the level of reading ability (or better the level of language skills
for written and spoken language) depends very much. mostly onthe fact
when the person has lost her hearing. if that has happened very early,
the ability to read and understand the written text may be limited
(also depending on the complexity of the text).

so for many deaf people the sign language is their mother language
(first language) and the written and spoken language (english) might
be comparable to a foreign language.

hope the brings some light on this case. although i might have missed
the context of the question.

cheers, michael