As you may already know by now, Captcha is a common approach to prevent access of users that aren’t human to all kinds of websites. The captcha is a kind of test that requires verification of distorted image plus textual content which is deciphered easily by humans but not by computer bots. 
However, it appears that this simple system may also prevent access from a certain group of human being: disabled people such as blind, low-vision, or people who suffer from cognitive problems (dyslexia). Once a low vision user cannot decipher the image displayed on the Captcha, he cannot access the specific website that uses the captcha protection and thus can not open an account, comment on a forum or blog and so on.

To make a long story short - Captcha can tell human and bots apart but cannot recognize human with disabilities who are not bots and fail the test because of their disabilities…

Captcha Alternatives
But do not worry. There other alternatives for Captcha:

One of the alternative methods is a sound test also known as “audio captcha“. It requires the potential user to listen to some sounds which are played on a noisy background, filter the sounds and repeat them. It is a good solution for low vision or blind users, but not for deaf people or users who don’t have a good hearing.

 Other captcha alternatives are as follows:
Spam filtering – software for blogs contain spam filters targeted to delete spam messages. More advanced systems are able to control spam attacks and filter permanently or temporarily spam contents using the IP address of the user.

Heuristic checks – are based upon the user’s data such as requests, pages he has visited, IP address, data entry or signature data and assist in detecting robot users. 

Single sign-on – This system is similar to a passport. It utilizes visual verification. No doubt – it’s highly recommended for people with disabilities.  

Public key infrastructure solutions – This is another alternative solution not used yet but may help low vision users – to use certifications issued by government or other trusted authorities that verify their identities. The disabled user will have to telegraph the site, inform his or her disability and get access to the site’s service.

Biometrics – Another tool, to be used together with the single sign–on system (using D.N.A / fingerprints). Microsoft has already developed a new system of Biometric verification but it will take years until the biometric hardware will penetrate into the market.

 To sum up, Captcha is a good solution, but not for all. When it comes to disabled people with low vision or hearing and other disabilities, one must consider some other security tools and / or software, such as the captcha-alternatives mentioned on this post (spam filters, heuristic checks and so on).