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Hyponatremia
Approaches to evaluating patients
and instituting treatment
BY Malvinder S. Parmar, MD
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Ms. H.L., a 64-year-old retired schoolteacher,
presents with six-month history of gradual onset dementia. She has
no other complaints aside from the memory loss and confusion. The
patient has hypothyroidism and hypertension, which are controlled
adequately with L-thyroxine 0.125 mg a day and amlodipine 5 mg daily.
She drinks about one ounce of alcohol every week and has been a one-pack-a-day
smoker for the past 35 years.
Presentation
Examination
Although Ms. H.L. isn't distressed, she is disoriented about
the time and place of her location. Her blood pressure is
140/80 mm Hg while supine and 135/85 mm Hg when seated, with
a pulse rate of 68. She is afebrile and there is no neck vein
distension. A cardiac assessment reveals a regular rhythm
with grade 1/6 systolic ejection murmur. A chest exam shows
a barrel-shaped chest with increased expiration, but no crackles
or wheezes. The abdomen is unremarkable. There's no peripheral
edema and her peripheral pulses are normal. A neurologic test
is non-focal.
Investigations
Initial laboratory data reveal:
- complete blood count (CBC): normal
- plasma glucose: 5.2 mmol/L
- blood urea nitrogen (BUN): 2.14 mmol/L
- serum creatinine: 45 µmol/L
- serum sodium: 124 mmol/L
- serum potassium: 4.2 mmol/L
- serum chloride: 89 mmol/L
- serum bicarbonate: 24 mmol/L
Further investigations show: normal
serum thyroid-stimulating hormone of 3.2 mIU/L, normal morning
serum cortisol, low serum uric acid of 136 µmol/L, normal
serum albumin of 38 g/L, low serum osmolality of 255 mOsm/kg
H2O. A urinalysis
reveals specific gravity of 1.030, but negative for blood
and protein, random urine sodium of 60 mmol/L and urine osmolality
of 200 mOsm/kg H2O.
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Making the case
Hyponatremia is defined as abnormally low concentration of sodium
ions in the blood (< 135 mmol/L), and occurs in up to 4% of hospitalized
patients. Hyponatremia often results from the body's inability to
appropriately excrete dilute urine, except in rare circumstances
where water intake exceeds the water-excreting capability of the
kidneys (i.e. as in patients with psychogenic polydipsia).
Hyponatremia can be associated with high, normal or low plasma
osmolality. With high or normal plasma osmolality, hyponatremia
occurs in patients with either hyperglycemia, or in those receiving
infusions of mannitol. Here, water moves from the cellular to extracellular
space because of the osmotic effect of glucose or mannitol, which
stimulates antidiuretic hormone (ADH) and thirst.
In most patients, however, hyponatremia is associated with low
plasma osmolality (hypo-osmolar hyponatremia), meaning excessive
amounts of water in the body in relation to sodium. This occurs
secondary to non-osmotic stimulation of ADH, which may arise on
a volume or non-volume basis.
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