Page 9 - Medicine and Surgery

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Chapter 1: Fluid and electrolyte balance 5

Table 1.1 Important causes of hyponatraemia Acollecting duct that is impermeable to water, so that
dilute urine is excreted. ADH acts on the collecting
Hyponatraemia with Vomiting, diarrhoea, drains, burns,
fluid depletion pancreatitis, sweat duct to make it permeable. Any cause of raised ADH
Renal failure and salt-losing renal will increase water reabsorption and hence tend to
disease cause hyponatraemia. ADH is increased in SIADH,
Addison’s disease hypovolaemia, reduced effective circulating volume
Euvolaemic Psychogenic polydipsia (e.g. cardiac failure, nephrotic syndrome), postoper-
hyponatraemia Alcohol excess with malnutrition
Syndrome of inappropriate atively (pain and stress), and increased ADH activ-
antidiuretic hormone (SIADH) ity is caused by certain drugs. Lack of glucocorticoid
Postoperative patients, particularly can also cause increased permeability of the collecting
after inappropriate fluids duct, because cortisol inhibits ADH.
Hyponatraemia with Congestive cardiac failure, cirrhosis,
In psychogenic polydipsia, patients drink such large
fluid overload nephrotic syndrome
Renal failure volumes of water that the ability of the kidney to ex-
Severe hypothyroidism crete it is exceeded. In chronic alcoholics who are mal-
Drugs Diuretics (e.g. thiazide diuretics, nourished, the renal ability to excrete free water may
amiloride, loop diuretics) be markedly impaired due to lack of dietary solutes
Nonsteroidal anti-inflammatory to as little as 4Laday and anyexcess fluid may cause
drugs
Many psychiatric medications (e.g. hyponatraemia.
haloperidol, selective serotonin As serum sodiumlevelsfall, water moves from the extra-
reuptake inhibitors) cellular compartment into cells. The brain is most sensi-
Opiates, ecstasy tive to this and if hyponatraemia occurs rapidly oedema
develops, leading to raised intracranial pressure, brain-
stem herniation and death. If hyponatraemia develops
it is acute or chronic and whether there is fluid depletion, more slowly, the cells can offset the change in osmolality
euvolaemia or fluid overload. by extrusion of organic solutes. This reduces the degree
Acute hyponatraemia is usually due to vomiting and
of water movement and there is less cerebral oedema.
diarrhoea and/or inappropriate intravenous fluids When treatment is initiated, if serum sodium levels are
such as dextrose or dextrosaline. corrected too rapidly in a patient who has had chronic
Common causes of chronic hyponatraemia include
hyponatraemia, cell shrinkage can occur due to move-
SIADH, Addison’s disease, congestive cardiac failure ment of water out of the cells, causing more damage.
and drugs.
Clinical features
Symptoms of hyponatraemia include lethargy, anorexia,
Pathophysiology
nausea, vomiting, fatigue, headache, confusion and a de-
Normally when serum osmolality falls, ADH production
creased conscious state. The severity depends on the
ceases and the kidneys rapidly excrete the excess water
degree of hyponatraemia and the rapidity at which
(up to 10–20 L/day). In order for the kidneys to excrete
it develops. In severe cases, the patient may have seizures
water there needs to be the following:
or become comatose. It is important to take a careful
Adequate filtrate reaching the thick ascending loop of

drug history, including the use of any illicit drugs such
Henle (where sodium is extracted to produce a dilute
as heroin or ecstasy. There may be symptoms and signs
urine). This is impaired in renal failure and hypo-
of fluid depletion or fluid overload (see page 2).
volaemia (reduced glomerular filtration rate) or re-
duced effective circulating volume such as in cardiac
failure. Investigations
Adequate active reabsorption of sodium at the loop of To determine the cause of hyponatraemia the following

Henle and distal convoluted tubule, this is impaired tests are needed: the plasma osmolality, urine osmolality
by all diuretics. and urine sodium concentration.