Intracranial hematoma, encephalitis, cerebral edema, hydrocephalus and brain tumor are all important causes of increased intracranial pressure (ICP), and timely and accurate determination of ICP changes, reasonable medication and timely surgery are important to improve the prognosis of patients. Therefore, ICP monitoring is an important prerequisite for the treatment of craniosynostosis. In recent years, with the development of modern imaging and biomedical engineering equipment, many new monitoring instruments and methods have emerged, especially the rapid development of noninvasive monitoring technology, and flash visual evoked potential (FVEP) technology is one of the research directions of noninvasive ICP monitoring, and its role in ICP monitoring is reviewed as follows. In 1981 and 1984, York et al. found that the N2 wave latency of FVEP was prolonged in patients with severe traumatic brain injury and hydrocephalus, and that there was a linear correlation between N2 wave latency and ICP. 2001, Desch suggested that regular monitoring of FVEP in patients with ventricular shunts could detect increased ICP by observing N2 wave latency before the appearance of clinical symptoms. Liasis et al. also demonstrated that the changes in FVEP were highly consistent with the changes in ICP in the temporal phase. In recent years, domestic scholars have conducted a large number of comparative studies between FVEP noninvasive ICP and invasive ICP, showing no difference between the two and proving the reliability of FVEP noninvasive ICP monitoring. 2.Basic principle FVEP is one of the earliest and most well studied clinical theories of cortical evoked potentials, which refers to the cortical (occipital lobe) potential changes induced by non-diffuse, non-patterned light stimulation, reflecting the integrity of the visual pathway from the retina to the occipital cortex. The visual conduction pathway is located at the base of the brain and has a long travel time. increased ICP can produce mechanical compression of the brainstem, resulting in deformation of the brainstem vessels by compression, impaired cerebral blood circulation, ischemia and hypoxia of neurons and nerve fibers, impaired metabolism of brain tissue, blockage of neuronal electrical signal conduction, prolonged FVEP wave latency, decreased wave amplitude, and increased wave width. When brain herniation is formed, the above changes are more obvious. In this way, the regression equation between FVEP and ICP can be established, so that ICP can be deduced by detecting FVEP. 3. Clinical application 3.1 Help to judge the change of condition at an early stage The condition of patients with craniocerebral injury is variable, even due to ICP is a more sensitive indicator and can detect changes in intracranial conditions earlier than general vital signs and state of consciousness, and patients who only show drowsiness may have elevated ICP. When the elevated ICP is detected by FVEP and CT is performed immediately, the time to definitive diagnosis is shortened and the patient can be treated early. In addition, the machine can calculate cerebral perfusion pressure (CPP) by entering blood pressure parameters while monitoring non-invasive ICP. Ensuring proper perfusion of brain tissue is critical in the treatment of craniocerebral injury. In normal physiological state, CPP is 80-100 mmHg. When ICP is normal and mean arterial pressure is 60-140 mmHg, cerebral blood flow can be kept constant by cerebrovascular mechanism through its own regulation. However, the level at which CPP is maintained after craniocerebral trauma has been controversial. Although the Brain Tranma Foundation (BTF) issued a new guideline in 2007 recommending a target value of 50-70 mmHg for CPP, with the evidence of impaired cerebrovascular autoregulation due to trauma, a uniform CPP standard is not suitable for all patients, so some scholars Therefore, some scholars have proposed the idea of “CPP-oriented therapy”, which is based on protecting or maintaining the ability of cerebrovascular autoregulation and ensuring the stability of cerebral blood flow after craniocerebral trauma. Since it is very difficult to determine the timing of cerebrovascular autoregulation impairment, early implementation of this treatment strategy according to the change of CPP is very important for the prognosis. Through FVEP monitoring, ICP and CPP parameters can be obtained in real time, which is conducive to controlling ICP, cerebral blood perfusion and preventing secondary brain damage, and is important for guiding clinical care and prognosis. 3.2 Guidance on the use of dehydrating agents 20% mannitol is the most widely used cranial pressure-lowering drug in clinical practice, and clinicians mostly use it by virtue of clinical experience, but there is no consensus on its optimal use dose to obtain the best cranial pressure-lowering effect, which easily leads to the abuse of mannitol: it often results in the use of large amounts and long duration, even in patients with normal cranial pressure or low cranial pressure. The use of mannitol with non-invasive ICP monitoring allows the dosage to be determined according to the changes in ICP. It has been shown that the prolonged N2 wave latency of FVRP is positively correlated with elevated ICP. Mannitol shortens the latency of the N2 wave of FVEP, which means that FVEP can observe changes in ICP after mannitol use. dehydration is not advocated when ICP is below 180 mmH20, and medication is only considered when ICP is greater than 200 mmH20. it is promptly discontinued when ICP approaches normal. Therefore, FVEP can help to observe the efficacy of dehydrating agents and facilitate the adjustment of drugs and doses. According to the monitoring results of ICP, the time of applying dehydrating agent can be mastered to avoid the blindness of dehydration treatment, reduce the dosage of mannitol, and reduce the complications such as electrolyte disorder and renal failure. 