Hello
Presuming that all tests are normal including X ray and C B C , E S R , sputum for A F B , tuberculin skin test, Montox's test ( fresh report of tests , necessary particularly you are having severe cough from last 20 years, this type of cough may be due to these reasons, these include:
1 Uncomplicated bronchitis , symptoms of lower respiratory infections: cough & /or increase in sputum production, breathlessness , wheeze, chest pain/aches, sweats & / or
sore throat. When such type of patient visit I only prescribe codeine +
chlorpheniramine maleat syrup ( but draw back is that this is a habit forming drug , so take care) . I also advise my patient to take monteleukast+
fexofenadine tablet once or twice in day depending on the condition of patient and disease.
Usually don't find in any thing abnormal in tests.
Only physical finding and clinical examination by a doctor is helpful.
2 Rhinitis allergic or non-allergic in this there develops itching in throat , nasal irritation due to inflammation of nasal lining membrane .
Nasal itching, watery rhinorrhea,
nasal obstruction, sneezing ,
post nasal discharge are common .
Physical examination by an E N T specialist find out the rhinitis .
3
Bronchiectasis is another such cause but in this there is huge amount of mucus production is there .
Sputum culture and X RAY chest find out the disease.
In my opinion consult a " PULMONOLOGIST " and get his opinion.
Good luck.