A 21 year old Asian woman presented with relapse of Neuromyelitis Optica (NMO) spectrum disorder, diagnosed aged 16. She had headache, dizziness, right hand weakness and severe hyponatraemia (serum sodium [Na+] 116 ). Emesis, dominant in previous relapses, was absent she was euvolaemic. Serum (Seosm) and urine (Uosm) osmolalities were 250 and 468 mosm/kg respectively, thyroid function normal. Medications included Prednisolone 20 mg daily, Azathioprine, and proton-pump-inhibitor converted to histamine2-receptor-antagonist. Hyponatraemia improved to 129 with fluid restriction, diagnosis being syndrome of inappropriate antidiuretic hormone (SIADH) secretion.
2-weeks later she reported increasing somnolence, increased frequency of urination without evidence of infection, and unchanged thirst. Documented fluid balance over a 24-hour period was normal as were repeat paired serum and urine test results (Na+141, Seosm293, Uosm698). She appeared emotionally labile, expressed suicidal ideation, and fell into deep sleep mid-consultation precipitating urgent admission for treatment of further relapse. Somnolence was likely secondary to central sleep apnoea confirming neurological progression as sodium was normal. MRI brain showed florid inflammatory change in the region of the hypothalamus.
NMO is an autoimmune disease, predominantly affecting optic nerves and spinal cord but also certain brain regions, is associated with the presence of IgG antibodies to aquaporin-4 (highly expressed in hypothalamus, brainstem, periventricle & spine). Minority proportion (case-series, case-reports) is associated with endocrinopathies involving hypothalamus-pituitary (hyperphagia and obesity, hyperprolactinaemia, amenorrhoea-galactorrhoea); and diabetes mellitus, hypothyroidism & hypoparathyroidism. Hyponatraemia due to SIADH, may occur only at the beginning of a relapse, or persist through it and later resolve. Diabetes Insipidus (central or nephrogenic-unclear) has been reportedly associated less commonly than SIADH.
KEY-POINTS: 1. Unlike autoimmune idiopathic hypophysitis which typically causes central-DI, SIADH has been reported more commonly with NMO.
2. SIADH may reportedly precede an exacerbation of NMO, or accompany & resolve after a relapse.
3. Increasing awareness of NMO-associated-endocrinopathy needed.