Your husband probably is going to have a vitrectomy surgery. There could be a vitreous hemorrhage probably due to abnormal leaky blood vessels as a result of diabetic retinpathy. If there is retinal detachment also then it may be combined with a scleral buckling surgery as well. During surgery he will be given local anesthesia and so pain is not likely. In the post operative period he may have pain for which he will be given pain killers. Vision recovery is possible and recovery time is variable but you can expect it to be around 2 weeks. You need to discuss all this with the treating ophthalmologist as he would be knowing the details and would give you a much better answer as prognosis will depend a lot on your husband's retina status. Thanks and best wishes.
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