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

## TL;DR

A rare case links chronic low sodium levels to Lyme disease, highlighting the need for careful diagnosis in tick-prone regions.

## Contribution

This paper reports a novel clinical case linking chronic hyponatremia to Lyme disease through central nervous system inflammation.

## Key findings

- A 16-year-old girl with chronic hyponatremia was found to have Lyme disease as the underlying cause.
- Lyme disease may induce SIADH through nervous system inflammation, a rare mechanism.
- Antibiotic treatment and fluid restriction improved sodium balance in the patient.

## Abstract

Lyme disease (LD), caused by the spirochete Borrelia burgdorferi, is prevalent in Europe, particularly in Romania, and can lead to a wide range of symptoms, including nervous system involvement. One extremely rare complication of this disease might be persistent hyponatremia, which can be associated with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). In this study, the authors report the case of a 16-year-old girl who developed severe, chronic hyponatremia which was later found to be linked to LD. The few cases published suggest that LD may cause this condition through inflammation in the nervous system. The clinical case presented aims to highlight the potential causative link between LD and SIADH, underlining the importance of differential diagnosis and a multidisciplinary approach in complex, intricate cases. The authors’ clinical experience among very few others may help raise suspicion, improve understanding, and help clinicians reach an early diagnosis and ensure that patients receive early treatment in similar cases in the future.

Lyme disease (LD), caused by the spirochete Borrelia burgdorferi, 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 Borrelia serologies, despite the absence of a suggestive history of exposure. SIADH likely symptomatology resulted from central nervous system inflammation induced by Borrelia, 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.