My Mother 73 Years Old,suffering From Chronic Cough For Last

My mother 73 years old,suffering from chronic cough for last 16/17 years.local doctors says it is allergy.after taking medicine she gets relief but after some months it revive again
but last 1-1/2 month she taking medicine according to doctors advice for allergy &indignation.but no recovery appears.yesterday she went to ENTspecalist.he says throat is clear.no infection im throat.beside severe cough problem,no other symtom arises.ENT specialist it is inflammation in food pipe.advice me what it may be.any serious disease?plz reply me