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I will say follow the symptoms and file for any symptoms that your doctors can link or connect to your military service like asthma, depression, sinus problems, or breathing problems, depression, acid reflux (GERD), IBS, headaches or any other symptoms that can be medically associated with your service. I know I typed depression twice, keep in mind that depression is not saying you are crazy, it simply means that you a dealing with mental health symptoms that can be linked to or connected to your service or the symptoms of your service-connected disability.
To make a long story short, I filed for Gulf War Syndrome (GWS), but my claim was denied but I began to file for the symptoms I was diagnosed and treated for while in service and post service and now I am rated 100% schedular P & T.
You will need 1. An in-service event, accident or incident. 2. A current diagnosis and 3. A nexus letter or statement connecting 1. And 2. The letter would only have to say something like it is as least likely as not that your (name symptom) is/was caused by or related to your military service.