Syndrome of Inappropriate Antidiuretic Hormone Secretion as a Presentation of Untreated Parkinson's Disease

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most common electrolyte disorder associated with neurological conditions. Parkinson's disease (PD) has not been known to be causative of SIADH. We present the case of a 71-year-old male patient with diabetes type II (T2DM) and hypothyroidism admitted with progressive confusion, slow speech, and severe fatigue for one week, accompanied by sluggish body movements for a few months. A neurological exam revealed mild arm rigidity, bradykinesia, resting tremors, and stiff gait. The exam was otherwise normal. Initial blood work showed hypo-osmolar hyponatremia (Na 122 mEq/L, serum osmolarity (Osm) 275 mOsm/kg, and urine Osm 672 mOsm/Kg). CT chest showed localized infiltrate. The initial diagnosis was SIADH secondary to pulmonary process, most probably pneumonia. After starting him on a fluid restriction of 1.5 L/day and urea 15 mg BID, sodium improved gradually to 133 mEq/L on discharge. Urine osmolality continued to be elevated ranging between 700 and 800 mOsm/Kg. An active pulmonary process was ruled out by a pulmonologist. Parkinsonism was diagnosed four weeks after discharge by Neurology who started carbidopa/levodopa. As extrapyramidal symptoms improved, urine osmolality improved as well to 400 mOsm/Kg. Sodium level was maintained between 135 and 137 while urea treatment was stopped and fluid restrictions removed. New-onset SIADH was thought to be secondary to Parkinson's disease. Parkinson's disease treatment (carbidopa/levodopa) is known to cause SIADH. In this case, the treatment itself and a dose increase led to improvement in sodium levels and urine osmolality concomitantly with the improvement of the patient's extrapyramidal symptoms.