Signed languages depend not just on movements of the hands, but also on subtle facial cues. Direction of eye gaze, puffed cheeks, and mouth movements are all essential elements of a signed language. Given that, wearing a full facial veil (as is the practice of many Muslim women) can significantly affect how one is able to communicate.
For those Muslims who consider the veil to be obligatory, are there any special provisions which would allow deaf Muslim women to communicate effectively via sign language?