Introduction

Chronic hyponatremia occurs in patients after surgery, or in association with other conditions, including psychiatric illnesses. The condition carries a substantial mortality, and is described by the authors of this study as being a common problem in postmenopausal women. In spite of its name, chronic hyponatremia is usually diagnosed within a week of onset. Recognition and prompt treatment can result in recovery for the majority of patients, but otherwise the outlook is poor. Fluid restriction has been a recommended treatment in the past, but the present study shows that the administration of intravenous (IV) sodium chloride produces a better outcome.

Methods

The study group consisted of 53 postmenopausal women who were consecutively analyzed by the authors, having been diagnosed as having hyponatremic encephalopathy, based on a plasma sodium level below 130 mmol/L together with central nervous system signs. The mean age of the patients was 62 years (range: 45 to 89 years). Hyponatremia was documented as existing for at least 48 hours, and the rate of decrease in plasma sodium was less than 0.5 mmol/L per hour over at least 48 hours. Cerebral edema was assessed using standard neuroradiological criteria.

The collective were divided into three groups based on the therapeutic regimen they received. Group 1 consisted of 17 patients given IV sodium chloride before the onset of respiratory insufficiency. Group 2 comprised 22 patients given sodium chloride after the onset of respiratory insufficiency, together with tracheal intubation and assisted ventilation. Group 3 was 14 patients who were treated with fluid restriction only. The authors of the article were directly involved in treatment of the first group, and they were consulted about patients in group 2 only after respiratory insufficiency had intervened. Group 3 patients were referred after they had developed respiratory insufficiency where the managing physicians had decided to use fluid restriction rather than IV sodium chloride - the reason for this was the fear of iatrogenic cerebral demyelinating lesions.

In Group 1, the IV sodium chloride used was hypertonic (514 mmol/L) in 12 patients and "normal" (154 mmol/L) in 2 patients. No such information is given for the remaining 3 patients in Group 1 and the 22 patients in Group 2, but the mean rates of correction for these two groups were 0.7 and 0.8 mmol/l per hour, respectively.

After four months (or longer) patients' outcomes were scored, using a cerebral performance category (CPC¹), as follows: 1 = normal function or only slight disability; 2 = conscious and alert with moderate disability; 3 = severe disability unable to care for self, usually institutionalized; 4 = persistent vegetative state; 5 = death.

Results

The mean age of the 53 postmenopausal women was 62 years (45 - 89), the mean duration of hyponatremia was 5.2 days, and the mean level of plasma sodium was 111 mmol/L.

The clinical presentation was interesting. Orthopedic injury led to the diagnosis in 10 patients - all these injuries followed seizures that were a consequence of the hyponatremia. The most common presenting clinical signs were headache, nausea, emesis, weakness and seizures. Ten patients had associated psychiatric disorders, 17 had hypertension treated with thiazide diuretics, and 26 had post-surgical conditions.

The effects of treatment in the three groups of patients are summarized in the following table:

	Group 1	Group 2	Group 3
Treatment	IV NaCl (early)	IV NaCl (late)	Restricted Fluids
Number of patients	17	22	14
Initial plasma sodium (mmol/L)	111	103	109
Final plasma sodium (mmol/L)	133	132	112
Outcome (mean CPC score)	1.0	3.0	4.6
Outcome - summarized results	Normal, or only slightly impaired	Severe disability, institututionalized	Dead or vegetative state

No correlation was found between the outcome and either the initial plasma sodium level or the rate of its correction. Eleven patients had neuroradiological evaluation before starting sodium chloride treatment; in 6 of these, there was evidence of cerebral edema. Postmortem examination on 5 patients who died (3 from Group 3) showed generalized cerebral edema with evidence of tentorial herniation; there was no evidence of central pontine myelinolysis in any of these five patients. Magnetic resonance imaging 4 months after recovery in 18 patients from groups 1 and 2 revealed no evidence of central pontine myelinolysis.

Comment

Chronic symptomatic hyponatremia is clearly not a benign condition - left untreated, seizure lead to orthopedic injuries, neurological impairment and death. The authors find no evidence to show that administration of IV sodium chloride causes demyelinization - on the contrary, this treatment clearly reduced morbidity and mortality in their patients, especially if started before the onset of respiratory insufficiency. In a larger collective they studied (158 patients), less than 4% of the brain damage found was associated with improper treatment.² The speed of correction may be relevant; the authors suggest that if there is respiratory distress or seizures plasma sodium should be raised by 8 mmol/L in the first hour, and this can only be accomplished using hypertonic (514 mmol/L) sodium chloride - otherwise normal saline may be used. Treatment should be continued until the plasma sodium level is above 130 mmol/L.

An accompanying editorial³ reviews the available information on hyponatremia, and reminds physicians of the need to watch for encephalopathic symptoms. These may be difficult to distinguish in an elderly patient - e.g. a mild alteration in mental status or behavior, illusions, confusion, incontinence, tremor, ataxia or falls. As a practical measure, all elderly patients with acute orthopedic injuries should be screened for hyponatremia. Seizures are a signal for urgent treatment. The development of hypoxia is critical in precipitating cerebral damage - up to this point the effects are potentially reversible.