Normal-Pressure Hydrocephalus Accompanied by a Syndrome of Inappropriate ADH Secretion: Differential Diagnosis of Hyponatremia

Abstract

Objective: To prompt clinicians to consider the syndrome of inappropriate antidiuretic hormone secretion (SIADH) in the differential diagnosis of hyponatremia in patients with idiopathic normal-pressure hydrocephalus (NPH). Theoretically, hyponatremia may occur in NPH, but only two associated cases of SIADH have previously been reported in the literature. Methods: A case report of an elderly woman with the clinical and radiologic features of NPH accompanied by hyponatremia is presented. The author reports his findings from the extensive workup of the hyponatremia and the treatment. Results: A 78-year-old woman was being evaluated for hyponatremia; she presented with gait disturbance, decline in cognitive function, and urinary incontinence, which constitute the classic triad of NPH, and brain magnetic resonance imaging showed ventriculomegaly and other features consistent with this syndrome. The serum sodium was 129 mEq/L, with osmolality of 270 mOsm/kg, and urine osmolality was 167 mOsm/kg. Serum uric acid level was 3.1 mg/dL, and urea was 5 mg/dL. Other metabolic workup was negative; renal, adrenal, and thyroid functions were normal. Cardiac workup was negative. Chest and brain imaging results were negative for an occult malignancy. SIADH was diagnosed. Serum vasopressin (antidiuretic hormone) measurement was 6.2 pg/mL. Based on the patient's preference, a conservative treatment was instituted for both SIADH and NPH, a novel approach for this clinical scenario; it resulted in modest improvement of the hyponatremia. Conclusion: Recognition of occurrence of SIADH in NPH, though an extremely rare metabolic manifestation of the neurologic disorder, is important for the appropriate management of the associated hyponatremia. Abbreviations: ADH = antidiuretic hormone CSF = cerebrospinal fluid NPH = normal pressure hydrocephalus SIADH = syndrome of inappropriate antidiuretic hormone secretion