There can be many causes for chronic cough. Most common cause is allergic airway disease such as asthma and allergic bronchitis. Many times all routine investigations can turn out to be normal in these conditions. Other cause for throat irritation with cough is reflux laryngopharyngitis. Here stomach acid spills into throat and causes irritation of throat and chronic spasmodic cough. Consult nearby ENT specialist or chest physician for further help