# What Do We Know about the Use of EEG Monitoring during Equine Anesthesia: A Review
## 4 Analysis of EEG and Its Usefulness in Equine General Anesthesia  
In equine, EEG was investigated in order to study sleep disorders and convulsions diagnostics [27; 34], but it did not find a wider clinical use due to the size of the animals and the rapid (and dangerous) course of such disorders which generated too much risk to personnel. This fortunately changes, as the new ways of safe measurements like telemetric EEG headsets have been described [35], which may open the possibilities of EEG use as a welfare test or impact of drugs [51]. Most veterinary clinics do not have the ability to perform head imaging using MRI and CT. However, it is worth the effort, because in the field of the detection of cortical disorders in horses (such as meningoencephalitis, neuronal necrosis, cerebral edema, abscess), EEG has 70% specificity and 100% sensitivity [52]. Still, in practical conditions, the clinical examination and the client interview are most common bases for diagnosis due to the limited usefulness of the EEG in the clinical practice. Apart from the use for neurological examination, EEG has also been used in anestesiology as a part of the anesthetic monitoring during surgery under general anesthesia. Assessment of the arousal, therefore, the depth of anesthesia through the CNS monitoring is a direct method that can significantly improve the quality of anesthesia in horses, which, due to high risk and huge difficulties in maintaining a proper level of anesthesia (especially during inhalation and balanced anesthesia), is still a challenge. Moreover, the EEG recording can be analyzed to measure the reactions on the noxious stimuli, which can help to assess a proper level of analgesia--a specific arousal pattern (increase of the frequency and decrease in amplitude of the EEG wave) has already been noticed in a man [16], but there is conflicting information about its occurrence in horses [25; 37; 38]. Studies have already been carried out using the EEG in small and large animals [25; 39; 44; 53], but in horses there is a need for further research introducing this method into everyday clinical practice. The EEG recordings have been used as a method of choice for decades to diagnose epilepsy and sleep disorders such as narcolepsy and to determine the REM phase deficit [54; 55; 56]. However, the issue of general anesthesia in horses requires the search for new methods to assess the depth of anesthesia to reduce high intraoperative mortality [57]. This reason has directed the EEG research to new paths of clinical practice. The breakthrough occurred when it was introduced during balanced anesthesia. The simultaneous use of intravenous and inhalant anesthetics forced a change in the concept of assessing the depth of anesthesia. Advanced monitoring technics were needed because the use of multiple medications influences the reflexes and physiologic functions. The application of EEG in the study of horses was conducted in specific order, so that the next research could be better, based on previous discoveries. First, the methods of electrodes' mounting and technical aspects were investigated, which were based on the previous studies on EEG as a diagnostic method for neurological disorders [56; 58] (Figure 1). In horses, the electrodes should be placed in the frontal part of the skull, above the line of the eyes and below the ears. There is no need of shaving (but it enables better contact) when placing the superficial electrodes when there is a proper medium which will conduct the signal (gel, glue). The invasive electrodes are not clinically useful because their invasiveness might not be ethically accepted just for anesthesia depth assessment. In the next step, the EEG of healthy horses anesthetized with specific type of agents were measured in research concerning inhalant anesthesia [37; 44; 46; 59]. They found there were significant variabilities between individuals and specific time frames of measurements.