Hello and thank you for your question.
Your mother is dealing with a difficult diagnosis and she has my sympathies.
Parafoveal telangiectasia many times presents with good vision or just mildly decreased (20/20 to 20/40). It sounds as if, given the history you've provided of her needing multiple Lucentis injections and her difficult with near vision tasks such as reading, that she may already be having some significant leakage from the retinal capillaries causing vision problems.
Unfortunately, not knowing what her exam looks like and what her fluorescein angiograms have looked like before and after the injections, I cannot say for certain what her chances of losing significant vision are. The vision in parafoveal telangiectasia can, over many years, decrease dramatically and can, in some cases, lead to legal blindness. If there is abrupt vision loss, this can be a sign that there are some new (and unwanted) blood vessels growing underneath the affected area of the retina, which would require these types of injections to stem the tide of vision loss. She may already be at this stage if she is receiving multiple Lucentis injections.
Approximately 5% of patients with this disorder go on to develop these new blood vessels (neovascularization). Unfortunately, however, there are no randomized, double-masked controlled clinical trials evaluating the efficacy of medicines such as Lucentis in the treatment of this disorder.
So, in summary, most people do not go on to legal blindness, but it is a distinct possibility. Depending on what type of pathology (new blood vessels versus chronic leakage of blood vessels) is causing her problems with near vision, her prognosis may be quite different. Unfortunately without knowing her exam and the other details of testing, it would be very difficult for me to give you odds; even with that information there is not a great deal of research trial information on this disorder to provide hard and fast prognostic odds. Sorry.
Does that help address your concerns?
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must see an eye doctor.