In this study, changes in ICP during screw activation were prospectively evaluated with US-guided ONSD measurements in adolescent patients who had undergone MARPE. In MARPE applications, as the forces are distributed to the palate by the microimplants, the tension on the teeth and supporting structures is decreased, which reduces the side effects of conventional expansion, such as gingival recession and dehiscence of the buccal bone in patients whose growth has ceased, and eliminates the need for osteotomies [24–26]. In a study comparing MARPE and conventional expansion in late adolescents, Lin et al. found that MARPE had more orthopedic effects and fewer dentoalveolar side effects [27]. Although MARPE has shown evidence of clinical success [27–29], it is difficult to predict exactly what occurs physiologically [25]. To date, no study has evaluated the effect of skeletal maxillary expansion on ICP. In their study on pig heads, Sun et al. proved that cumulative expansion forces applied to the maxilla also induced a tension beyond the physiological limit in peripheral maxillary sutures [30]. Although the current literature suggests that palatal expansion devices may have extensive craniofacial outcomes, this information is often overlooked in clinical practice. However, the extent to which expansion forces affect brain hemodynamics has not yet been extensively investigated [31].
Some studies have emphasized the importance of age in palatal expansion procedures [32]. Although a study of cadaver skulls showed that the ossification of the midpalatal suture widely varied across various age groups [33], the consensus in the literature is that conventional expansion is no longer possible after adolescence, since the facial skeleton’s resistance to enlargement significantly increases with age [34, 35]. Resistance to skeletal separation increases after the pubertal growth spurt; therefore, significant suture separation cannot be expected with tooth-supported expanders [36]. Previous studies have revealed a significant relation between the CVM index and suture maturation [37]. The mean age of the patients participating in our study was 16.77 ± 1.90 years, and their skeletal maturity was CS4 and above, according to the CVM index. Patients at a skeletal maturation stage of CS3 or below were excluded from our study because it is usually possible to treat them with conventional expansion [35]. However, the radiological visualization of midpalatal suture opening at a different chronological age is recommended, considering that there may be early or late fusion of sutures in some cases.
Both optic nerves are surrounded by the dural sheath, which is an extension of the meninges protruding into the orbit [38]. This causes similar cerebrospinal fluid pressure changes between the intracranial and the intraorbital subarachnoid spaces, suggesting that sheath dilatation reflects increased ICP [38, 39]. Various methods such as computed tomography (CT), magnetic resonance imaging (MRI), ICP monitoring, and lumbar puncture are used for the measurement of ICP [40]. However, these methods have limitations in terms of invasiveness, contraindications, radiation exposure, availability, and requirement for patient transport [38, 41]. In recent years, the measurement of ONSD using transorbital US has been described as a non-invasive alternative for assessing ICP in children and adults, with no complications observed during post-operative follow-up [42]. A recent systematic review and meta-analysis of studies comparing ONSD values measured on US with direct ICP monitoring measurements showed a strong correlation between the two [43]. In our study, ONSD was measured by US, a rapid and non-invasive method, to evaluate the effect of MARPE on ICP, and no complications were observed in our patients during the post-operative follow-up. However, a couple of studies report that optic disc swelling takes time in cases of high ICP; therefore, this technique should not be applied when an acute increase in ICP is suspected [40, 41]. Although studies have shown a good correlation between the measurements from the left and the right eyes [44], in our study, we used the mean value of ONSD measurements from both eyes to exclude individual anatomical variations and the possibility of any unilateral pathology. Nevertheless, it should be noted that ONSD is not perfectly circular, and its extensibility varies in individuals [39]. It is very important for the results’ reliability that the diagnostic methods used are reproducible and independent of the operator performing the measurements [44]. Therefore, in our study, the measurements were undertaken by two experts to minimize operator-related differences. Regarding the interobserver reliability, Lochner et al. found a strong correlation between the measurements of two operators [45]. Ballantyne et al. found that sonographic measurement of ONSD is a reproducible technique with low intra- and inter-observer variation [46].
