Ticking Down Sodium Levels—An Atypical Link Between Chronic Hyponatremia and Borreliosis

Lyme disease (LD), caused by the spirochete <i>Borrelia burgdorferi</i>, is the most prevalent tick-borne disease in Europe, including Romania, where endemic areas are well documented. It has a wide range of clinical manifestations and severity, including rare neurological complications. Persistent hyponatremia is an atypical presentation of Lyme neuroborreliosis and can be associated with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH is characterized by unregulated antidiuretic hormone release, leading to impaired water excretion, dilutional hyponatremia, and low serum osmolality. We report the case of a 16-year-old female with clinically well-tolerated, but severe, refractory hyponatremia, who was poorly responsive to intravenous sodium supplementation and fluid management. Complex investigations ruled out multiple causes of hyponatremia; neuroborreliosis was confirmed via positive <i>Borrelia</i> serologies, despite the absence of a suggestive history of exposure. SIADH likely symptomatology resulted from central nervous system inflammation induced by <i>Borrelia</i>, a mechanism rarely documented in the medical literature. Treatment with antibiotics and fluid restriction led to a gradual improvement in fluid balance and sodium homeostasis. This case emphasizes the importance of considering rare infectious causes, such as LD, in patients with unexplained SIADH, especially in endemic areas. It highlights the importance of a multidisciplinary approach in intricate, complex cases.