Case Presentation: 83 year old female with a history of SLE, rheumatoid arthritis, B-cell lymphoma in remission, hypothyroidism presented with a 4 day history of atraumatic lower back pain, rash to left lower back, nausea, retching, few episodes of small volume, clear vomitus and fatigue. She also reported one year of 25 pound unintentional weight loss and decreased appetite. No fever or infectious symptoms.Exam was significant for cachexia and an erythematous macular rash on her left lower back, extending to left hip in a dermatomal distribution with no open vesicular lesions, warmth or tenderness. She appeared euvolemic and vitals were normal. Investigations were significant for euvolemic hypotonic hyponatremia (Na 129, serum osmolarity 260), high urine osmolarity of 534 and urine Na of 100. Normal BUN, creatinine, CBC, LDH, glucose, morning cortisol level and TSH. 3 months prior, Na was 137. CXR and CT Abdomen and Pelvis were both unremarkable. Prior to obtaining results of urine studies, 0.9% saline and plasmalyte fluids were given as hyponatremia thought to be due to dehydration/vomiting initially. Despite this, Na decreased to 124. Following results suggestive of syndrome of inappropriate antidiuretic hormone secretion (SIADH), management included fluid restriction, valacyclovir re: likely herpes zoster viral (HZV) infection, in addition to pain management. Hyponatremia did not resolve and prompted treatment with urea and 1 dose of tolvaptan 7.5 mg IV which led to improvement of Na to 134. Her sodium remained stable 2 months later, rash improved but she experienced post herpetic neuralgia.

Discussion: SIADH is a disorder of water balance characterized by hypotonic hyponatremia and impaired water excretion due to an abnormally elevated level of ADH (1). Common causes include neuropsychiatric disorders, drugs, pulmonary diseases, postoperative complications, infections, malignancy, and exogenous use of ADH (2). Common symptoms include nausea, headaches and malaise, but in rare cases seizures or coma can occur in patients with severe hyponatremia.A thorough work up excluded the most common causes of SIADH, leaving cutaneous HSV as the most likely etiology.  That diagnosis was further supported by resolution of the resistant hyponatremia with treatment of her underlying HZV infection; leaving us to conclude that HZV was the likely culprit.While the association between SIADH and HZV is not common, there have been several case reports dating as far back as 1983 (3). Some involved HZV meningoencephalitis (3), while others were limited to cutaneous involvement (4,5). Although most cases reported resolution of hyponatremia shortly after treatment of HZV infection, there were cases where patients required long term tolvaptan therapy(6).SIADH diagnosis can easily be missed in the setting of a seemingly unrelated acute illness. Early identification in such cases prevents diagnostic errors which have the potential to cause harm or even fatal outcomes. Gait instability due to persistent hyponatremia can also lead to falls and significant morbidity in the elderly who are more prone to HZV infections.

Conclusions: Hyponatremia secondary to SIADH can be caused by multiple factors including infections, brain pathology, lung pathology, malignancy, pain and electrolyte abnormalities. There is increasing evidence limited to case reports showing an association of HZV and SIADH. Recognition of this association is important to help identify and treat hyponatremia early in these patients to reduce morbidity and mortality.