The inability to produce concentrated urine due to either antidiuretic hormone deficiency or resistance, which leads to extreme thirst and polyuria. The diagnosis may be confirmed using a water deprivation test.
Sudden onset is typical with central DI, though familial nephrogenic DI presents in early childhood and psychogenic polydipsia may have a very long history.
Diabetes insipidus is typified by unquenchable thirst for any fluid, with polyuria and nocturia.
Ask specifically whether the patient prefers certain drinks for example tea or coffee. This is common with habitual drinking, whereas in diabetes insipidus the thirst is for any available liquid - usually water.
Habitual over-drinking or psychogenic polydipsia may in time lead to chronic dilution and loss of collecting tubule concentrating capacity. This mimics the biochemistry of DI and may require a formal water deprivation test to elucidate.
Arrange for the patient to keep a diary of their intake prior to attending clinic. This is useful to quantify the volumes involved at first assessment.
Ask whether they take a glass of water to bed with them, how large it is and whether they drink this in one go, or just sip it during the night. Patients with DI will drink very large volumes and tend to finish a glass in one gulp rather than sipping it over a period of time.
Patients who drink habitually may drink in certain situations for example when bored or anxious, but do not tend to drink when busy, or during the night.
Ask the patient exactly what it is that wakes them. If they drink immediately on waking, before using the toilet, this is typical of DI. If they wake to use the toilet, and then have a drink afterwards, this is less indicative of DI.
Prostate pathology may lead to nocturia though volumes are low and other symptoms are usually obvious. Nocturia in DI typically involves very large volumes but passes in a smaller number of visits.
Patients with prostatic hyperplasia may present with nocturia but also tend to complain of poor stream and other obstructive symptoms.
In patients already taking replacement therapy, nasal desmopressin tends to control symptoms for approximately 16 hours.
Symptoms occurring before this time require an increase in the size or frequency of desmopressin dose.
DI occasionally occurs following head injuries.
Psychogenic polydipsia is more common in patients with anxiety and other psychiatric disorders.
Central DI is commonly seen with peripituitary disease, or after surgery for intrinsic pituitary tumours.
DI may also occur with structural neurological lesions, for example septo-optic dysplasia.
Loss of acuity or failing vision at night are worrying features suggesting optic nerve involvement. Visual field loss, for example homonymous hemianopia, indicates chiasmal impingement from a pituitary or peripituitary lesion.
Headaches present on waking and worse on coughing or leaning forward are suggestive of increased intracranial pressure which demands urgent imaging with an MRI scan. Pain across one section of the head or face only, are more suggestive of cranial nerve involvement particularly with disease in the cavernous sinus.
Weight loss may indicate the development of hypopituitarism but should also alert the clinician to the possibility of malignant disease, germ cell tumours or peri-pituitary metastases.
Vague symptoms of tiredness, nausea and dizziness may indicate the development of hypopituitarism with ACTH or TSH deficiency.
This could indicate the possibility of underlying malignancy or a loss of thyroid function.
Oligomenorrhoea may occur with modest elevations of prolactin.
Amenorrhoea and hot flushes indicate loss of the gonadotropin release.
Loss of libido may have many causes but in this circumstance suggests a loss of gonadotropin release.
Symptoms of erectile dysfunction may not be forthcoming and should be specifically asked about, as should early morning erections.
DI does not tend to occur with intrinsic pituitary disease. Stalk dysfunction may occur in isolation or with para sellar lesions such as germ cell tumours and metastases. DI may also occur when granulomatous and inflammatory conditions affect the posterior pituitary such as Wegener’s granulomatosis, sarcoidosis, tuberculosis or auto-immune disease.
Hypercalcaemia may also occur in malignancy and cause nephrogenic DI.
Hyperprolactinaemia is common with non-functioning and para-sellar tumours due to disinhibition of prolactin release.
Ask about breast swelling, galactorrhoea, oligomenorrhoea, libido and erectile function.
Lithium is a common cause of nephrogenic DI.
Diuretics may also complicate the presentation and history, and can lead to hypokalaemia leading to transient loss of concentrating capacity and nephrogenic DI.
Indigestion remedies and dietary supplements may be associated hypercalcaemia which may also lead to nephrogenic DI.
There are various forms of familial nephrogenic DI which tend to present in infancy.