I m sorry that your mother suffers with this illness and wish you the best for both her and your family. Discoloration of the cheeks could possibly indicate some form of organ failure or metabolic disorder. Hepatic OR LIVER failure is certainly one possibility and there are others. Medications can also cause overpigmentation especially if she is exposed to sunlight. For example, elderly patient who have urinary tract infections or pneumonia may be treated with Bactrim and then, exposed to the sun which can cause pigmentation issues. She will need a laboratory workup if you choose to pursue a diagnosis. I would greatly appreciate a STAR RATING on this question if you feel the answer provides useful information.