I am sure all who have studied ophthalmic and physiological optics will have encountered a diagram similar to figure 1. This shows a pseudo-isometric view of an astigmatic pencil of light. I am also sure, in view of this journal’s target readership that it requires little explanation. It depicts the ray paths along the principal meridians in astigmatic refraction corresponding to a distant point object source (not shown) located on the central optical axis.
Clinical regular astigmatism of the eye is characterised by where the retinal plane intercepts the astigmatic ray pencil. The traditional classification of astigmatism acknowledges five types of astigmatism:
1 Compound Hyperopic Astigmatism (CHA): The retinal plane intercepts the astigmatic pencil anterior to the first focal line. At this location the rays in both principal meridians are convergent, ie hyperopic.
2 Compound Myopic Astigmatism (CMA): The retinal plane intercepts the astigmatic pencil posterior to the second focal line. At this location the rays in both principal meridians are divergent, ie myopic.
3 Simple Hyperopic Astigmatism (SHA): The astigmatic pencil is intercepted at the first focal line. One principal meridian is focused on the retinal plane, the other is convergent.
4 Simple Myopic Astigmatism (SMA): The astigmatic pencil is intercepted at the second focal line. One principal meridian is focused on the retinal plane, the other is divergent.
5 Mixed Astigmatism (MxA): The retinal plane intercepts between both focal lines where one principal meridian is convergent and the other is divergent.
However, I feel the category of mixed astigmatism is too vague as it encompasses three distinct refractive states which should be recognised individually.
Firstly let us compare and contrast the classification of non-astigmatic with astigmatic refractive states (figure 2). Here the different background colours denote uncorrected refractive states in which application of accommodation reduces blur (pink) or increases blur (blue). In both non-astigmatic and astigmatic errors blur is least when the retinal intercept is located at the transition of these two zones. In non-astigmatic refraction we know this as emmetropia, however, in astigmatic refraction there seems to be no recognised term for the analogous state.
Figure 2: Comparison of non-astigmatic (top) and astigmatic (bottom) refractive states with respect to the region where retinal intercept occurs. It should be understood the red and green astigmatic raypaths are in mutually perpendicular meridians.
Hyp = Hyperopia (a.k.a. Hypermetropia). Emm = Emmetropia. Myp = Myopia. CHA = Compound Hyperopic Astigmatism. SHA = Simple Hyperopic Astigmatism. MxA = Mixed Astigmatism. SMA = Simple Myopic Astigmatism. CHA = Compound Myopic Astigmatism
This, to me, is a curious omission as it has a unique physiological property. It is the only location of retinal intercept along the astigmatic pencil which results in a circular blur patch. All other classes of astigmatism result in an elliptical blur patch (including SHA and SMA since a finite straight line is a special case of ellipse). In other words this is the special case of mixed astigmatism in which the circle of least confusion lies on the retinal plane. I propose this should be known as Neutral Mixed Astigmatism (NMxA).
Having argued there is a special case, what of the remaining range of mixed astigmatism? If the retinal plane locates between the first focal line and the circle of least confusion, application of accommodation will have the hyperopic property of reducing blur, here I propose the term hyperopic mixed astigmatism (HMxA). Conversely, if the retinal plane locates between the circle of least confusion and the second focal line, application of accommodation will have the myopic property of increasing blur, hence I suggest Myopic Mixed Astigmatism (MMxA) (figure 3). There is another physiological distinction between HMxA and MMxA in that the elliptical bur patches have opposing perpendicular orientations.
Figure 3: The three distinct states of mixed astigmatism
Thus, what I argue is that astigmatism should be classified in terms of retinal plane intercept in relation to the ‘landmarks’ of the astigmatic pencil of rays. This modification is completely compatible with the traditional classification of astigmatism but subdivides mixed astigmatism into three physiologically distinct categories.
To summarise
- Compound Hyperopic Astigmatism (CHA): The retinal plane lies anterior to the first focal line.
- Simple Hyperopic Astigmatism (SHA): The retinal plane lies at the first focal line.
- Hyperopic Mixed Astigmatism (HMxA): The retinal plane lies between the first focal line and the circle of least confusion.
- Neutral Mixed Astigmatism (NMxA): The retinal plane lies at the circle of least confusion.
- Myopic Mixed Astigmatism (MMxA): The retinal plane lies between the circle of least confusion and the second focal line.
- Simple Myopic Astigmatism (SMA): The retinal plane lies at the second focal line.
- Compound Myopic Astigmatism (CMA): The retinal plane lies posterior to the second focal line.
Classification of ametropia according to distance prescription
In non-astigmatic refractive errors, the task is trivial: plus equates to hyperopia, minus equates to myopia. In astigmatic refractive errors there is always a cylinder component which has to be taken into account.
Figure 4: Flowchart to classify astigmatism<
Fortunately the process only involves simple arithmetic. Following the flowchart in figure 4 will disclose both constituent names of the class of astigmatism corresponding to any spherocylindrical correction in standard notation.
For example consider: +1.75/-2.00 x 45.
Here A = +1.75 and B = -0.25.
The product of A times B is negative, hence the astigmatism is mixed.
Figure 5: Astigmatic classification in terms of standard form spherocylindrical correction within the range ±1.00 D.S. and ±1.00 D.C.
The sum of A plus B is positive, hence the astigmatism is hyperopic.
Thus, the given prescription is a correction for Hyperopic Mixed Astigmatism.
It should be noted that were the prescription given in transposed form (-0.25/+2.00 x 135), the same result would be obtained. Figure 5 gives further examples of astigmatic classification.
Henry Burek is an optometrist resident in South Yorkshire and an examiner for the College Of Optometrists.
