# What Do We Know about the Use of EEG Monitoring during Equine Anesthesia: A Review
## 5 Limitations of EEG Use and Further Research in Equine Anesthesia  
Considering the fact that the cerebral cortex lies on the surface and is much pleated, which affects the orientation of neurons, the reading of signals can be significantly different. The most important problem of EEG and MEG (magnetoencephalography) is to determine the location of neurons responsible for the propagation of the examined electrical impulses or the formation of a magnetic field in a given region. The range of potential sources of change in electric or magnetic potential that is difficult to determine is a real challenge. This problem started the interest into the combination of anatomical, physiological, and biophysical diagnostics [16].  
Other technical limitations are the dependence of the EEG record on the position of the electrodes on the front part of the skull (in horses the fragment is quite wide, there are few possibilities where to place the electrodes), the effect of muscle activity around the electrodes, and the presence of reflexes (including palpebral reflex, which activates the surface head muscles) (Figure 1). Errors in evaluation can also be caused by a thick hair coat in the animal. It is also worth mentioning that because of the fact that recordings are altered by muscle activity, it is difficult to assess the EEG on a conscious (or loosing/regaining consciousness) horse. The only possibility to reach a clear recording is to use invasive deeply implanted electrodes, which application may be limited because of their invasiveness [35].  
The EEG is not a completely objective method because the activity of the brain wave is also influenced by the choice of drugs administered during general anesthesia [43]. An understanding of the mode of action of each drug used for the procedure is very helpful. However, the relationship between the effects of these drugs in a predictable way has not yet been determined. A frequently noticeable change is the effect of all drugs acting via GABA receptor on the slowing down of the EEG rhythm [35]. Anesthetic drugs also affect the spinal cord independently of the cerebral cortex. This means that the analysis of only the activity of the cerebral cortex does not always allow one to predict the motor and vascular response [18]. This was seen with the development of potent sedatives such as xylazine and detomidine. They had both potent neurologic and cardiovascular effects.  
It became obvious in the 1990s that the neurologic and cardiovascular evaluation of the combinations of sedatives, analgesics, and anesthetics was needed to reduce risk to the horse. A question frequently asked: is there adequate cerebral blood flow during profound sedation? The use of compressed spectral analysis of the EEGs combined with radioisotope cerebral blood flow evaluation provided the needed answers. The brain wave activity was depressed before reductions in blood flow and the desired recovery to aware state confirmed the brain had sufficient perfusion to provide needed oxygen and glucose. The duration of responses was influenced by dosages and if combined with anesthetics [66].  
Now we have advanced technologies to conduct even more detailed evaluation of neurologic responses to anesthetics and analgesics. Finally, we have the monitors needed to determine the cerebral activity in the horse during anesthesia and surgery (Figure 2). It is expected the results of current research will make anesthetic management during surgery, full recovery, and return to normal activity of the horse more promising than ever.