Macular Function tests

Moderator: Dr.GKG.Prasad Presenter Dr.Vaibhav

Macula is a round area at the posterior pole temporal to the optic disc 5.5mm in diameter.

Recently term area centralis has been given to this.

It corresponds to 15 degrees of visual field,photopic vision,colour vision.

It is yellowish color derived from the

presence of xanthophyll pigment

Anatomical background

Fovea centralis is the central depressed part of macula.1.85mm diameter,0.25 mm thick.

Most sensitive part of retina.

Foveola forms the central part Umbo is the tiny depression in the centre of

foveola Foveal avascular zone is located inside the

fovea but outside the foveola.

Densest concentration of cones a one to one photoreceptor-ganglion

cell relationship Cones more elongated and slender Absence of rods at the foveola RPE cells are taller, thinner and

deeply pigmented Presence of xanthophyll pigment

Specific anatomic cofiguration of the Fovea

This special anatomy enable the fovea for:

highest discriminative ability(VA) color perception

Diagnosis and Follow up of macular diseases

For evaluating the potential macular

function in eyes with opaque media such as cataract and dense vitreous hemorrhage

Uses of macular function tests

Symptoms

• Central vision impairment • Metamorphopsia• Micropsia• Macropsia

Clinical assessment of the macula

Tests utilizing gross recognisation responses. Based on entoptic phenomenon Interference fringe techniques Based on hyperacuity Electrophysiological tests Direct visualisation & assessment of anatomic

integrity.

Macular function tests classification

Psycho physical tests1. Visual acuity2. Colour vision3. Photostess test4. Amslers grid test5. Two point discrimination test6. Entoptic imagery7. Maddox rod test8. Perinauds near vision chart testing9. Self illuminated near vision chart

Visual evoked potential Electro retinogram.

Electrophysiological tests

Slitlamp examination. OCT Presence or absence of perception of light history

Others

They utilise gross recognisation response. A. Two light discrimination test: The ability to distinguish two illuminated

points of light 2mm diameter in size 2 inches apart placed 2 feet away suggests good functions.

< or equal to 12.5cm- VA of HM/PL < or equal to 7.5cm-VA of CF to 6/60 <5cm –VA >6/60

Rudimentary clinical techniques

Pinhole increases the depth of focus and limits scattering effect of corneal scars, lenticular opacities.

In macular disorders pinhole worsens the visual acuity.

Pinhole test

Maddox rod test Simple and reliable test and can

be used in semi opaque media Pt is asked to fixate light at a

distance of 1/3m through M.R. with opposite eye occluded

Any breaks/holes; discoloration/distortion indicates a macular lesion

Vryghem macular function test Parinaud near reading chart 8 D trial lens First position the chart 12cm

from trial lens with best correction for long distance and hyper addition of 8D in trial frame.

Patient is asked to read smallest number on the chart. If patient reads it give + score

If not give – score.+ score indicates good macular

function.

Evaluates the 10 ̊ of visual field centered on fixation

Used in screening and monitoring macular diseases

square 10*10 cm divided into 400 5*5 mm squares to be held at 30 cm

Amsler grid

The First Grid Has White Lines On A Black Back Ground And Central White Dot On Which The Patient Is To Fixate.

If The Patient Reports On The First Chart They Cannot See The Central White Spot. This Would Indicate A Positive Scotoma. The Following Chart Should Be Used On Which Diagonal Lines Help Maintain Central Fixation. This Helps Them Point Out The Limits Of The Scotoma. This Chart Also Has White Lines On A Black Back Ground And Central White Fixation Dot.

The Third Chart Has Red Lines On A Black Back Ground And Is Very Helpful In Diagnosis Of Optic Nerve, Chiasmal, Or Toxic Amblyopia Related Problems.

Central Scotoma As Seen By A Patient With A Positive Or Absolute Scotoma. For Example This Might Be Secondary To Central Areolar Choroidal Dystrophy Or Congenital Toxoplasmosis .

A Space-Taking Pathology Such as A Tumor That Forces The Cones Closer Together Will Cause The Grid To Be Seen Distorted. The Retinal Image Will Fall On More Cones Than Normal And The Lines Of The Amsler Grid Will Be Seen As Larger And Bend Outward As In The Above. This Is Known As "Macropsia".

