Usefulness of optic nerve sheath diameter measurement in hypertensive patients during pregnancy and the puerperium

Abstract Pre-eclampsia/eclampsia during pregnancy and the puerperium are major risk factors for maternal and perinatal morbidity and mortality worldwide. Early diagnosis followed by appropriate treatment can prevent neurological disorders, considered one of the most serious sequelae of the disease. The detection of increased intracerebral pressure through the use of ocular ultrasonography could be considered an effective method for its diagnosis, since it has the advantage of being a noninvasive technique, easy to perform at the patient’s bedside and with a high sensitivity and specificity for the detection of intracranial hypertension.


Editorial
Hypertensive disorders are one of the most frequent medical complications encountered during pregnancy and the puerperium, being one of the most significant causes of prematurity and maternal-fetal morbidity and mortality. The worldwide incidence varies from 6 to 30% of all pregnancies [1].
Preeclampsia is the most common cause of hypertension during pregnancy [2], affecting 5 to 10 percent of pregnant women and accounting for about 12 percent of maternal mortality. In fact, it is the third most common cause of maternal death worldwide [3]. In high-resource countries, the incidence of eclampsia is low and has decreased or remained stable at 1.5 to 10 cases per 10,000 deliveries. However, in low-and middle-income countries, the incidence varies widely: from 19.6 per 10,000 delivery [4].
In the United States in a 2014 series that included nearly four million hospitalized deliveries, 11% had a hypertension-related diagnosis, including 4.7% with preeclampsia, 3.8% with gestational hypertension, and 1.7% % with chronic hypertension. Of the 176,925 pre-eclampsia/eclampsia deliveries, approximately 47 percent were mild pre-eclampsia, 37 percent were pre-eclampsia with severe features, 1.4 percent were eclampsia, and 15 percent were pre-eclampsia superimposed on chronic hypertension [5].
In Cuba, the maternal mortality rate in 2019 was 37.4 deaths per 100,000 live births, of which 2.7 per 100,000 inhabitants were associated with hypertensive disorders during pregnancy, childbirth and the puerperium [6].
Preeclampsia/eclampsia is currently considered a potentially serious disease associated with maternal complications such as pulmonary edema, placental abruption, cardiac and renal complications, hemolysis, increased liver enzymes, low platelet count syndrome, and neurological complications associated with hypertension. intracranial [1].
The rapid diagnosis of intracranial hypertension is urgently neede for therapeutic reasons in different clinical situations. In patients with increased intracranial pressure, the optic nerve sheath diameter (ONSD) increases due to its close association with the flow of cerebrospinal fluid. The identification of this situation implies a clinical and complementary assessment that allows us not only to diagnose it, but also to approximate the value of intracranial pressure [1].
Increased intracranial pressure (ICP) is a serious, lifethreatening condition that can occur in both patients with neurological and non-neurological pathologies and whose diagnostic gold standard is monitoring with an intracranial device. However, the latter is an invasive method, not exempt from risks and complications [2]. Therefore, noninvasive methods are proposed as an alternative for managing these patients, such as computed tomography, magnetic resonance imaging, transcranial Doppler ultrasound, spectroscopy of near infrared and visual evoked potentials [1]. Computed tomography has been widely used in the assessment of neurological complications, being a difficult method to perform in critically ill patients in intensive care units, it is expensive and has the risk of exposure to the radiation. Spectroscopy, Doppler ultrasound, and evoked potential studies require specialized equipment and experienced examiners. Ocular ultrasound for optic nerve diameter measurement is considered a noninvasive, safe, and easy-to-perform procedure to assess the anatomical characteristics of the optic nerve [2]. Compared to measurements using axial tomography, it shows similarity for the diagnosis of increased diameter of the optic nerve, with no differences in statistical analysis. Being an accessible, sensitive and specific technique for detecting increased optic nerve ICP noninvasively [7].
The optic nerve is an extension of the central nervous system, it is covered by meninges and cerebrospinal fluid, an anatomical concept that explains the papilledema that occurs with an increase in ICP and is the basis for changes in the measurement of the diameter of the optic nerve as a reflection of ICP fluctuations [2].
The optic nerve is identified as the hypoechoic structure with a regular course posterior to the eyeball. The measurement standard requires drawing a vertical line that begins at the junction of the optic nerve with the eyeball; this line is just a reference and should measure 3 mm. Although the measurement at different distances from the junction of the eyeball with the optic nerve has been studied, dilation of 3 mm of this junction has shown greater sensitivity for assessing fluctuations in nerve dilation. Once these 3 mm has been located, a horizontal line is drawn from edge to edge of the optic nerve; this second line is the one that measures the value in millimeters of the optic nerve [2].
Some studies have shown that the measurement of the diameter of the optic nerve with a standardized cutoff point of 0.5 cm can achieve a sensitivity and specificity for detecting intracranial hypertension of 80 to 95% and 80 to 100%, respectively [2].
Currently, ocular ultrasound has been used for the diagnosis of intracranial hypertension in patients with preeclampsia/eclampsia. Clement Dubost and et. in a study conducted with 26 pre-eclamptic women and 25 healthy pregnant women who had their optic nerve sheath measured; found that median ONSD values were significantly higher in patients with pre-eclampsia compared to healthy pregnant women at delivery (5.4 mm (95% CI: 5.2, 5.7) vs. at 4.5 mm (95% CI: 4.3, 4.8), p < .0001). At delivery, 5/26 (19%) of the pre-eclampsia patients had ONSD values greater than 5.8 mm while none of the healthy pregnant women had ONSD values that high [8].
However, Kumar Singh and et al. in their 2018 article on ultrasonographic optic nerve sheath diameter as a surrogate measure of elevated intracranial pressure in patients with severe pregnancy-induced hypertension, concluded that OSND is a surrogate marker of elevated ICP in patients with Severe pregnancyinduced hypertension [9].
Recently, another investigation by Brzan Simenc and et al. which examined the correlation between brain plasma biomarkers (S100B and neuron-specific enolase (NSE)) and ultrasonographic optic nerve sheath diameter in patients with pre-eclampsia and control groups. The results indicated that both S100B and NSE are elevated in severe pre-eclampsia. NSE correlated with increased ONSD suggestive of cerebral edema [10].
Salaheldin Omran and et al. in a prospective pilot study with thirty pregnant patients suffering from severe preeclampsia, decided to perform an ultrasound measurement of ONSD before starting magnesium sulfate and at 1, 6, and 24 h after administration. Its objective is to detect the effect of magnesium sulfate administration on ICP in patients with severe preeclampsia by measuring changes in the ONSD. The authors present as results that the periodic measurement of ONSD is a useful tool to detect the increase in intracranial pressure and guide the therapy with magnesium sulfate [11].
Other proposals on the usefulness of the ONSD expose its benefits for evaluating fluid status and guiding peripartum fluid therapy in patients with severe preeclampsia [12]. To this is added the prognostic value in the neurological evaluation that authors show in limited studies on the measurement of the ONSD in the assessment of intracranial hypertension [13][14][15].
Although there are several studies [8][9][10][11] that support the use of ocular ultrasound as a diagnostic method for intracranial hypertension in obstetric patients, other research still needs to be carried out since, the scientific evidence is not sufficient. Future explorations in relation to the subject would be of great importance for the prevention, diagnosis and treatment of neurological disorders during pregnancy and the puerperium.

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