Rathke’s Cyst and Adrenal Insufficiency: A Rare Case of Hyponatremia

Abstract

Abstract Background: Hyponatremia is the most common electrolyte abnormality encountered in clinical practice, the diagnosis of which can be challenging. A holistic approach with appropriate investigations is imperative to its diagnosis and management. We report this peculiar case of a patient with hypopituitarism secondary to Rathke’s cyst resulting in secondary Adrenal Insufficiency, thus causing Hyponatremia. Case Details: A 32 year male was admitted to hospital with an intractable headache and deterioration in his general wellbeing. His admission parameters revealed a GCS of 15 with normal observations. He had no evidence of meningism and examination of all other systems was unremarkable. He was not on any regular medications and there was no history of trauma. Bloods revealed a Sodium of 122mmol/l, the rest being normal. Investigations including a CT brain, CXR, paired serum & urine osmolality, urinary sodium, Thyroid functions and a 9am cortisol were conducted. Radiological imaging was unremarkable. Subsequently, he was fluid restricted due to a euvolemic fluid status. Serum osmolality was 246 mmol/kg (275-295 mmol/kg), Urine osmolality 847 mmol/kg, urine sodium 85 mmol/l and a 9 am serum cortisol of 29 nmol/L, in keeping with Adrenal Insufficiency. A further decline in his Sodium to 116 mmol/L resulted in Hyponatremic Encephalopathy. In view of impending Adrenal crisis, IV hydrocortisone was commenced in conjunction with hypertonic Saline. This was followed by strict monitoring with cautious correction. An anterior Pituitary profile, serum testosterone and Prolactin were requested. Serum Testosterone levels were 0.4nmol/L (10.0-36nmol/L), FSH- 2.5 IU/L (1.0-11.0 IU/L) and LH- 1.9 IU/L (1.5-9.3 IU/L). Serum prolactin was 85mU/L (45-375 mU/L), TSH- 0.59 mU/L (0.30-6.00 mU/L), Free T4- 13.0 pmol/L (10.4-24.5 pmol/L), IGF1-135 ug/L (60-197 ug/L). This was suggestive of partial hypopituitarism. An MRI Pituitary revealed appearances in keeping with a Rathke’s cyst occupying the pituitary fossa without compromise of the Optic Chiasma. Once stable, he was safely discharged with follow-up appointments planned with the Endocrine and Neurosurgical teams. Discussion: Rathke cleft cysts are non-neoplastic cysts arising from the embryological remnants of Rathke’s pouch in the pituitary gland. Hormonal and visual manifestations can occur if these cysts enlarge and interfere with the Pituitary production and optic chiasma. A proposed mechanism causing Hyponatremia in the context of Cortisol deficiency is increased hypothalamic secretion of Corticotropin releasing hormone (CRH), a Vasopressin secretagogue. Cortisol feeds back negatively on CRH, which is removed with adrenal insufficiency. Hyponatremia is thus often seen in patients with adrenal insufficiency, caused by an inappropriate increase in vasopressin due to cortisol deficiency and inability to excrete free water.