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Few adults tollerate a complete correction of a severe astigmatism if they haven’t, first, worn a partial correction. In these cases the complete correction cannot be prescribed.The general rule is to prescribe about ¾ of the required correction and tell the patient to return after a few months for the complete correction. He also has to be told of the difficulties that he may encounter: images that appear to be distorted but that these difficulties will disappear after little time. In any case one should recur to lenses with the most adequate curvature (See lens shapes).
Similar considerations are valid for most corrections that are worn for a first time, even for spherical lenses.
In general, unless the corrections is slight (1-2D) it is better not to prescribe the complete correction but to follow the earlier discussed rule arriving gradually at the total correction.

Related article: The prescription of corrective lenses: Severe astigmatism (1/2).

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two).

For the correction of a severe astigmatism and practically in all cases where it hasn’t previously been corrected, a considerable experience is required. Although there are exceptions, few can tollerate a complete correction of 4.0D of cylinder (or even less), if the astigmatism has not been corrected before, especially when the cylinder has an oblique axis.
The spatial orientation is altered as well as the compensation that the mind forces itself to impose for a comfortable condition.
The fact that the images appear to be inclined, that a circle seems oval, etc…, is the consequence of the correction of the astigmatism and this is understandable if one thinks that till that moment there was a compensation in act. Analogously, a circle projected on the retina of an uncorrected eye is oval but is still interpreted to be round given that experience had tought that it had to be like that.
The correction’s effect is that to subvert this compensation, creating confusion for a period of time. The image that is now projected on the retina, is not inclined and can be interpreted as pending while the circle, which is now a circle, could be interpreted as being oval, given that the compensation persists for a while when it is not necessary.
In some cases, according to the patient’s age, as well as the time the compensation took place, the patient will tollerate a complete correction in little time. This is true for children till 10-12 of age and the problem is for them almost non existent.

Related article: The prescription of corrective lenses: conditions which require a modification

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two)

The general rules that we have listed before, though valid in most cases, are not always applicable.
For example, in the case of:

1. high astigmatism while the patient has never worn glasses;
2. high myopia or aphakia;
3. aniso- and antiametropia;
4. disturbance of the binocular equilibrium (eteroforia);
5. strabysm.

Before prescribing, it may be necessary to modify the refraction correction that has been found before: in the cases 1, 2 and 3 in order to avoid the patient’s discomfort, and for the cases 4 and 5 in order to influence the development of the relation between convergence and accomodation. When one can inhibit or stimulate the accomodation in relation with the convergence or, like in case 4, according to age and other factors, recurring to the use of prisms to compensate for the insufficient convergence, or to inhibit the convergence in relation with the accomodation.
Let’s consider first the conditions that require the corrections of the refraction vice.

Related article: The prescription of corrective lenses for special purposes (2/2)

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two).

In prescription glasses, besides the spheric addition to prevent an excessive accomodation effort, we need also prisms at the nasal base to reduce an excessive convergence. We have seen all this in the posts on prescribing prisms. Let’s remember that young workers complain that the images of the objects lose sharpness outside of a narrow interval and that they feel unwell and dizzy. In that case the bifocals can be useful even though in some cases the “half-high glasses” are more practical.
Naturally, the general rule is that these glasses must be used for the appropriate purposes.

Relared article: The prescription of corrective lenses for special purposes (1/2)

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two).