Polyuria (oxford clin cases) Flashcards
How do you clinically differentiate polyuria from nocturia or increased urinary frequency?
Ask the patient if they have been passing increasingly larger VOLUMES of urine every time they go.
The only way to actually know would be to do a 24hr urine sample but this is impossible in practice.
You can ask them about how often they go and if they find themselves waking up to go if they can’t recall.
What is polyuria?
Passing increased volumes of urine
What are the top differentials you will have in mind when someone presents with polyuria?
Diabetes mellitus Diuretics (medication, caffeine, alcohol, lithium) Heart failure Primary polydipsia Hyperthyroidism Hypercalcaemia Hypokalemia
What are some other causes of nephrogenic diabetes insipidus?
Chronic renal failure
Hypercalcaemia
What are some other causes of diabetes mellitus?
Cushing’s syndrome
Steroids
Why is nocturia concerning?
At night the kidney’s concentrate urine to increase fluid retention because fluid intake is zero. This results in a lack of needing to wake up to urinate. If nocturia arises it is one of the first signs something is wrong with the concentrating mechanism.
What symptom does the presence of nocturia make less likely?
Polyuria
What are other symptoms of diabetes mellitus asides from polyuria?
Fatigue
Weight loss
Recurrent infections
What lower urinary tract symptoms should you ask about when someone presents with polyuria?
Urgency Frequency Dribble Hesitancy Incomplete voiding
What pathology do lower urinary tract symptoms indicate?
Pathology of the bladder or outflow tract eg prostatism (men), detrusor instability/collapse (women)
What symptoms might indicate a UTI?
Dysuria
Change in urine colour (especially cloudy urine)
Change in urine smell
New nocturia
What should you ask about in the history when someone has polyuria? Explain why you are asking about each thing?
Temporal pattern of urine output- if nocturia is present primary polydipsia is unlikely
Weight loss, fatigue, recurrent infections- diabetes mellitus
Lower urinary tract symptoms- bladder or outflow pathology
Past medical history- cancer, renal problems
Drug history- diuretics
Family history- cancer, diabetes mellitus
What are the 2 mechanisms by which polyuria arise? How do they affect urine osmolality and volume?
Inability to reabsorb solutes= solutes remain in urine, high urine osmolality and high volume
Inability to reabsorb water= low urine osmolality and high volume
Which cells are affected if water cannot be reabosrbed?
Cells in the distal tubules
What hormone is affected if water cannot be reabsorbed?
ADH
What are some tests a GP might order when someone presents with polyuria? Explain why each one is done
Blood glucose- to check for diabetes mellitus
Thyroid function tests- to check for hyperthyroidism
UEs- to check for urine osmolality
Urinalysis- to look for infection/rule out UTI
Renal function tests- to look at eGFR
Serum calcium- to look for hypercalcaemia
What is the brief pathophysiology of diabetes insipidus?
A problem in the ADH pathway leads to an inability to reabsorb water resulting in polyuria with normal glucose levels and a low urine osmolality
What are the 2 types of diabetes insipidus? How do they differ in terms of pathophysiology?
Cranial- reduced ADH production (brain doesn’t respond to signals to concentrate urine
Nephrogenic- reduced receptivity to ADH in the kidney
What are some causes of cranial diabetes insipidus?
Trauma Tumor Surgery Vascular lesion Meningitis
What are some causes of nephrogenic diabetes insipidus?
Chronic kidney damage- can be due to lithium Hypercalcaemia Hypokalemia Pyelonephritis Hydronephrosis Genetic