The Hong Kong Practitioner

Case Report	*The Hong Kong Practitioner* VOLUME 29 / July 2007

Two asymptomatic patients with hyponatraemia
Carroll K L Chan 陳家樂，Kin-sang Ho 何健生，Wai-man Chan 陳慧敏

Summary

Hyponatraemia is not uncommonly found in routine blood investigation in asymptomatic patients. In this article, we report two cases of patients with asymptomatic hyponatraemia in general practice. The general approach by family physician to patients with hyponatraemia is also discussed.

摘要

無症狀病人的血常規檢查發現低鈉血症並不罕見。本文報告2例基層醫療中無症狀低鈉血症的病例，並討論家庭醫生一般的處理方法。

HK Pract 2007;29:278-281

Carroll K L Chan, MBBS (HK), FHKCFP, FRACGP, FHKAM (Family Medicine)
Medical Officer,
Elderly Health Service, Department of Health.
Kin-sang Ho, MBBS (HK), FHKAM (Medicine), FHKAM (Family Medicine)
Consultant (Family Medicine),
Elderly Health Service, Department of Health.
Wai-man Chan, MBBS (HK), MPH (USA), FHKAM (Community Medicine)
Assistant Director of Health,
Family and Elderly Health Services, Department of Health.

Correspondence to:
Dr Carroll Ka-lok Chan,
Shaukeiwan Elderly Health Centre, 8 Chai Wan Road, Shaukeiwan, Hong Kong.

Introduction

Elderly health centres of Elderly Health Service provide comprehensive health assessment for our elderly members aged above 65. Routine blood investigation is one part of our health assessment. Hyponatraemia is not uncommonly found in routine blood investigation. In this article, we would like to use two elderly patients of our centre to illustrate the general approach to manage patients with asymptomatic hyponatraemia.

Case 1

M/76
Past Health

Thyroidectomy for benign thyroid nodule 20+ years ago
Old pulmonary TB 40+ years ago
Bilateral hernia repair done
BPH with TURP done in 2004
Left unilateral hearing loss with follow up by ENT specialist clinic
Not on any medication

He attended our elderly health centre for health assessment in 2005. He was asymptomatic. We found the following on routine blood investigation:

Na 132		mmol/L		(137-145)
K 5.0		mmol/L		(3.7-5.2)
Cr 94		mmol/L		(63-112)
Ur 4.7		mmol/L		(3.0-8.1)
FBS 5.8		mmol/L		(3.6-6.1)
TC 5.4		mmol/L		(<5.2)
LFT, Albumin, Globulin normal
TSH normal

So, we rechecked the sodium and potassium levels and found the following:

Na 131		mmol/L		(137-145)
K 4.3		mmol/L		(3.7-5.2)

Patient was not on diuretic or any medication. He was clinically not dehydrated and no ankle oedema was found. Further investigation revealed:

Serum osmolality 274 mOsmol/kg (275-295)

Urine sodium <20 mmol/L

The low serum osmolality indicated that the patient had relative water excess. Further detailed history found that he did not have any history of psychiatric disorder and did not have features of compulsive drinking. However, he had started drinking more than 2 L of water per day after his TURP operation because he was told by his Urologist to drink more water after the operation. Ever since then, he had been drinking this amount of water until this health assessment. As such, over-compliance to doctor's advice appeared to be the only reason for his drinking of excessive amount of water and hyponatraemia! We then advised him to drink an appropriate amount of water and the subsequent plasma sodium level returned to normal.

Diagnosis: Hyponatraemia due to overdrinking of water.

Case 2

F/75
Past Health

Right ear drum perforation with right conductive hearing loss followed up by ENT specialist
Cataract with bilateral extraction done
Diagnosed to have 'anxiety neurosis' by a private family physician

She attended our elderly health centre for health assessment and was also asymptomatic. We found the following on routine blood investigation:

Na 128		mmol/L		(137-145)
K 4.3		mmol/L		(3.7-5.2)
Cr 59		mmol/L		(59-98)
Ur 3.7		mmol/L		(3.0-8.1)
FBS 4.9		mmol/L		(3.6-6.1)
TC 4.7		mmol/L		(<5.2)
LFT, Albumin and Globulin normal
TSH normal

We rechecked the sodium and potassium levels and found the following:

Na 129		mmol/L		(137-145)
K 3.9		mmol/L		(3.7-5.2)

She was clinically not dehydrated and no ankle oedema was found. Further investigation revealed:

Serum osmolality 268 mOsmol/kg (275-295)

Urine sodium 46 mmol/L

In this case, urine sodium concentration was inappropriately high as compared with low serum osmolality. So the clinical diagnosis of syndrome of inappropriate antidiuretic hormone ( SIADH) was made.

