Amsler grid

What it measures

The Amsler grid samples the central visual field, roughly ten degrees around fixation, which corresponds to the macula. The patient fixates the central dot and reports how the surrounding grid looks.

Two findings are sought. Metamorphopsia makes lines appear wavy, bent or pulled, and points to displacement of photoreceptors by fluid or by a membrane. A scotoma appears as a patch where the grid is grey, blurred or absent, and points to loss of function in the matching retina.

The test is subjective. The patient reports the perception, and the regular grid supplies the reference against which any irregularity becomes visible.

How it is administered

The patient wears near correction: a reading add or the full near prescription. Testing without it is the commonest error.

The patient sits about 30 to 40 cm from the screen and keeps that distance, because it fixes how much of the field the grid samples. One eye is tested at a time, the other covered with the palm, not squeezed shut.

The patient fixates the central dot throughout and does not look around the grid. While fixating, they answer: are all four corners visible, is the whole grid present, are the lines straight and unbroken, are there holes, grey patches or wavy areas. Affected areas are drawn on a paper grid or described in the notes, then the second eye follows.

Lighting stays even, with no glare on the screen, and display brightness is set to a comfortable level.

Reading the result

The screen presents the grid. The result is what the patient reports, and the clinician records it, on a paper grid or as a note.

Record the location and the rough extent of each finding relative to fixation. Position matters. Distortion touching or surrounding fixation affects reading differently from a patch at the edge of the grid. Wavy lines indicate metamorphopsia, missing or grey areas indicate scotoma, and the two often coexist.

Compare the two eyes, and compare against the grid kept from the last examination. A new area, or an existing area that has grown, is the finding that changes management.

The record is qualitative: read it as a map of symptoms, not as a field measurement.

When it is used

The grid is used as a quick first look when a patient reports distortion or central blur. It is used to monitor known macular disease, particularly the fellow eye in age-related macular degeneration, and in central serous chorioretinopathy, epiretinal membrane, macular oedema and macular hole.

It is also taught for home monitoring between appointments, and used in drug toxicity monitoring alongside more sensitive tests rather than instead of them.

Limitations

Sensitivity is low. The visual system fills in missing areas, so a patient can be unaware of a real scotoma and report a normal grid. A normal Amsler does not rule out macular disease.

The test depends on steady central fixation. A patient who scans the grid produces an unreliable map, and patients with a central scotoma often adopt an eccentric fixation point without knowing it.

Results are subjective and hard to reproduce. The same patient can describe different areas on consecutive days.

Nothing the patient reports is captured by the app. There is no marking layer and no stored result, so a finding exists only in the note made at the time.

Screen delivery does not fix the viewing distance, so the angular size of the grid varies. Squares intended to subtend one degree may subtend more or less, which changes the field sampled and makes sessions hard to compare. Screen brightness, contrast and pixel structure differ from a printed chart, and on a scaled or low resolution display fine lines can look irregular by themselves. Ambient light is not controlled.

This is a screening test. It does not replace macular imaging or formal central threshold perimetry, both of which detect changes the grid misses.

Validated status means this implementation is considered ready for clinical use on this platform. It does not mean a published study has shown equivalence to the printed chart.