Although a review of the literature demonstrates the extensive effects of the forces produced during the rapid expansion of the palate, little is known about the exact nature of these forces and how they are transmitted through the craniofacial complex [47–50]. During rapid expansion, the pterygoid processes are strongly bent laterally, resulting in yet-to-be-clarified effects on the sphenoid and foramina, through which important nerve and vascular structures pass, as well as on the entire cranial base [51]. While neural structures are not expected to be damaged due to relatively low stress in juvenile cranial structures during rapid maxillary expansion, this damage cannot be ignored due to much greater stresses caused by increased stiffness and decreased elasticity in skeletal structures with age [9, 52]. In a case report by Lanigan and Mintz, partial oculomotor nerve paralysis occurred in an adult after surgically assisted rapid maxillary expansion without separating the pterygomaxillary junction [53]. A study conducted in monkeys showed that changes occurred in the surrounding structures as a result of the rapid expansion of the midpalatal suture [49]. It is unclear whether this high stress distribution at the cranial base caused by rapid expansion will affect ICP. Some clinically serious complications in the cranial base after rapid expansion were also described in two studies [54, 55].
In our study, the ONSD values measured 1 minute after screw activation (T1 and T4) in the beginning and at the end of the expansion treatment were relatively higher than those obtained before screw rotation (T0 and T3). The relative increase between T0 and T1 and between T3 and T4 was found to be statistically non-significant (p > 0.05). Although the pathophysiology of the increase in ICP is not clearly known, we consider the possibility that it is the result of varying degrees of stress, minimal bone displacement, and cerebral hemodynamic changes in the intracranial region. However, some studies suggest that major changes in ICP are required to cause significant changes in ONSD [56]. In our study, there was a relative decrease in the ONSD values measured 10 minutes after screw activation (T2 and T5) compared with those measured 1 minute after activation (T1 and T4), but the differences were found to be statistically non-significant. This is probably due to the autoregulatory capacity and mechanism of the brain [57]. A previous study showed that the Boyle-Davis mouth plug increased the ONSD value measured on US during adenotonsillectomy, and the ONSD value decreased after the mouth plug was removed, possibly due to autoregulation [42]. In another study using CT, cerebral blood flow was shown to increase at the early stages of rapid palatal expansion therapy and then return to normal [50].
Changes in blood pressure, pulse, and respiration are clinically important as they cause high ICP [58, 59]. Additionally, low blood oxygen saturation (hypoxia) and high blood carbon dioxide levels (hypercarbia) increase blood volume and result in the undesirable elevation of ICP due to the dilation of vessels connected to the brain [60, 61]. In our study, changes in heart rate, SpO2, and MAP were also evaluated to exclude the effects of all these ICP-related components. A significant decrease was found between T0 and T5 in the pairwise comparison of heart rate measurements (p < 0.05). There was a significant decrease in the MAP values measured at T4 (p < 0.05) compared with T1, as well as in those measured at T5 compared with T1 (p < 0.05). Thus, these hemodynamic parameters, which have possible effects on ICP[59], could not explain the main reason for the relative increase in the ONSD values between T0 and T1 and between T3 and T4 or the increased ICP. We attribute these hemodynamic findings, which do not support our study’s hypothesis, to the manual measurement of the data by different individuals and consider this a limitation of our study.
Studies have shown that pain elevates ICP and increases blood pressure, resulting in respiratory irregularity [62, 63]. In our study, the patients’ perceptions of pain during screw activations were evaluated with the VRS-4 at T1 and T4. Pain scores during screw activations (T0–T1 and T4–T5) were non-significant at both the first and the last sessions of active rapid maxillary expansion treatment (p > 0.05). Therefore, we conclude that the cause of the relatively elevated ICP was not pain.
As a result of all these findings, the statistically non-significant relative increase between T0 and T1 and between T3 and T4 was also not significant in clinical terms. Therefore, we can state that MARPE has no significant effect on ICP. Long-term ICP changes after screw activation in MARPE therapy cannot be predicted based on our data. When tension-related bone changes are considered, the duration of therapy is an important variable for mechanical stress [64]; therefore, longer-term studies are needed. There is also a need for more comprehensive studies with a larger sample size to ensure that physicians apply MARPE without any concerns about ICP, especially in patients with intracranial pathologies, which has not yet been evaluated.