What does Metamorphopsia mean?
metamorphopsia, means ‘seeing mutated shapes’
Definition of Metamorphopsia
Metamorphopsia in patients is the perception deviation of the horizontal or vertical line, and it sometimes may appear before the clinical manifestation of macular disease
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What is metamorphopsia and how can it be treated?
In metamorphopsia, patients perceive distorted images of the object. This results due to alteration in the retinal contour caused by a raised patch of choroiditis.
What does metamorphopsia look like?
There are 2 types of metamorphopsia
Specific types of metamorphopsia include macropsia and micropsia.
Micropsia, in which the human eye perceives an object to be smaller than it actually is, is more common.
In macropsia, an object is perceived to be larger than it actually is.
Causes of Metamorphopsia
It is mainly associated with macular degeneration, particularly age-related macular degeneration with choroidal neovascularization. Other conditions that can present with complaints of metamorphopsia include: pathological myopia, presumed ocular histoplasmosis syndrome, choroidal rupture and multifocal choroiditis
Method of Measuring Metamorphopsia
To evaluate metamorphopsia, many functional examinations can be conducted, such as the Amsler grid and the mobile handheld home surveillance devices
The Amsler grid was the first functional test to be evaluated for visual distortion. This test enables simple and fast qualitative assessment of changes in the visual function in the field of view of central 10°, which might appear prior to certain macular diseases or together with the development of these diseases.
However, the standard Amsler grid, as a suprathreshold trial stimulus, cannot be used to detect early metamorphopsia because of its poor sensitivity; thus various trials have been inspired by the Amsler grid for the application of the quantification and early detection of metamorphopsia.
Most of these tests, however, still need full clinical validation. For example, M-CHARTS, differential perimetry, SDH (shape discrimination hyperacuity), and PHP (preferential hyperacuity perimeter) tests have been applied to metamorphopsia assessment
Amsler Grid
The Amsler grid is a black piece of cardboard of 10 square centimeters that is divided into small squares of 5 mm with white lines. A white point in the center is used as the fixed point. Black lines on a white background are also useful.
When the grid is placed 28–30 cm in front of the patient’s eyes, the whole grid is equivalent to 20°, and each small square is equivalent to 1°. This square was therefore designed by Amsler to test for diseases in the range of 10°. The blind spot is located at the nasal boundary approximately 5° outside the grid. Although the Amsler grid has been extensively applied to visual distortion, quantifying the severity of metamorphopsia is still difficult [34, 37, 38, 58].
M-CHARTS
In 1999, Matsumoto et al.proposed quantifying the seriousness of visual distortion using M-CHARTS (Inami Co., Tokyo, Japan). The basic principle of the method is that when the dotted line substitutes for the straight line and the coarse intervals substitute the dot intervals, line distortion is reduced.
Furthermore, when the dot intervals are enhanced, line distortion continues to be reduced until the line becomes nearly straight. M-CHARTS includes nineteen dotted lines, and the visual angle of dot intervals ranges from 0.2 degree to 2.0 degrees.
Moreover, M-CHARTS is divided into two types: the type 1 single-line M-CHARTS (for general use) and the type 2 double-line M-CHARTS
; the type 2 is composed of 2 lines that have an intervening fixation point for the purpose of MH patients. In this test, the patient is first asked to gaze at the target at the center and then vertical straight lines (0°) are shown.
If the straight line is perceived as straight by the patient, then the metamorphopsia score of the patient is 0, and the test is complete. In contrast, if the patient sees the straight line as a curve or other irregular shapes, then he/she will be shown many interrupted lines composed of different dots, from fine dots to coarse dots.
If the dotted line is recognized as straight by the patient, then the metamorphopsia score of the patient will be determined to be the visual angle used to separate the dots. Next, the same test is conducted using the horizontal lines after rotating the M-CHARTS at 90°.
These tests are performed three times, with the mean value applied to data analysis. The advantages of M-CHARTS are its low cost, its portability, and its ease of use.
However, it also has disadvantages of not being applicable to patients with serious central dark spots or low vision due to its design purpose of detecting and quantifying subtle deformations and is thus very demanding of visual quality.
Nowomiejska et al. made a comparative analysis of the Amsler grid and M-CHARTS in evaluating bevacizumab for the treatment of neovascular AMD in metamorphopsia patients using OCT, and they concluded that the sensitivity of M-CHARTS might be better than that of the Amsler grid [50].
3.4. Different Manifestations of Visual Distortion
The Amsler grid can be used to detect a large number of metamorphopsia in different forms. A straight line can break into a small/large wavy line or an angular deformity. In micropsia, the horizontal lines and the vertical lines appear to bend towards the center, while in macropsia, the lines bend from the center. In addition, researchers found correlations between macular diseases and metamorphopsia of varied types.
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