Binocular vision anomalies investigation(cover test)

what methods of investigation are there

objective and subjective

what does objective test mean

practitioner makes judgement- not relying on px responses

what doe subjective test mean

relies on patient responses or perceptions 

what are objective examples

• cover test

• general observation

• Hirschberg, krimsky and Bruckner

subjective test examples

• diplopia testse e.g vertical prism

• tests with independantt objects.g Maddox wing

• distortion testse.g Maddox ring

advantages of subjective

• detects smaller deviations( vertical)

• requires less observer skill

• more precise sometimes

what is the cover test

differentiates between phobia an tropias

there are two types

what are the types of cover test

cover/uncover

alternating cover

how to carry out general cover test

• Monocular VA measure

• px focus on letter of line with weaker eye

• if target appears to move follow it with eye 

• performed at DV and NV

• performed with and without glasses

cover/uncover test

• observe uncovered eye was I cover and uncover other eye

what will practitioner observe

if when cover is introduced , uncovered eye moves out for fixation = esotropia( this means its usually inwards but the cover corrects)

Moves in to correct = exotropia

no movements = trope rules out (no manifest)

which eye are we covering

the ‘good eye’ for deviation to fixate

alternating cover test

used when presence of tropia is ruled out

how is it used to investigate phobias

• cover eye and observe covered eye as you swap the cover between the eyes

• Esophoria= moves in behind cover →( may observe it moving back out as cover removed)= recovery movement

• exophoria = moves out behind cover

what’s important to remember with alternating cover tests

do not allow binocular fixation

what is intermittent tropia

starts with latent deviation- becomes manifest after cover ( first time perform cover test looks like a phobia then a tropia)

how do we measure the deviation size

prism bar

what does a prism do

bend light rays towards the base, object appears to move towards apex

1 prism dioptre causes 1 cm displacement for an object at 1metre

what’s a prism cover test

neutralising movements with a prism bar( by moving retinal image to fovea), its used alongside a cover test to measure size of deviation ( magnitude of phorias/tropia)

how is it carried out

• you carry out cover tests first and subjective if required

• prism bar in front of deviating eye

• do alternating cover test and when cover in front of fixating eye no movement is seen when correct prism in place

• find lowest prism power required to stop the deviation

when is base out used

for esophoria and esotropia

when is base in used

for exotropia and exophora

what about vertical phobias

they are less common than horizontal but more problematic- look at corneal reflexes or associated lid movement to help identify

what tests are used to estimate manifest strabismus- objective

• hischberg- compares position of 1st purkinje images of two eyes formed by pen torch

• krimsky- uses prism of increasing power to position corneal reflex in centre of squinting eye

• Bruckner test

diplopia principle wat tests

vertical prisms

distortion tests(Maddox rod)

independant objects(Maddox wing

what does diplopia principle test

subjective localisation of a single point object imaged on fovea of fixating eye and extrafoveal retinal area in other

Maddox rod

can be used with horizontal/ vertical deviations

on screen a line is shown 

practitioner shines pen torch in mirror (DV) or at px(NV)

small glass used before RE -

Ask’ is line to the left or right of the spotlight,

if lien is to the left?

= exophoria

correct with base IN

if line Is to the right

=esophoria

correct with Base OUT

what do we do next

use prism to correct and align spotlight and line

what about vertical madden rod

if line above spotlight= hypophoria(below fovea)

if line below spotlight = hyperphoria (imaged above fovea)