Weight loss - think cancer
Hyponatremia - SIADH from small cell lung cancer
Edema + JVD - SVC syndrome
The initial presentation looks like cancer: weight loss and progressive dyspnea over the course of months in a heavy smoker. Then, the question describes extra-pulmonary symptoms and a paraneoplastic syndrome.
More acutely, there is development of edema of the face and jugular venous distention. Because this is localized to upper body, we should think of a mechanical obstruction to venous flow as opposed to some cardiogenic reason. This is an extra-pulmonary symptom of lung cancer.
[ Superior Vena Cava Syndrome caused by the medial spreading of the tumor. Can be exacerbated in the physical exam by asking patient to raise both arms. ]
Finally, there is a single highlighted lab value -> hyponatremia. Small cell lung cancer can release inappropriate levels of antidiuretic hormone -> SIADH. ADH will retain water and decrease sodium concentration possibly leading to cerebral edema and seizures.
[ Syndrome of Inappropriate ADH (due to small cell carcinoma, a neuroendocrine tumor of the lung) decreases sodium concentration. Paraneoplastic syndrome. ]
In SketchyPath: SVC syndrome is the red balloon near the mediastinum ship and SIADH is the guy trying to carry the water cooler and getting water all over himself.