A Patient With Macular Edema Or Any Other Pathology That Forces The Cones Apart The Retinal Image Will Stimulate Fewer Cones Than Normal And The Lines Of The Amsler Grid Will Be Seen As Smaller And Tend To Bend Away From The Patient. This Condition Is Termed "Micropsia".

A Combination Of Squeezing And Spreading Of The Cones Causes An Overall Distortion Of The Image. The Lines Of The Amsler Grid Become Distorted And Non-Uniform. This Can Occur In A Number Of Macular And Retinal Conditions. This Condition Is Termed Metamorphopsia.

Colour vision is the function of three populations of retinal cones

Blue ( tritan) 414-424 nm Green ( deuteran) 522-539nm Red (protan) 549-570nm Normal person possess all these three

cones and called trichromat

Colour vision

Acquired macular diseases tends to produce blue yellow defects and optic nerve lesions red green defects

Deutran anomaly is the most common and those subjects can not differentiate between red and green colours

Colour confusion tests

Ishihara charts it is useful as screening test

Farnsworth-Munsell 100 Hue test is the most sensitive but seldom used

Hue discrimination tests

It is refer to visual perceptions that have their origin in the structure of an observer's eye.

Three types are used for testing the macula in opaque media

1/ PURKINJE VASCULAR E.P 2/ Flying spot( blue field entoptic phenomenon) 3/Haidinger’s Brushes

Entoptic phenomena

The Purkinje’s vascular entoptic test is a simple method which elicits the response by placing a penlight against a closed eyelid or the globe and moving it back and forth, creating images of the patient’s retinal vascular tree

PURKINJE VASCULAR E.P

Blue field entoptoscopy relies on the observation of leucocytes flowing in the macular retinal capillaries. The leucocytes appear as ‘Flying Corpuscles’ when the retina is diffusely illuminated with a bright blue light.

Flying spots( blue field entoptic phenomenon)

Subject looks at a surface illuminated with blue light through a polarizer

hourglass shaped yellowish brushes seen radiating from the point of fixation. On rotating polarizer, brushes rotate

Haidinger’s Brushes

Phenomenon caused by variations in absorption of plane polarized light by oriented molecules of xanthophyll pigment in foveal retina

Used to sensitize the fovea in amblyopic child with eccentric fixation

limited by the patient’s subjective interpretation

May yield false negatives if the retina cannot be sufficiently illuminated through a dense cataract

Limitations of EP

Interferometer & potential acuity meter Utilizes coherent white light or helium-neon

laser generated interference stripes or fringes that are projected onto the retina through the ocular media.

Useful in mild to moderate media opacities. Principle: light interference.

Projection of target directly on macula after bypassing the opacities of media is done

Since these are not the images in usual sense they are not affected by ordinary optical defects, defects of focus, and imperfections of refraction what ever observer sees depends only on the ability of retina to conduct signals from photoreceptors into nervous system.

Limitations 1. subjective

2. Laser fringe vision>vision of letter acuity.

3. over predicts visual potential in amblyopes

Potential visual acuity meter PAM introduced in1983 This is attached to a slit lamp and projects a

reduced Snellen’s chart via narrow beam of light through a pinhole clear area in the cataract towards the macular region

The resulting potential acuity is the smallest line where the patient was able to read three characters

Subjective

methods that require an alert and cooperative patient and skilled compassionate examiner

But it is easier than laser interferometry

Limitation

Hyperacuity: visual performance that is a level above achieved by measurement of visual acuity.

Physiological basis of use of hyperacuity as macular function test is vernial hyperacuity is highly dependent on retinal stimulus location with lowest threshold being at macula.

Tests based on hyperacuity

PHP relies on the concept of hyperacuity which is the ability to discern a subtle misalignment of an object.

This can be explained by the fact that an extended edge will stimulate an array of cones and when there is a break in this line the fovea can perceive it.

Preferential hyperacuity perimeter

Stereo acuity: detection of difference in depth or distance of pair or more objects in space

Vernial acuity: consists of targets like butting vertical lines,2 dots or vertical lines separated by gap, misalignment is to be recognised.

