A Tick-Borne Cause of Hyponatremia: SIADH Due to Lyme Meningitis

      An 83-year-old woman was incidentally found to have hyponatremia (125 mmol/L) when undergoing an inpatient colonoscopy because of unspecific gastrointestinal symptoms and weight loss; no pathology was found on endoscopy. Further biochemistry was in keeping with the syndrome of inappropriate antidiuretic hormone (SIADH) secretion (urinary sodium 66 mmol/L, urinary osmolality 321 mosmol/kg, low uric acid); adrenal and thyroid function as well as glucose were normal. The patient was remarkably fit for her age; she was a lifelong nonsmoker; and her only medication was candesartan for hypertension. Clinical examination was reportedly unremarkable. A battery of tests, including gastroscopy, computed tomography of the brain, chest and abdomen, mammogram, and gynecologic examination, yielded normal results. Drug-related hyponatremia was surmised and the antihypertensive held; as sodium levels were slightly higher when controlled 5 days later, amlodipine was prescribed instead. However, when the patient's general practitioner repeated electrolytes after 1 month, profound hyponatremia (126 mmol/L) was evident again, and the patient was referred to nephrology clinic, where a detailed history was obtained. One month prior to the first admission, around Easter, the patient had presented to the emergency department of another hospital because of stabbing back pain of subacute onset, localized to the left shoulder. No cardiopulmonary cause was found at that time, and she was discharged on analgesia. The shoulder pain gradually subsided but lower back pain ensued, worsening at night. In parallel, she noticed difficulties in concentrating, unsteadiness, and poor appetite. The patient, a keen gardener, denied headache or arthralgia, did not recall any tick bites, and had not noticed a rash. On examination, she appeared euvolemic, and there were no skin or focal neurologic signs. Laboratory results were essentially unchanged to the previous admission. In view of the history, nocturnal back pain and obscure hyponatremia, she was admitted for a lumbar puncture. Cerebrospinal fluid showed pleocytosis (> 90% lymphocytes), elevated protein and normal glucose; intrathecal Borrelia burgdorferi-specific antibodies and CXCL13 were strongly positive, while syphilis serology was negative. On these grounds, a diagnosis of Lyme meningitis was reached. A 3-week course of ceftriaxone and urea (15 g twice daily for SIADH) were prescribed and on her next clinic appointment, 1 month later, the patient was well and eunatremic and has remained so on follow-up. In hindsight, the lancinating shoulder pain prompting the patient's first hospital visit likely represented Bannwarth syndrome, a radiculoneuritis occurring early in the course of Lyme disease.
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