Oh dear! I really do feel sorry for you. You've certainly been through a lot of investigations and I cannot think of a single additional investigation to exclude another causes. You have a condition called idiopathic chronic cough. There are a variety of mechanisms including a neurogenic set point, hereditary factors, and hitherto unknown pathophysiological factors. I have a couple of suggestions:
1. At UCT Private Academic Hospital there are excellent respiratory therapists and I suggest that they see you for some breathing synchronisation exercises.
2. There are several medications, including those used to treat certain types of epilepsy, that may be useful in cases of idiopathic chronic cough. You should talk to your pulmonologist about this.
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