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Old 02-25-2011
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Help primary polydipsia vs diabetes insipidus

I am getting confused between primary polydipsia also known as psychogenic polydispia and Diabetes insipidus ...can anyone clearly explain the difference between them?
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Old 02-25-2011
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Psychogenic polydipsia is an excessive water intake seen in some patients with mental illnesses such as schizophrenia, and/or the developmentally disabled. It should be taken very seriously, as the amount of water ingested exceeds the amount that can be excreted by the kidneys,and can on rare occasions be life-threatening as the body's serum sodium level is diluted ( in other words dilutional hyponatremia ) to an extent that seizures and cardiac arrest can occur.
The excessive levels of fluid intake may result in a false diagnosis of diabetes insipidus since the chronic ingestion of excessive water can produce diagnostic results that closely mimic those of mild diabetes insipidus

Excessive urination and extreme thirst (especially for cold water and sometimes ice or ice water) are typical for DI. Symptoms of diabetes insipidus are quite similar to those of untreated diabetes mellitus, with the distinction that the urine does not contain glucose and there is no hyperglycemia (elevated blood glucose). Blurred vision is a rarity. Signs of dehydration may also appear in some individuals since the body cannot conserve much (if any) of the water it takes in.

In order to distinguish DI from other causes of excess urination, blood glucose levels, bicarbonate levels, and calcium levels need to be tested. Measurement of blood electrolytes can reveal a high sodium level (hypernatremia as dehydration develops). Urinalysis demonstrates a dilute urine with a low specific gravity. Urine osmolarity and electrolyte levels are typically low.

Habit drinking (in its severest form termed psychogenic polydipsia) is the most common imitator of diabetes insipidus at all ages. While many adult cases in the medical literature are associated with mental disorders, most patients with habit polydipsia have no other detectable disease. The distinction is made during the water deprivation test, as some degree of urinary concentration above isosmolar is usually obtained before the patient becomes dehydrated.
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Old 02-27-2011
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Quote:
Originally Posted by TAAOM View Post
I am getting confused between primary polydipsia also known as psychogenic polydispia and Diabetes insipidus ...can anyone clearly explain the difference between them?

I think the main confusion is there is a similarity between the two diseases. In both patients we would see a high urine output. But the patient with polydipsia the high urine output is from drinking excessive amounts of water usually because of psychogenic causes like schizophrenia mentioned earlier.

However, in Diabetes Insipidus patients urinate excessive amounts because of a lack of ADH (central DI) or a lack of sensitivity (Nephrogenic DI). Central DI can be caused inadvertently by surgeries that are near the posterior pituitary where ADH is secreted (like a pituitary adenoma). Nephrogenic DI can be caused by a patient on demeclocycline or lithium which both decrease the kidney's sensitivity to ADH in the collecting tubule.


I think clinically, you differentiate between the two patients in an outpatient setting by a water deprivation test. In a patient with psychogenic polydipsia (like one with a history of psychiatric disorders), water deprivation should cause the patient patient to urinate less (because water intake was the cause of the excessive urination). But in DI patients will continue to urinate excessive amounts (and secondarily increase their serum osmolality) because the problem of ADH insensitivity has not been addressed by the water deprivation.



In the previous post there was a mention of Diabetes Mellitus. In both DI and DM patients urinate excessive amounts. But DM is because of a high serum glucose that overwhelms the kidney's ability to reabsorb filtered glucose in the proximal tubule (Na /Glucose transporter). So those patients urinate large amounts of water IN ADDITION TO glucose. But DI patients urinate large amounts of water but WILL NOT have glucose in their urine (glucose is normally 98-100% reabsorbed from the kidney).
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Old 03-01-2011
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thxxx for such a clear explanation ....one more

water deprivation test
person with psychogenic polydipsia urine osmolarity remains normal ( unsure abt this)

where as person with DI osmolarity is decreased


am i correct ???
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Old 03-01-2011
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Quote:
Originally Posted by TAAOM View Post
thxxx for such a clear explanation ....one more

water deprivation test
person with psychogenic polydipsia urine osmolarity remains normal ( unsure abt this) where as person with DI osmolarity is decreased
am i correct ???
In someone with psychogenic polydipsia, once their water intake were restricted, their normal kidneys would begin to concentrate the urine again.

In someone with DI, the urine would remain dilute even in the absence of water intake.
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Old 03-01-2011
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Good explaination!

How about biochemically in serum and urine in these two conditions?
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Old 06-04-2011
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Primary polydipsia:

Serum [Na] decreased, Serum osmolality decreased, Urine osmolality decreased, water deprivation increases urine osmolality (there was vague detail about the kidney losing its ability to respond to ADH if medulla has been under water dilution for so long, but thats not important for USMLE purposes)

DI:

Serum Na concent. increased, Serum osmolality increased, Urine Osmo decreased, water deprivation worsens serum osmolality and do not change urine osmol.
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