Neurophysiology of the visual system: basics and intraoperative neurophysiology techniques

4.1

Introduction

Neurosurgical procedures for treatment of tumors or vascular lesions along the visual pathways carry a risk of damage to the visual pathways. These procedures include parasellar tumors, pituitary adenomas, craniopharyngiomas, and tuberculum sellae meningiomas. Risk of visual impairment also exists in surgeries involving temporal and occipital lobe tumors as well as intraorbital lesions. Additionally, surgical intervention for the treatment of aneurysms, such as internal carotid artery (ICA) aneurysms, can pose a risk. Surgically related visual morbidity is higher in patients who already have an impaired visual pathway. A reliable method for intraoperative visual evoked potential (VEP) monitoring can assist in decision making regarding: (1) the aggressiveness of tumor resection and (2) modifying approach strategies in real time when near the optic nerve and/or visual pathways, reducing the likelihood of permanent iatrogenic-induced neurologic injury.

VEPs are utilized for evaluation of visual pathway integrity from the retina to the visual cortex. Two types of stimulations are used for eliciting VEPs: (1) flash and (2) checker board pattern reversal stimulation. However, it is impossible to perform pattern reversal stimulation under general anesthesia, because patients are unconscious and cannot fix their gaze at an object. Thus flash stimulation is the only option at present. Limiting the use of intraoperative flash eliciting VEPs has been their intra- and interindividual varieties. Recent technological advancements in light-emitting diodes (LEDs) manufacturing and size reduction have helped bring VEP monitoring back into the operating room.

4.2

Historical review

In 1973 Wright et al. reported the first use of intraoperative flash VEP monitoring during an intraorbital tumor surgery under general anesthesia. The clinical use of flash VEP monitoring under general anesthesia, for preservation of visual function, was observed subsequently, though no clear utility is reported yet. One of reasons was the fact that VEPs obtained under general anesthesia were unstable, and showed poor reproducibility; furthermore, due to inter- and intra-individual variations caused by the influence of anesthetics as well as nonoptimal light delivery sources. All of these factors were thought to be the reasons why there was no consistent correlation between intraoperative VEP changes and postoperative visual outcome . Recent breakthroughs in both patient management during surgery and microelectronics have helped establish VEP monitoring as a way to functionally interrogate the visual system during surgery. First, as with most of the neuromonitoring, the introduction of total intravenous anesthetics has reduced the intraindividual variability of VEPs obtained in the operating room. Second, generating a bright flash of light to the patient during surgery has been difficult due to the size of the hardware needed to generate a high enough intensity flash. Recent technologic advances in light delivery system size reduction, switching speed, and intensity increases have improved the stability and quality of light delivery under general anesthesia . The development of brighter LEDs allowed the retina to be stimulated in supramaximal fashion. In 2010 Sasaki et al. as well as the authors reported a high reproducibility and utility of intraoperative flash eliciting VEP monitoring. The authors reported that reproducible flash-elicited VEPs were possible in 93.5% (187/200) and 97.2% (103/106) of eyes tested, respectively, and that flash-elicited VEPs, using the new technology developed for the operating room, were also useful in the clinical laboratory setting, because VEPs could be recorded with a remarkably higher reproducibility compared to previous methods . They also reported the correlation between intraoperative VEP changes and postoperative visual function. The third intraoperative breakthrough was the simultaneous recording of the electroretinogram (ERG) and VEP to control and monitor stimulus quality. Including the ERG in the recording methodology can help detect possible insufficient photostimulus delivery to the retina, dislocation of the stimulating diode/goggles, and/or insufficient intensity of light, which are all major causes of insufficient photostimulus delivery. Information of correct light delivery to retina based on information from simultaneously recorded ERG improved predictive power of postoperative visual deterioration from 60% to 100% . This allows the flash VEP to become a standard intraoperative monitoring method at some institutions .

4.3

Neurophysiology of the visual pathway

The photostimulus delivered to the retinal surface stimulates primary neurons (photoreceptor cells) in the outer nuclear layer of retina (photoreceptor layer). This stimulation is transmitted to secondary neurons (bipolar and horizontal cells) and finally reach tertiary neurons (ganglion and amacrine cells), which transmit the information to the optic nerve, consisting of the axons of the ganglion cells, to the central nervous system. Light information input from the nasal retina propagates to the contralateral lateral geniculate body by intersecting the optic chiasm ( Fig. 4.1 ). Light information input to the temporal retina propagates to the ipsilateral lateral geniculate body. Most neurons in the lateral geniculate body receive information from one eye only. The optic radiation conveys the information from the lateral geniculate body to the primary visual cortex (Brodmann area 17) of the occipital lobe. A large part of the primary visual cortex of the human brain is embedded in the medial surface of the occipital lobe. The synthesis of information from both the eyes primarily occurs in the cerebral cortex. Information is conducted from Brodmann area 17 to Brodmann areas 18 and 19 . Brodmann areas 18 and 19 consist of (1) direct input from the lateral geniculate body bypassing the optic radiation and (2) input from the superior colliculus and pulvinar of the thalamus bypassing the lateral geniculate body .

For the rest of this chapter, we will divide the visual system into the anterior and posterior visual pathways. The anterior visual pathway arises from the retina and includes all structures up to the lateral geniculate body. The posterior visual pathway includes all structures from the lateral geniculate body to, and including, the primary visual cortex.

4.4

Recording of intraoperative flash visual evoked potentials