Each square measures 5 mm and when the grids are held at 30 cm from
the patient each square subtends 1 degree on the retina. The amsler grid
is a very elementary and easy test to run on patients who have reduced
vision or subjective complaints that cannot be explained. There are times
when you might want to run the test on patients who have no complaints
but you want to use the test to confirm your diagnosis. The Amsler should
be run whenever a patient's best visual acuity is reduced even with pinholes,
when macular or optic nerve disease are suspected, a patient has abnormal
color vision, or the macular area has a suspect appearance. Some practitioners
feel it should be run on every patient, but most certainly on the elderly
patient, i.e., patients 65 years and over. It is important the test be
run before dilation or Goldmann applanation tonometry. The patient is wearing
their best correction and add if the patient is presbyopic.
The first grid is basically a standard grid that tests for any general
subjective patient responses to faults or distortions in the pattern. The
following grids will give more detailed information regarding the macular
foveal area. The amsler grid booklet goes through in good detail when and
why certain grids should be used.
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.
The instructions to the patient should always be the same. Can you see
the central white dot? If you can see the central white dot keep looking
directly at it. Do you see all four corners and all four sides of the chart?
Are there any areas of the chart that are missing or distorted in any way
and are any of the lines not straight or unequal in size? If not, describe
what you see then draw what you see.
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
.
Above Is An Arcuate Scotoma As Seen By An Advanced Glaucoma Patient.
Above A Positive Or Absolute Paracentral Scotoma As Seen By The Patient That Might
Be The Result Of A Healed Chorioretinal Scar.
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.
Each 5mm Square Of The Grid Subtends A Visual Angle Of One (1) Degree When
The Chart Is Held At 30 Cm. In This Position The Entire Chart Tests Up
To Ten (10) Degrees From Fixation In Both The Horizontal And Vertical Meridians
From The Fovea.
UHTHOFF'S SIGN OR PHENOMENON: This is a temporary worsening of vision
and other neurologic functions commonly seen in patients with multiple
sclerosis just after exertion or in situations where they are exposed to
heat. The condition is of importance because multiple sclerosis (a demyelinizing
condition which can be related to optic neuropathy) may present with optic
neuritis. Patients may report they notice their central vision becomes
blurry and will show a positive defect when tested with an amsler grid.
The condition is commonly reported after exercising or simply sitting in
front of a fireplace, it is important if you suspect multiple sclerosis
that you ask questions related to this phenomenon.
PULFRICH PHENOMENON: There have been reported cases of patients with
multiple sclerosis who experience this phenomenon. Using the test diagnostically
is somewhat questionable, however, suspect patients should be questioned.
It is the perception (a binocular stereo phenomenon) that objects they
know are moving in a straight line appear to be moving in an elliptical
pattern. These patients may suddenly start having motion sickness problems
unrelated to an ear infection. They may also complain that things just
look odd to them or they are having problems with depth perception.
