hi this is tom from zero finals comm in this video I'm going to be going through diabetes insipidus you can find written notes on this topic at zero finals comm slash diabetes insipidus or in the endocrinology section at the zero finals Medicine book let's jump straight in diabetes insipidus is caused by either a lack of antidiuretic hormone or ADH or a lack of response to antidiuretic hormone remember the antidiuretic hormone acts on the collecting ducts in the kidneys and allows them to reabsorb water from the urine so having diabetes insipidus prevents the kidneys from being able to concentrate the urine and this leads to polyuria or excessive amounts of urine and polydipsia or excessive thirst because the blood is so concentrated it can either be classified as nephrogenic or cranial diabetes insipidus depending on whether the problem is in the kidneys or in the brain and a key differential diagnosis of diabetes insipidus is primary polydipsia and this is where the patient has a normally functioning ADH system but they're drinking excessive quantities of water and this is leading to excessive urine production they don't have diabetes insipidus but they present with polyuria and often polydipsia firstly let's talk about nephrogenic diabetes insipidus and this is where the collecting ducts of the kidneys don't respond to the ADH that's in the bloodstream and this can be caused by a number of different factors one of the key causes is drugs particularly a medication called lithium that's used in bipolar affective disorder there's also a genetic abnormality that leads to nephrogenic diabetes insipidus and this is mutations in the AV pr2 gene on the X chromosome intrinsic kidney disease so really any chronic kidney disease that's affecting the internal functioning of the kidneys can lead to nephrogenic diabetes insipidus and electrolyte imbalances can also be a cause particularly hypokalemia or a low potassium and hypercalcemia or a high calcium next let's talk about cranial diabetes insipidus and this is where the hypothalamus gland does not produce ADH for the pituitary gland to secrete and this can be idiopathic without any clear cause or it can be caused by brain tumors head injuries brain malformations infections such as meningitis and colitis and TB or brain surgery or radiotherapy the presentation is all linked to the excessive loss of water in the urine so patients are polyuria or excessive water production polydipsia or excessive thirst dehydration postural hypotension so their blood pressure drops as they stand up and if you check their blood tests they'll also have hyponatremia or high sodium concentration in the blood how do you investigate them you can do your user knees blood test to check the kidney function and you might find a hyponatremia you can also check the urine osmolality and you'll find a low urine osmolality because there's a low concentration of solutes in the urine they've all been diluted by water and you'll find a high serum osmolality as there's a high concentration of solutes in the blood the next test is called a water deprivation test the water deprivation test is also known as the desmopressin stimulation test and this is the tested choice for diagnosing diabetes insipidus so how do you do it well firstly the patient needs to avoid taking any fluids or food for 8 hours prior to the test and this is referred to as fluid deprivation then the urine osmolality is measured and synthetic ADH or a desmopressin is given eight hours later the urine osmolality is measured again how do you interpret the results well in cranial diabetes insipidus the patient lacks ADH so the kidneys are still able to respond to ADH so initially after that first 8-hour period of water deprivation the urine osmolality will still be low as it continues to be diluted by the excessive water secretion by the kidneys then after the synthetic ADH has been administered the kidneys will start to respond to that synthetic ADH by reabsorbing water and concentrating the urine so that 8 hours after the ADH is given the urine osmolality will be high and that way you know that it's cranial diabetes insipidus and the kidneys are still functioning normally in nephrogenic diabetes insipidus the patient is unable to respond to that ADH what's happening is they're diluting their urine with excessive water secretion by the kidneys and they're unable to respond to ADH therefore the urine osmolality will be low after the initial eight-hour water deprivation and will remain low after this synthetic ADH is given in primary polydipsia the eight-hour water deprivation will cause the urine osmolality to be high even birth or the synthetic ADH is given if you have a high urine osmolality eight hours after water deprivation you know that they don't have diabetes insipidus so there's no purpose for giving ADH and doing the full test you can exclude at this point that they have any diabetes insipidus so just to briefly summarize those results in cranial diabetes insipidus you'll have a low urine osmolality after deprivation and a high urine osmolality after the ADH is given in nephrogenic diabetes insipidus you have a low urine osmolality after deprivation and it remains low after the ADH is given and in primary polydipsia you'll have a high urine osmolality after deprivation and of course it will remain high after the ADH is given so how do you treat diabetes insipidus well if possible is worth correcting the underlying cause often mild cases can be managed conservatively without any treatment desmopressin which is synthetic ADH can be used as a treatment in cranial diabetes insipidus to replace the ADH that they're not already making and in nephrogenic diabetes insipidus it can also be used but often very high doses are required and close monitoring is necessary in order to get an adequate response from the kidneys so thanks for watching I hope you found this video helpful if you did don't forget there's plenty of other resources on the zero to finals website including loads and loads of notes on various different topics that you might cover in medical school with specially made illustrations there's also a whole test section where you can find loads of questions to test your knowledge and see where you're up to in preparation for your exams there's also a blog where I share a lot of my ideas about a career in medicine and tips on how to have success as a doctor and if you want to help me out on YouTube you can always leave me a thumbs up give me a comment or even subscribe to the channel so that you can find out when the next videos are coming out so I'll see you again soon