3.3 Early warning of brain herniation The ability of FVEP to monitor ICP on both sides separately at the same time is a feature that is not available with invasive ICP monitoring. Because FVEP can measure ICP on both sides separately, it can reflect the intracranial fractional pressure gradient. When there is a brain contusion, intracranial hematoma and other occupying effects on one side, its potential conduction is slowed down, the N2 wave latency is prolonged, the ICP value is higher than the opposite side, and there is a pressure difference between the two sides. When the pressure difference reaches more than 180 mmH2O, due to the large pressure gradient, part of the brain tissue on the high pressure side is displaced to the low pressure side, and the risk of forming brain herniation increases greatly. Through repeated and intermittent monitoring, the values of ICP and pressure difference on both sides can be understood and a curve trend graph can be made to determine whether intracranial occupancy is developing, and the transient phase before the appearance of brain herniation, i.e., the prodromal phase of brain herniation, can be detected in time. When the ICP or pressure difference is monitored to gradually increase, CT should be reviewed and the cranium should be opened as soon as there is an indication for surgery. Active interventions at this stage can improve the cure rate and reduce the disability and morbidity and mortality rates. 3.4 Judgment of prognosis Increased ICP is one of the most common causes of deterioration, poor prognosis and even death in patients with acute craniocerebral injury. In the ICP monitoring of 58 patients with heavy TBI, Shi Dongliang et al. found that all 8 of them with an initial ICP greater than 70 mmHg died. ICP monitoring is important for indicating the prognosis of craniocerebral trauma. Yuan Qiang et al. divided 535 cases of heavy craniocerebral injury into ICP and non-ICP groups according to whether or not ICP monitoring was performed, and the results showed that the in-hospital morbidity and mortality rate of the ICP group was 16.7%, which was significantly lower than the in-hospital morbidity and mortality rate of 32.2% in the non-ICP group. Although ICP monitoring by itself does not improve the prognosis of patients, ICP monitoring is strongly recommended for patients with severe TBI in both national and international guidelines for the management of TBI. Moreover, it has been demonstrated that standardized treatment based on ICP changes must be implemented under the premise of ICP monitoring to improve the prognosis of patients. 4. Localization of N2 waves The determination of ICP by FVEP is based on the latency of N2 waves. However, there is no unified standard for the identification of N2 waves and the determination of N2 wave latency so far. Due to the large variability of FVEP waveforms and amplitudes, the identification of N2 waves is often difficult. The correct identification of N2 waves and the selection of N2 wave latency in noninvasive ICP monitoring are the most critical factors related to the accuracy of noninvasive ICP monitoring. It is known that there are four types of N2 wave latencies: onset latency, peak latency, midpoint latency, and extension latency. However, other scholars believe that the measurement of latency is related to the accuracy of ICP measurement, and thus the measurement point of latency should have the most stable value in consecutive measurements in the same patient, rather than the most convenient measurement. Among the four latency measurement methods, peak latency has the largest difference in latency variation in three consecutive measurements in the same patient, and midpoint latency has the smallest difference in variation, so it is obvious that using a reference value with a large variation as the ICP measurement would yield a highly variable ICP value. Therefore, they concluded that the peak latency should not be used for noninvasive ICP monitoring, but rather the midpoint latency with the smallest variation in the difference should be chosen as the standard latency for ICP monitoring. 5. Advantages and disadvantages 5.1 Advantages of FVEP (1) It can avoid the trauma caused by invasive methods and the resulting infection that is difficult to control, and it can prevent serious complications such as brain herniation and hypocranial pressure induced by surgery. (2) It can be operated at the bedside and has the advantages of being safe, simple, timely, effective, controllable, and widely applicable. (3) FVEP reflects the integrity of the visual pathway from the retina to the occipital cortex, is less affected by visual acuity, and can be completed regardless of the patient’s cooperation, making it suitable for monitoring of critically ill, especially comatose patients. 5.2 Disadvantages of FVEP (1) FVEP calculates ICP value mainly by the length of N2 wave latency, but cerebral edema, hematoma, local hypoxia and ischemia, lactic acid accumulation and other factors can cause prolonged N2 wave latency, so FVEP cannot distinguish the cause of intracranial hypertension; (2) The accuracy of the N2 wave latency selected by the operator of FVEP monitor directly affects the measurement results, and the current (2) the accuracy of N2 wave latency selection by the FVEP monitor operator directly affects the measurement results, and there is no unified scientific standard for the selection of N2 wave latency; (3) ocular diseases such as severe visual impairment and fundus hemorrhage affect flash visual evoked potentials; when intracranial occupying lesions compress and destroy visual pathways, the reflection of flash visual evoked potentials on intracranial pressure will be affected. (4) Age also has an effect on nerve conduction velocity, and the latency period is prolonged with increasing age in patients over 60 years of age; FVEP is also not suitable for monitoring pediatric patients with increased ICP. Currently, invasive monitoring techniques (including lumbar puncture, epidural manometry, and ventricular manometry) are still recognized as the “gold” indicators for ICP detection, but their common disadvantage is the risk of intracranial infection, hemorrhage, and even death, as well as the high technical requirements for operation. The non-invasive monitoring technique is less risky, relatively simple to operate, and can reflect ICP changes more objectively and accurately, which has attracted attention in recent years. More and more detailed research data are needed for wider application.