Resistant syndrome of inappropriate antidiuretic hormone (SIADH) secondary to atonic bladder in an older woman

Abstract

Syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of hyponatraemia. There are many causes of SIADH, but investigation tends to focus around the most common causes—particularly diseases of the brain and lung, malignancy and medication-induced SIADH [Ellison and Berl (2007, The Syndrome of Inappropriate Antidiuresis. N Engl J Med., 356, 2064–72]. We describe a case of SIADH secondary to atonic bladder in an 83-year old woman, which was discovered on MRI of the abdomen, performed for further characterisation of a known pancreatic lesion. Insertion of a urinary catheter alleviated retention and resulted in prompt resolution of hyponatraemia. This is an under-recognised cause of this common condition, with important implications for investigation and management.

syndrome of inappropriate antidiuretic hormone (SIADH), urinary retention, hyponatraemia, older people

Key points

• Syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of hyponatraemia.

• Urinary retention is an under-recognised cause of Syndrome of inappropriate antidiuretic hormone (SIADH) in older people.

• Urinary retention occurs in both older men and older women.

• Abdominal palpation is unreliable in detecting or excluding urinary retention.

Case

An 83-year-old lady was admitted with a fall which felt to be a result of orthostatic hypotension and a higher-level gait disorder. The patient also had another fall in the past week. She had reduced appetite for 6 months. There were no urinary symptoms.

Past medical history included hyponatraemia which was first identified 2 days prior to presentation by a geriatrician in another hospital. Her sodium at that time was 121 mmol/l and lansoprazole was discontinued. She had an intraductal papillary mucinous neoplasm of the pancreas, under surveillance for the past 5 years. She also had a history of hypothyroidism, an aneurysm of the left middle cerebral artery, prior haemorrhagic stroke, transient ischaemic attack, mild cognitive impairment, hysterectomy and a femoral hernia repair. She was clinically euvolaemic. No abnormality was recorded on abdominal examination, specifically the bladder was not palpated, despite slim body habitus.

Admission bloods showed serum sodium (Na⁺) to be 115 mmol/L (reference range 135-145 mmol/L). Serum potassium (K⁺) was 2.7 mmol/L (reference range 3.5-5.0 mmol/L). Serum phosphate (PO4^-) was 0.50, while serum magnesium (Mg²⁺) was 0.56. Serum creatinine was 38 and eGFR >90 ml/min/1.73 m². Serum osmolality was 237, while urinary osmolality was 365, urinary sodium was 79 (Figure 1).

Figure 1.

Trend of serum sodium level during admission

The cause of hyponatraemia was felt to be most likely syndrome of inappropriate anti-diuretic hormone (SIADH) secondary to a proton-pump inhibitor. The patient was commenced on 0.45% saline and intravenous K⁺, PO4^- and Mg²⁺. On further advice from Endocrinology, she was changed to 1l fluid restriction. Na⁺ failed to normalise with these measures.

CT Brain was performed and showed chronic changes, but no acute lesion. A chest radiograph showed bilateral lower lobe infiltrates, no solid mass.

The patient proceeded to MR cholangiopancreatography on day 8 of admission to assess for malignant transformation of her pancreatic lesion. This showed stable appearances of the pancreatic lesion, but gross distension of the urinary bladder with symmetrical thickening of the bladder wall and hydronephrosis, suggestive of chronic urinary retention. Bladder scan revealed a 900 ml post-void residual. A urinary catheter was inserted and drained 1,000 ml of urine stat. The patient subsequently maintained a neutral fluid balance. Na⁺ normalised within 9 days of catheter insertion. CT abdomen and pelvis was performed on the advice of Radiology to further evaluate the MRI findings. This showed marked improvement in hydronephrosis. The diagnosis was made of SIADH secondary to urinary retention in the context of atonic bladder. The patient was reviewed by Urology and long-term urinary catheterisation was arranged.

Discussion

SIADH is the most common cause of hyponatraemia, with incidence rising with increasing age. There are myriad causes of SIADH, though investigation often focuses on diseases of the brain and lung, as well as looking for malignancy and reviewing medications. Hyponatraemia is associated with serious adverse neurological sequelae and overly rapid correction of chronic hyponatraemia can cause central pontine myelinolysis.

This is an unusual case, as urinary retention has been noted as a cause of SIADH, but this is rare. To our knowledge, it has only been described in one case series and a few case reports [2, 3]. Clinical examination is notoriously unreliable in detecting urinary retention, especially in cases where overflow incontinence masks the issue [4]. In addition, those practicing outside geriatric medicine may be unaware that urinary retention is at least as common in women as in men in geriatric rehabilitation settings [5]. It is therefore pertinent to perform a bladder scan in cases of resistant hyponatraemia secondary to SIADH, especially if no other cause can be found. Urinary retention should be investigated appropriately and Urological input may be helpful. It can be effectively treated with urinary catheter insertion.

Declaration of patient consent: Written informed consent for publication of their clinical details was obtained from the patient.

Declarations of Conflicts of Interest: None.

Declarations of Sources of Funding: None.