La miopía y el astigmatismo afectan a la visión.

Because the subject is La miopía y el astigmatismo — two things joined by y.

• la miopía = one thing
• el astigmatismo = another thing

Together, they make a plural subject, so the verb must be plural:

• afecta = affects
• afectan = affect

So:

• La miopía afecta... = Myopia affects...
• La miopía y el astigmatismo afectan... = Myopia and astigmatism affect...

In this sentence, afectar is being used with a: afectar a algo / a alguien.

So:

• afectan a la visión = they affect vision

This a is not the same as the so-called personal a used only with people. Here, it is part of a very common pattern with afectar.

You may also sometimes see afectar la visión, especially in some varieties of Spanish, but afectar a la visión is very natural and common, including in Spain.

Because Spanish nouns have grammatical gender, and the article must match the noun.

• miopía is feminine singular → la miopía
• astigmatismo is masculine singular → el astigmatismo
• visión is feminine singular → la visión

This is something English speakers often need to get used to, because English does not usually mark noun gender this way.

Because the two nouns have different genders:

• miopía = feminine
• astigmatismo = masculine

So each noun needs its own singular article:

• la miopía y el astigmatismo

If both nouns had the same gender, Spanish sometimes allows one article to cover both, but here that would not work because one is feminine and the other is masculine.

Yes, you may hear or read that too.

Both of these are possible:

• afectan a la visión
• afectan la visión

In Spain, afectar a is very common and often sounds especially natural in sentences like this. So for a learner of Spanish from Spain, afectan a la visión is an excellent model to follow.

Both can relate to seeing, but they are not always interchangeable.

• visión often refers to vision, visual ability, or eyesight in a more medical or technical way.
• vista can mean sight, eyesight, or even view.

In a medical sentence like this one, visión sounds very natural because the topic is eye conditions and their effect on visual function.

So afectan a la visión is a very suitable choice here.

The accent marks show where the stress falls.

• miopía → stress on the í
• visión → stress on the final -sión

They also help show the correct syllable structure:

• mi-o-pí-a
• vi-sión

In miopía, the accent helps prevent the i from blending into a diphthong with the surrounding vowels. It tells you to pronounce it clearly as a separate stressed syllable.

A simple pronunciation guide is:

la myo-PEE-a ee el as-tee-gma-TEES-mo af-EK-tan a la bee-SYON

A few useful points:

• miopía = mi-o-pí-a
• astigmatismo has the stress on -tis-
• afectan has the stress on fec
• visión has the stress on the last syllable

In Spain, the c in visión is actually not relevant here because the word has s, but if you are learning Peninsular Spanish, remember that pronunciation differences mostly affect letters like z and soft c, not these words in this sentence.

Because y only changes to e before words that begin with an i sound.

For example:

• padre e hijo
• España e Italia

But astigmatismo begins with an a sound, not an i sound, so the normal conjunction stays:

• la miopía y el astigmatismo

Yes. Spanish often allows several natural alternatives. For example:

• La miopía y el astigmatismo afectan a la visión.
• La visión se ve afectada por la miopía y el astigmatismo.

The first version is more direct and straightforward.
The second means roughly Vision is affected by myopia and astigmatism and sounds a bit more impersonal or formal.

Both are correct, but the original sentence is simple, clear, and very natural.