versa?
L eye for L eye, R eye for R eye
Should the fellow eye be occluded in retinoscopy and why?
no, best to fog it rather than occlude to discourage involuntary accommodation
When might you have to occlude the fellow eye in retinoscopy?
manifest squint, to achieve steady fixation with the non-dominant eye
When might you have to occlude the fellow eye in retinoscopy?
manifest squint, to achieve steady fixation with the non-dominant eye
Who should be given a cycloplegic drug for retinoscopy and why?
young children or children with latant or manifest squint, to paralyse ciliary muscle
What type of lens should you add if the retinoscopy reflex is ‘against’ the movement of the retinoscope?
minus (concave) lens
How can the axis of astigmatism be foundwith retinoscopy?
rotate axis of cylindrical small amounts rechecking retinoscopy each time; if the axis of the cylindrical lens lies outside the axis of astigmatism, the reflex will move obliquely; reflex will only align with the axis of the cylindrical lens if they all lie in the axis of the astigmatism
Why can it be beneficial to try to correct astigmatism with plus cylinders with plus spheres, and minus cylinders with minus spheres?
most patients don’t accept the full value of the cylinder found on retinoscopy; if plus cylinder is reduced e.g. by +0.5 D, the higher value meridian would now be undercorrected which is better tolerated than overcorrection
What is an example of when refraction varies between the central and peripheral parts of the pupillary aperture?
nucleosclerosis - central zone relatively myopic compared with periphery
What can cause ‘scissor shadows’ with retinoscopy, giving the apearance of two reflexes in the pupil?
difference in refraction between different zones of the pupillary aperture
What should be used as the end point in the case of scissor shadows on retinoscopy?
one blade of scissors usually brighter than the other - end point taken when brighter reflex reverses
What can cause the oil drop sign on retinoscopy?
keratoconus - swirling reflex, reflex from apex of cone darker than periphery
How is subjective refraction performed?
- occlude fellow eye, using distance test type, verify sphere achieved with objective refraction by offering small plus and minus additions until no further improvement can be made
- then verify axis of cylinder before adjusting its power
- if patient myopic, duochrome test should be done monocularly and binocularly
How should the result from the duochrome test be interpreted?
- If the images on the green are clearer, add plus +0.25 DS until you obtain balance.
- If the rings on the red look clearer, add minus -0.25 DS until you obtain balance
If clarity is better from green to red or red to green with one 0.25 change in lens power, how should you leave the patient (i.e. slightly clearer red or slightly clearer green)?
leave on the red
At what degree of refractive error must you emausre and record back vertex distance?
power of cylindrical lens exceeding 5 dioptres
What is the most frequent reason that a patient seeks a refraction retest?
too strong a near additino ha been prescribed
What is the maximum reading add that should be given in normal circumstances?
no more than +2.50 DS
except pseudophakic patients - often prefer +3.00 DS
What is the maximum reading add that should be given in normal circumstances?
no more than +2.50 DS
except pseudophakic patients - often prefer +3.00 DS
What is the Maddox wing used for?
used to check near muscle balance
What does orthophoria (perfect alignment of the 2 eyes) for distance but a large exophoria for near indicate?
convergence insufficiency
What should be the initial management of convergence insufficiency that is symptomatic and why?
- convergence should be strengthened by means of convergence exercises
- if base-in prisms are prescribed, convergence may become weaker still and progressively stronger prisms will be required
What is a common cause of intolerance of aphakic spectacles?
poor central of spectacle lenses causing prismatic effect - can be due to incorrect interpupillary distance
How is the anatomical interpupillary distance measured?
patient looks at examiner L eye and zero of rule set to pt R eye nasal limbus, then asked to look at R eye and measurement taken to L eye temopral limbus (alternatively look at fixation light in front of eaminer’s eyes and corneal light reflexes measured on patient’s eyes)
What measurement is used when making spectacles for IPD?
distance between visual axes for distance vision; approx 1mm less than anatomical IPD
How should you leave refraction of hypermetropes and why?
don’t overcorrect - best to leave 0.25 DS undercorrected so can read bus numbers in far distance
How should you leave refraction of myopes and why?
do not fully correct - best to leave 0.25 DS undercorrected so don’t need accommodation for distance
When does hypermetropia, which is very common in infants and children, require full correction?
in presence of esophoria or esotropia
What degree of hypermetropic anisometropia must be corrected and why?
> 1 dioptre, to prevent refractive amblyopia
Why might it take a few days for a hyeprmetropic child to adapt to new spectacles?
uncorrected hypermetropic child overcomes some or all hypermetropia by exercising extra accommodation; when glasses worn for first time, accommodation may not relax and vision will be blurred initially
Why might it take a few days for a hyeprmetropic child to adapt to new spectacles?
uncorrected hypermetropic child overcomes some or all hypermetropia by exercising extra accommodation; when glasses worn for first time, accommodation may not relax and vision will be blurred initially
What are 2 reasons why myopia should be corrected in a child?
- if sufficient to prevent child fom seeing what is written on the blackboard
- in presence of exophoria or exotropia, to stimulate accommodation and convergence
Why do myopic children need to have stronger glasses as they grow?
eye also grows increasing axial myopia
How do young children tolerate spectacles for uniocular high myopia?
young brain copes with greater degrees of aniseikonia than adult is able to
When does astigmatism require correction?
if visual acuity reduced in affected eye
How common is astigmatism in babies and infants?
present in many to high degrees, but usually reduces or disappears by age of 2.5 -3 years
How often should refraction be reviewed in children once glasses are established?
annually
What is an example of when refraction varies between the central and peripheral parts of the pupillary aperture?
nucleosclerosis - central zone relatively myopic compared with periphery
