Ultrasonographic Assessment of Optic Nerve Sheath Diameter as a Tool for Correction of Hyponatraemia in the Intensive Care Unit: A Prospective Study

INTRODUCTION: The extracellular fluid in brain consists of a complex electrolyte equilibrium composed of sodium, potassium and calcium. Any degree of imbalance amongst these electrolytes results in pathological disorders. In critical care setup, hyponatraemia is one of the commonest electrolyte abnormalities encountered with the prevalence ranging from 3–10%(1). The European clinical practice guidelines define hyponatremia as a serum sodium (Na) level below 135 mmol/L(2). The amount of water in the body determines the serum sodium concentration and subsequently its serum osmolality(3). The related association is a feedback cycle involving anterior hypothalamic osmoreceptive neurons which perceives serum osmolality changes(4). Relative hyponatremia and difference of effective osmolality between brain and plasma induces water movement from the extracellular to the intracellular compartment; brain cells begin to swell and consequently intracranial pressure increases(3). Raised intracranial pressure may result in a plethora of symptoms ranging from mild nonspecific symptoms to severe life-threatening cerebral edema(5). Hypertonic saline is commonly used as the remedy. However judicious correction of sodium levels is essential to prevent the development of central pontine myelinolysis or osmotic demyelination syndrome(6). OBJECTIVE: Determine the relationship between the change of Optic Nerve Sheath Diameter (ONSD) and Correction of Hyponatraemia following treatment with hypertonic saline in terms of clinical improvement and alleviation of symptoms of raised intracranial pressure. MATERIAL AND METHODS: The study is being conducted at the parent hospital following a quasi-experimental research design with a sample size of 80. Patients with symptomatic hyponatremia (Na < 135 mmol/L) with history of symptoms less than 48 hours, age being more than 18 years are being included in the study. Hyponatremia is diagnosed with Point of Care (POC) testing of electrolytes by blood gas analysis machine and clinical features and later by laboratory findings. Serum Na level is assessed and hyponatraemia classified as mild (130- 134 mmol/L), moderate (120- 129 mmol/L) and severe (120 mmol/L). ONSD measurement (before and after hypertonic saline treatment) is recorded. ONSD would be reassessed after 24 hours, and any variability will be noted. DISCUSSION: With advancing technologies, non-invasive methods such as measuring Optic nerve sheath diameter (ONSD) is preferred for assessing bedside raised intracranial pressure (Fig 1). Because the optic nerve is surrounded by CSF, which in turn is connected to the brain's ventricular system, an increased ICP is thought to cause transmission of force through these spaces, leading to distention of the ONSD [7]. Bedside sonography as a tool for determining ONSD has gained popularity as it is simple, rapid, easily accessible and repeatable [8]. Also, compared to computed tomography (CT), it has a sensitivity of 95.6% and a specificity of 92.3%(9). Timely and appropriate correction of hyponatraemia can not only improve the symptoms but also prevent serious complications associated with increased intracranial pressure resulting from cerebral oedema. Limited research has been done in this field and many aspects are yet to be explored. RESULT AND CONCLUSION: This is an interim report of an ongoing study at our centre. The measurement parameters’ differences before and after hypertonic saline treatment is being evaluated. Until date, readings of more than half the sample size have been recorded and the preliminary findings have shown noticeable difference in ONSD before and after treatment with hypertonic saline. The final result with conclusion will be presented at the scientific conference.