Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): This condition is characterized by the excessive release of antidiuretic hormone (ADH), leading to water retention and hyponatremia (low sodium levels). Chronic headaches and occasional blurry vision can be symptoms of SIADH due to the hyponatremia causing cerebral edema.

Hyponatremic Encephalopathy: This condition occurs when the brain swells due to low sodium levels, leading to symptoms such as headaches and vision changes. The severity of symptoms can vary, but it's a plausible explanation given the patient's sodium level.
Pituitary Tumor: A tumor in the pituitary gland can cause an overproduction of ADH, leading to SIADH and subsequent hyponatremia. Headaches and vision changes (including blurry vision) are common symptoms due to the tumor's location near optic nerves.
Hypothyroidism: Although less directly linked to hyponatremia, hypothyroidism can cause headaches and, in severe cases, vision disturbances. It's also associated with changes in serum sodium levels, though this is less common.

Subarachnoid Hemorrhage: Although less likely given the chronic nature of symptoms, a subarachnoid hemorrhage can cause sudden severe headache and, if there's associated hyponatremia (e.g., due to cerebral salt wasting), could fit the presentation. It's critical to rule out due to its high mortality rate.
Meningitis: This infection can cause severe headache, vision changes, and, in some cases, alterations in mental status that might indirectly affect sodium levels through SIADH or other mechanisms. It's a medical emergency.
Brain Tumor: Any tumor within the brain can cause increased intracranial pressure, leading to headaches and vision changes. Some brain tumors can also cause SIADH, leading to hyponatremia.

Cerebral Salt Wasting: A rare condition characterized by the excessive excretion of sodium in the urine, leading to hyponatremia. It's often associated with intracranial disorders and can cause symptoms similar to those described.
Adrenal Insufficiency: Though more commonly associated with hyperkalemia, in some cases, adrenal insufficiency can lead to hyponatremia. Headaches and vision changes are not primary symptoms but could occur in the context of severe disease.
Pseudohyponatremia: Certain conditions (like hyperlipidemia or hyperproteinemia) can cause laboratory artifacts leading to falsely low sodium readings. While not a disease per se, it's a rare cause of hyponatremia readings that could explain the symptoms if the actual sodium level is normal.