Further detailed history found that she was prescribed with escitalopram by her family physician and there was no other medication. CXR was done to rule out possible chest pathology known to be associated with SIADH and the result was found to be unremarkable. Her mood was stable and the Geriatric Depression Score (GDS) done in our centre was 3. A letter was issued to inform her family physician of our finding. Escitalopram was subsequently withdrawn and her plasma sodium level resumed to normal later.

Diagnosis: SSRI-related SIADH

Discussion

During health assessment or body check, family physicians usually arrange routine blood investigations for their patients; and incidental findings of hyponatraemia are not uncommon. Common causes of hyponatraemia include laboratory error and due to diuretic effect. Therefore the initial management of hyponatraemia should include rechecking the serum sodium to confirm hyponatraemia as well as reviewing the drug history. Common diuretics include Lasix, Dyazide, Moduretic, Natrilix, Spironolactone. Less common "diuretics" include carbonic anhydrase inhibitor e.g. acetazolamide (Diamox).

After confirming hyponatraemia and excluding the use of diuretic, we should assess the hydration status of the patient to look out for any accountable causes.^1-5

Table 1: Causes of hyponatraemia

Clinical extracellular volume status		Pathogenesis		Aetiology (examples)
Hypovolaemia		Deficit of total body water, larger deficit of extracelluar Na		Renal: Diuretic, sodium-losing nephropathy, Addison's disease, mineralocorticoid deficiency Non-renal: Vomiting, diarrhoea, burn, fistula, excessive sweating
Normovolaemia		Normal or slight excess of total body water		SIADH, water overload, hypothyroidism, glucocorticoid deficiency, inappropriate intravenous therapy
Hypervolaemia		Excess of extracellular Na, larger excess of total body water		Renal: Nephrotic syndrome, acute and chronic renal failure Non-renal: Cardiac failure, cirrhosis, inappropriate intravenous therapy

If the patient is dehydrated i.e. sodium depleted, we should consider extra-renal causes like vomiting, diarrhoea, fistula, or low sodium intake. In these cases the urine sodium should be low (<20 mmol/L) indicating that there is no excessive renal loss of sodium as a cause.

If the patient's urine sodium is high (>20 mmol/L), we should consider excessive renal sodium loss such as use of diuretic, sodium-losing nephropathy, Addison's disease. If patient is oedematous, we should consider nephrotic syndrome, cirrhosis, and heart failure as cause of hyponatraemia. If patient is not dehydrated, we should also think of compulsive drinking or overdrinking of water when urine sodium is low as in Case 1 and we have to document this from the history. When urine sodium or urine osmolality is high and yet serum osmolality is inappropriately low, we should then think of the syndrome of inappropriate antidiuretic hormone (SIADH).

SIADH is characterized^1,3,4 by:

Dilutional hyponatraemia due to excessive water retention
Low plasma osmolality when with higher 'inappropriate' urine osmolality
High urinary sodium excretion
Absence of hypokalaemia (or hypotension)
Normal renal and adrenal and thyroid function

Table 2: Causes of SIADH

CNS		meningitis, abscess, tumor, subdural haematoma etc
Lung		TB, lung abscess, pneumonia, malignancy etc
Tumour		lymphoma, leukaemia, pancreas carcinoma, bladder carcinoma
Endocrine/metabolic		Hypothyroidism, porphyria
Drug		antidiabetic: chlorpropamide antiepileptic: carbamazepine(Tegretol) psychotropic: phenothiazine, TCA & SSRI cytotoxics; vincristine cyclophosphamide narcotic: morphine, pethidine

It should be noted that hyponatraemia related to SIADH has been associated with all types of antidepressants. However, it has been reported more frequently with SSRI than with other antidepressants and is also more common in elderly patients⁶as seen in our Case 2 patient.

Uncommonly, the combination of hyponatremia and normal plasma osmolality can be caused by pseudohyponatraemia.^1,4 Severe hypertriglyceridemia and hyperproteinemia are two causes of this condition in patients with pseudohyponatraemia. These patients are usually euvolemic.

Conclusion

The general approach to asymptomatic hyponatraemia include rechecking blood to confirm hyponatraemia, reviewing medication and past health as well as assessing hydration status to look out for common reversible causes. Besides, checking plasma and urine osmolality is also important to ascertain causes so that family physician can manage the underlying cause accordingly. Last but not the least, in general practice, family physicians should always beware of iatrogenic cause.

Key messages

In general practice, hyponatraemia is not uncommonly found in routine blood investigation in asymptomatic patients.
Based on the clinical extracellular volume status, causes of hyponatraemia can be divided into three groups, namely hypovolaemia, normovolaemia and hypervolaemia.
So, after confirming the hyponatraemia and reviewing the drug history, we should assess the hydration status to look out for any accountable causes.
Appropriate use of investigation like checking plasma and urine osmolality is also useful in helping family physician to ascertain the underlying cause and to manage accordingly.
Family physicians should always beware of iatrogenic cause.

References

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Oxford Textbook of Medicine, 4^th edition 2003. Oxford University Press.
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