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
What is the name of the test done to differentiate between nephrogenic and cranial diabetes inspidus?
Water deprivation test
What is the water deprivation test used for?
To determine whether diabetes insipidus is nephrogenic or cranial
How is the water deprivation carried out?
The patient is deprived of water and given desmopressin (which is a synthetic ADH analogue), and their urine output is measured. If diabetes insipidus is cranial urine volume will decrease but if nephrogenic urine volume will not decrease
What must you be careful when carrying out a water deprivation test?
You must not induce hypovolemia
This is done by measuring the patient’s weight throughout the 24 hrs and stopping if it falls by more than 3%
If someone has polyuria and raised plasma glucose what is the likely diagnosis?
Diabetes mellitus
If someone has polyuria, normal blood glucose and normal urine osmolality what is the likely diagnosis?
Solute diuresis due to drugs (diuretics), mannitol, contrast agent or urea
What is the likely diagnosis if someone has polyuria, normal plasma glucose, low urine osmolality and low serum sodium?
Psychogenic polydipsia
What is the likely diagnosis if someone has polyuria, normal plasma glucose, low urine osmolality and normal serum sodium?
Diabetes insipidus- either nephrogenic or cranial which is differentiated by doing a 24hr water deprivation test
If someone’s urine osmolality rises after the administration of desmopressin in a water deprivation test what is the likely diagnosis?
Cranial diabetes insipidus
If someone’s urine osmolality is low after the administration of desmopressin in a water deprivation test what is the likely diagnosis?
Nephrogenic diabetes inspidus
How is cranial diabetes insipidus managed?
Ensure an adequate fluid intake- this may be enough if symptoms are mild and warn them to look for signs of dehydration
Look for underlying cause- an MRI head should be done to look for common pathology eg pituitary tumors
ADH analogues- giving desmopressin should be good treatment
How is nephrogenic diabetes insipidus managed?
Drugs such as chlorpropamide or carbamazepine
What symptoms and who will classically present with type 1 diabetes mellitus?
Polydipsia, polyuria, weight loss, recurrent infections
Usually young people will present with a family history, with other autoimmune conditions etc
How is type 1 diabetes mellitus diagnosed?
Via fasting glucose (>7mM) or 2 hrs after 75g glucose levels are >11.1 mM
These measurements must be met twice or once with symptoms
Hba1c may also be raised
What is a complication of type 1 diabetes mellitus but not type 2?
Diabetic ketoacidosis
Why does diabetic ketoacidosis not occur in type 2 diabetes mellitus?
Even a small amount of insulin is enough to prevent it and in type 2 there is not a problem with insulin production but with insulin sensitivity
When does a patient with type 1 diabetes need to increase their insulin dose and why?
When they are ill
The body is under stress it produces corticosteroids which act as an antagonist to insulin so more insulin is required to maintain a normal blood glucose
What do patients with type 1 diabetes need to do when they are ill?
Increase their insulin doseage and measure their blood glucose more
What might you observe clinically when someone has diabetic ketacidosis?
Kussmaul breathing
Dry tongue
Drowsiness
Flushed appearance
What is Kussmaul breathing and when will you see it?
Rapid deep inspiration
Patients will breath this way during a metabolic acidosis to try and increase oxygen intake to correct the imbalancw
What are symptoms of hypercalcaemia?
Constipation
Impaired renal function- causes nephrogenic diabetes insipidus
What cells be upregulated in bony metastases vs multiple myeloma?
Bony mets= obsteoblasts
Mutiple myeloma= osteoclasts
What might raised ALP levels with hpercalcaemia suggest? Explain why
Bony metastases, these activate osteoblasts which produce ALP
Who is primary polydipsia most commonly seen in?
Schizophrenics
How is primary polydipsia diagnosed? Explain the result of the test
Water deprivation test- as the patient is deprived of fluid their urine osmolality will rise
What is primary polydipsia?
Increased urination due to overhydration
How is primary polydipsia managed?
Fluid restriction
Antipsychotic treatment/ review if they are schizophrenic as primary polydipsia often arises when patients become more unwell
What complication will arise in patients with type 2 diabetes instead of diabetic ketoacidosis?
Hyperosmolar hyperglycaemic state
What are the 2 main effects of hyperosmolar hyperglycaemic state?
Cellular dehydration- water moves from extravascular compartments to intravascular
Hypovolemia- there is an osmotic diuresis causing water loss in the urine
What might hyperosmolar hyperglycamic state result in?
Shock due to hypovolemia, neurological impairment, thrombosis