Oscillatory displacement threshold: tasks requires smallest change in position of oscillating object, judged in relation to 2 stationary reference bars.

These are objective Stimulus is presented and a response

parameter is measured by electrophysiological means.

One of the most effective modes for macular function assesment in total media opacities.

Electrophysiological tests

VEP Measure of the electrical potentialgenerated in response to a visual stimulus

it represents integrity of entire visual pathway from retina to occipital lobe so can not differentiate between macula ,ON and cortical pathology

Visually Evoked Potential

It measures the cortically evoked electrical activity providing about integrity of optic nerve and primary visual cortex.

There is a large representation of macula in occiptal cortex.

Types: flash VEP & pattern VEP. Pattern VEP has a measure denoted as P100 a

positive wave at 100 milli sec. This wave occurs more than 100 milli sec

indicates delay due to optic nerve demyelination.

If the issue is the V/A then the amplitude is measuredIf the issue is the lesion in the visual pathway then the latency is measured

ERG is only abnormal when a large area of retina is functionally impaired

Focal ERG needs a stimulus localized to one area without scattering of light to stimulate the rest of the retina

Focal Electroretinogram

This is the ERG response to a pattern stimulus

Luminance is constant but contrast reversing of pattern is seen.

N35,P50,P95 waves are analysed. This is abnormal in macular disorders

irrespective of peripheral retinal function.

Patterned ERG

It is a hand held foveal ERG

It employs a 3-4 degrees white flickering light focused on the fovea with a 10 degrees annulus of constant white light to desensitize

surrounding retina

Maxwell ophthalmoscope(foveal flickering sensitivity)

The luminance of flickering test light is sinusoidally modulated i.e. it increases and decreases in in a sinusoidal fashion above and below the mean luminance.

For each temporal frequency value the threshold is the minimum modulation depth at which detection of flicker occurs.

The reciprocal of threshold is sensitivity.

This is highly sensitive macular function test.

It is a simple clinical test that can differntiate between retinal (macular) & post retinal (optic nerve disease)

Introduced by bailliart in 1954.

Photostress test

Principle: Basis of test is to utilise induced fatigue of

macula. It is a gross test of dark adaptation in which

visual pigments are bleached by light.

Causes a temporary state of retinal insensitivity perceived by patient as scotoma.

Recovery of vision is dependent on ability of photoreceptors to resynthesise visual pigments.

A) best corrected distance visual acuity is determined.

B) pt fixes light of pentorch held at 3cm away. C) photo stress recovery time:time taken to

read any 3 letter of pretest acuity line & is normally between between 15 and 30 sec.

Test is performed on other presumably normal eye & results are compared.

It is prolonged in relation to normal eye in macular diseases (50 sec or more) but not in optic neuropathy.

Technique:

Advantages: It is easiest to perform Opd procedure Gives fairly accurate results Doesn’t need expensive instruments.

The principle of microperimetry rests on the possibility to see —in real time— the retina under examination (by infrared light) and to project a defined light stimulus over an individual, selected location

Microperimetry

SLO microperimetry was the first technique which allowed to obtain a fundus-related sensitivity map

SLO uses a near infrared diode laser (675nm)beam that rapidly scan the posterior pole.

The reflected light is detected by a confocal photodiode and the digitized image is stored in a computer

Scanning Laser Ophthalmoscope microperimetry

SLO fundus perimeter did not allow to perform fully automatic examination.

Moreover, automatic follow-up examination to evaluate exactly the same retinal points

tested during baseline microperimetry was not available with this instrument.

Limitations of SLO microperimetry

The limitations of SLO have been overcome by MP1 microperimetr a recently developed automatic fundus perimeter

MP1 microperimeter automatically compensates for eye movements during the examination via a software module that tracks the eye movements

MP1 microperimetry

To know the anatomical integrity of macula. Can perform cross sectional images of biological

tissues within less than 10micron resolution. OCT scan protocols in macula:1. Line scan2. Radial line3. Macular thickness map4. Fast macular thickness map5. Raster lines6. Macular scan

Optical coherence tomography

Evaluation of the macular function of a patient with opaque media is a challenging problem commonly faced by us

No single test is infallible

Home message

Thank you