Diabetes insipidus
BMJ 2019; 364 doi: https://doi.org/10.1136/bmj.l321 (Published 28 February 2019) Cite this as: BMJ 2019;364:l321
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We would like to point out a typographical error in Levy et al.'s review of diabetes insipidus. The flow chart titled "Biochemical assessment of polyuria and polydipsia" suggests that serum osmolality <295 mosmol/kg is an indicator of possible diabetes insipidus. However, it is high serum osmolality that is of clinical significance, as described in the main body of the article. Serum osmolality greater than (not less than) 295 mosmol/kg points towards the possibility of diabetes insipidus(1).
In the same figure, we also wonder if 24 hour urine volumes described in hours should be stated in litres.
1. Di Iorgi N, Napoli F, Allegri AE, Olivieri I, Bertelli E, Gallizia A, Rossi A, Maghnie M. Diabetes insipidus–diagnosis and management. Hormone research in paediatrics. 2012;77(2):69-84.
Competing interests: No competing interests
In their excellent Practice Pointer ‘Diabetes Insipidus’, there are 2 important areas that Levy and colleagues might have usefully included.
Firstly, whilst a ‘diabetes insipidus card and booklet to carry with them’ may be empowering for many patients, the use of modern technology such as ICE [in case of emergency] on mobile phones is an additional or alternative modality that also has an important role to play in patient safety.
Secondly, pregnancy can be a particularly challenging time to diagnose and manage Diabetes Insipidus. Physiological changes of normal pregnancy lead to thirst and increased fluid intake as well as increased urine output, so the clinical history may be less clear, although it is unusual for a healthy pregnant woman to drink more than 3litres of water daily.
Placental vasopressinase increases the metabolism of antidiuretic hormone, and can result in gestational Diabetes Insipidus especially in women with previous subclinical disease and or altered liver metabolism [with reduced antidiuretic hormone(ADH) breakdown] as for example in pre eclampsia, HELLP [haemolysis, elevated liver enzymes, low platelets] syndrome and acute fatty liver of pregnancy: treatment is with Desmopressin and delivery, as removal of the placenta allows ADH levels to begin to return to normal.
For pregnant women already diagnosed with Diabetes Insipidus, enhanced antenatal, intrapartum and immediate postnatal care is required. Desmopressin dose increment should be anticipated in pregnancy due to accelerated metabolism of endogenous ADH [as Desmopressin has a different N terminal to ADH it is not metabolised by vasopressinase], with reduction again after birth; it has no oxytocin-like properties and is safe in pregnancy and breastfeeding.
Competing interests: No competing interests
Re: Diabetes insipidus
I had initially planned to write to point out the incompatible advice in this article: The text states that serum osmolality >295mOsmol/Kg is significant whilst the highlight box states that <295mOsmol/Kg is significant. The correct advice being that a high serum osmolality due to excessive water loss is important [i.e. >295mOsmol/L]. This point had already been made.
There is however another simple test that can differentiate polyuria due to psychogenic polydipsia from polyuria due to diabetes insipidus [an important distinction because the treatments are diametrically different - restrict water vs. do not restrict water]. Patients with diabetes insipidus tend to have high (above upper limit of reference range) serum sodium concentrations whilst those with psychogenic polydipsia have low sodium concentrations. In my experience, this simple test is just as effective as a water deprivation test at identifying those who have to urinate to get rid of their excess water load, from those who have to drink to keep up with their excessive urination.
Competing interests: No competing interests