Nephro Flashcards
A 71 year old lady was admitted by ambulance to hospital following a fall. She was found to have a fractured right neck of femur.
She is a smoker of 10 cigarettes per day.
Her medication is lisinopril for hypertension.
Her initial blood tests on admission were
Na 130 mmol/l
K 3.8 mmol/l
Urea 2.3 mmol/l
On day 2 her sodium fell to 124 mmol/l. On clinical examination what will you need to assess?
The patient should be examined to assess her fluid status - is she hypovolaemic, hypervolaemic or normovolaemic.
Check her heart rate (pulse) looking for increased heart rate indicating hypovolaemia.
Blood pressure: this should be assessed lying and standing to look for a postural fall in blood pressure which would suggest hypovolaemia.
Look for evidence of fluid overload - peripheral odema, pulmonary oedema, ascites.
Clinically the lady is euvolaemic. What laboratory investigations will be useful to establish the cause of hyponatraemia?
Measure plasma osmolality - if low this confirms true hyponatraemia. If it is normal measure triglycerides and protein as elevated levels of these can result in laboratory measuring a lower sodium concentration than is actually present in vivo (pseudohyponatreamia).
If there is true hyponatraemia urine osmolality should be measured. When water excretion is impaired this will be inappropriately high at over 100 mosm/kg.
- *Urine sodium** should also be measured.
- If urine Na less than 20 mmol/l* this indicates reduced circulating volume which may be due to dehydration, heart failure, hepatic failure.
- If urine Na more than 20 mmol/l* the following should be considered:
SIADH
Adrenal insufficiency
Diuretics
Hypothyroidism
In this patient plasma osmolality was 265 mosm/kg (275-295mOsm/kg), urine osmolality was 345 mosm/kg (assume high)(200-900 mOsm/kg) and urine Na was 45 mmol/l (20mEq/L). Given the clinical findings and laboratory results what are the possible causes of her hyponatraemia?
What further investigations would be useful to establish which of these is the diagnosis?
Differential diagnoses for hyponatraemia in this patient who is euvolaemic with an inappropriately high urine osmolality and sodium in the presence of low plasma sodium and osmolality are:
SIADH
Adrenal insufficiency (Addison’s)
Hypothyroidism.
Short synacthen test should be performed to rule out adrenal insufficiency (Addison’s disease). Baseline cortisol level is measured then synthetic ACTH is given subcutaneously. Cortisol is measured again 30 minutes after this has been given.
A normal result is an increase in cortisol at 30 minutes by 200 nmol/l and to over 600 nmol/l.
Thyroid function should be measured, that is TSH and free thryoxine (free T4)
SIADH can only be diagnosed after these conditions have been ruled out.
If SIADH is the diagnosis what other tests may be helpful in establishing the cause?
SIADH can occur when there is stress, pain, as a paraneoplastic syndrome (particularly small cell carcinoma of the lung), injury to the chest wall, pneumonia, head injury or other cerebral lesions. This lady is not taking any drugs that are likely to cause the condition.
A chest X-ray would be helpful to look for pneumonia, chest wall injury and lung neoplasm. If there is any suggestion of abnormality a CT chest should be performed. This is particularly important in the lady with a history of cigarette smoking. A CT brain may also be indicated if there are any neurological signs.
The chest X-ray revealed that the right hilum was bulky and a CT chest revealed a lung tumour in this area. How would you manage this patient?
Treat cause if possible. She should be assessed by an oncologist and thoracic surgeon.
Fluid restriction to 1000 ml in 24 hours may allow sodium to increase. Often in patients with SIADH secondary to a neoplasm demeclocycline may be required. This blocks the action of ADH in the kidneys and allows excess water to be excreted.
Hypertonic saline is occasionally required if patients have severe symptoms including seizures and reduced consciousness.
With any treatment plasma sodium must be closely monitored to ensure that increases are at the rate of 1-2 mmol/l/hour and by no more than 10-12 mmol/l in 24 hours.
What are the causes of polyuria?
There are several causes of polyuria which should be considered.
Diabetes mellitus
Diabetes insipidus - this is when insufficient ADH is secreted from the posterior pituitary (central or cranial diabetes insipidus) or the kidneys are unresponsive to ADH (nephrogenic diabetes insipidus).
Hypercalcaemia - this can cause the kidneys to be unresponsive to ADH thus preventing reabsorption of water (nephrogenic diabetes insipidus).
Excessive fluid intake (psychogenic polydypsia).
A 40 year old man with a history of alcohol abuse is admitted having been found unconscious. On examination he is found to have a head injury. He is not on any prescribed medication. He is noted to have a high urine output passing 170 ml/hour.
The following are the results of his blood tests:
Na 168 mmol/l
K 4.1 mmol/l
Urea 6.7 mmol/l
Creatinine 67 umol/l
Glucose 4.3 mmol/l
Calcium 2.2 mmol/l
Clinically his heart rate is 72 beats/min, BP 126/74 mmHg. Skin turgor appears normal.
What other investigations would be helpful and what is the likely diagnosis?
From the above the patient is clinically euvolaemic and diabetes insipidus should be considered.
Measurement of plasma osmolality will confirm the hyperosmolal state, urine osmolality would be expected to show a very dilute urine.
What is the appropriate initial management of diabetes insipidis?
IV fluids (usually 5% dextrose) are administered to reduce the plasma sodium and replace the urine losses.
Sodium levels should be closely monitored to ensure that replacement is not too rapid (aim for 1-2 mmol/l/hour and not more than 12 mmol/l in 24 hours).
Plasma osmolality is 350 mosm/kg H2O, urine osmolality 80 mosm/kg H2O.
How would you confirm the diagnosis of diabetes insipidus?
Water deprivation test:
Before test is carried out ensure plasma sodium and osmolality are normal.
Patients are weighed and baseline urine osmolality measured and this is repeated hourly. When urine osmolality reaches a plateau ADH is given.
In normal patients there has already been maximal ADH secretion and effect so no further urine concentration is noted.
If there is a lack of ADH secretion (as in central or cranial diabetes insipidus) ADH administration allows the urine concentration to increase and volume to decrease.
If the defect is in the renal response to ADH (nephrogenic DI) then ADH administration will have no effect.
In this patient the urine osmolality plateau was at 310 mosm/kg H2O and then increased to 600 mosm/kg H2O with ADH administration.
What is the cause of the polyuria and hypernatraemia?
The diagnosis is central diabetes insipidus possibly secondary to the head injury.
How is ADH administered?
This patient may require ADH administration long term and there are several methods of administration. These include subcutaneous or intravenous injection, nasal spray (absorbed across mucous membrane) and oral administration.
