Outline your approach to the diagnosis and management of severe hyponatraemia.
Severe hyponatraemia implies either a very low level (eg. < 120 mmol/L) or one associated with significant symptoms (eg. neurologic). Approach should allow determination of aetiology by history, examination and simple investigations (and/or repetition of test). An approach involves measurement of plasma osmolality, urine osmolality and urine sodium concentration. Causes are multiple, and include:
Factitious: contaminated by hypotonic intravenous fluid
Isotonic: pseudohyponatraemia (eg. hyperlipidaemia, hyperproteinaemia)
Hypertonic: (eg. hyperglycaemia, mannitol) where hypertonicity induces movement of water out of cells, and lowers Na by dilution. No specific treatment is usually required.
• Water retention: (urinary Na is usually > 40 mmol/L) SIADH, inappropriate antidiuresis (eg. hypovolaemia, cardiac failure, pain, post-operative, renal failure), psychogenic polydipsia
• Salt depletion: (urinary Na is low, eg. < 20 mmol/L) adrenocortical failure, diuretic excess
Management includes diagnosis and, if appropriate, specific treatment of underlying cause. Most patients are asymptomatic, with plasma Na > 120.
Initial treatment obviously depends on the specific cause (eg. corticosteroids), but water restriction and isotonic saline is usually sufficient. More aggressive therapy (eg. hypertonic saline) is indicated if Na < 110, or if patient is symptomatic (eg. confusion, coma, seizures). Relationship of rate of correction of Na and risk of osmotic demyelination (central pontine myelinolysis) is controversial, but appears reduced if rate of correction of Na is less than 10-
12 mmol/L over the initial 24 hours (ie. < 0.5 mmol/hr). Desmopressin (dDAVP) may be required to slow the rate of water excretion. Consider even administration of sterile water to lower sodium if rising too quickly.
This is an easy question for the sodium enthusiast.
Somewhere, a chapter about the diagnosis of hyponatremia is waiting for me to finish it. However, that classification and diagnostic algorithm is based around urine osmolality rather than volume assessment, and thus is not the canonical view. Classically, hyponatremia is separated into classifications according to serum osmolality and volume status. In the answer, the college goes even further towards raw practicality and separates hypoosmolar hyponatremia into disorders which waste sodium , and disorders which retain water.
In any case, this question calls for a systematic approach.
The following bits of historical information are important:
The following standard battery of tests can be launched; particularly if history is unhelpful, or one cannot bring oneself to interview the patient or their family.
This is essentially the content of Box 93.1 from Anthony Delaney and Simon Finfer's chapter for Oh's Manual.
High urine sodium
Low urine sodium
Diagnosis on the basis of the above lab tests and historical findings:
Chung HM, Kluge R, Schrier RW, Anderson RJ. Clinical assessment of extracellular fluid volume in hyponatremia. Am J Med. 1987 Nov;83(5):905-8.
Milionis, Haralampos J., George L. Liamis, and Moses S. Elisaf. "The hyponatremic patient: a systematic approach to laboratory diagnosis."Canadian Medical Association Journal 166.8 (2002): 1056-1062.