When Sodium Falls With Pineal Gland Involvement: Severe Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) as a Presenting Manifestation of CD5-Positive Primary CNS Large B-cell Lymphoma

Primary CNS lymphoma (PCNSL) is an uncommon extranodal non-Hodgkin lymphoma that often presents with nonspecific neurologic symptoms and may be complicated by metabolic derangements. Hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a clinically significant but underrecognized manifestation of CNS pathology, and presentation as severe symptomatic hyponatremia in pineal-region PCNSL is particularly unusual. We report the case of a 76-year-old man with a biopsy-proven pineal-region mass who presented with severe symptomatic hyponatremia and progressive encephalopathy. Histopathologic evaluation demonstrated a CD5-positive large B-cell lymphoma with a markedly elevated proliferative index and no evidence of double-hit or triple-hit lymphoma on fluorescence in situ hybridization. The biochemical profile demonstrated hypotonic hyponatremia with inappropriately concentrated urine and elevated urine sodium, supporting SIADH in the setting of intracranial malignancy. The patient required intensive care unit monitoring and was treated with fluid restriction, sodium supplementation, oral urea, and carefully monitored hypertonic saline, resulting in gradual correction of serum sodium and clinical stabilization. This case illustrates how severe SIADH may serve as the presenting clinical manifestation of deep midline PCNSL and underscores the importance of integrating neuroimaging, neuropathology, and electrolyte physiology in medically complex neuro-oncologic presentations.