Optic nerve monitoring

Abstract

Orbital and anterior skull base surgery is generally performed close to the prechiasmatic visual pathway, and clear strategies for detecting and handling visual pathway damage are essential. To overcome the common problem of a missed clinical examination because of an uncooperative or unresponsive patient, flash visual evoked potentials and electroretinograms should be used. These electrophysiologic examination techniques can provide evidence of intact, pathologic, or absent conductivity of the visual pathway when clinical assessment is not feasible. Visual evoked potentials and electroretinograms are thus essential diagnostic procedures not only for primary diagnosis but also for intraoperative evaluation. A decision for or against treatment of a visual pathway injury has to be made as fast as possible due to the enormous importance of the time elapsed with such injuries; this can be achieved additionally using multislice spiral computed tomography. The first-line conservative treatment of choice for such injuries is megadose methylprednisolone therapy. Surgery is used to decompress the orbital compartment by exposure of the intracanalicular part of the optic nerve in the case of optic canal compression. Modern craniomaxillofacial surgery requires detailed consideration of the diagnosis and treatment of traumatic visual pathway damage with the ultimate goal of preserving visual acuity.

Keywords: flash visual evoked potentials (VEPs); megadose methylprednisolone; optic nerve decompression; optic nerve trauma; visual pathway damage.

Figure 1

Axial computed tomography view showing typical radiologic signs of a retrobulbar hematoma: globe-shaped bulb, axial proptosis, and retrobulbar fluid accumulation (marked with asterisk).

Figure 2

Clinical images of single and parallel electroretinogram and visual evoked potential performed using a mobile neurophysiologic measuring station with a dual-channel lead: position of the needle electrodes including grounding (A) and a threadlike electrode (marked with arrows; B), overview of the examination protocol (C).

Figure 3

Posttraumatic ERG (A₁ right eye, A₂ left eye) and VEP (B₁ right eye, B₂ left eye). Note the physiologic VEP with a clear amplitude (blue bar) for the right eye (B₁) and the pathologic VEP with a missing amplitude for the left eye (B₂); ERG on both sides without pathologic findings. Abbreviations: ERG, electroretinogram; VEP, visual evoked potential.

Figure 4

Posttraumatic secondary reconstruction of the left orbit: preoperative planning. The unaffected right side (red) is reflected onto the affected left side (blue), creating a virtual template for reconstruction of the left orbit. The orbital floor fracture is marked with a green arrow. Abbreviation: CT, computed tomography.

Figure 5

Posttraumatic secondary reconstruction of the left orbit: intraoperative navigation. Intraoperative multiplanar display shows pointer-based surface matching after titanium mesh insertion for left orbital reconstruction. Correct implant position is checked by comparing the pointer tip (green line) with the virtual reconstruction template (blue segmentation). Abbreviation: CT, computed tomography.

Figure 6

Decompression of the orbital compartment: surgical procedure. Four incisions are placed in natural creases directly above the supra- and infraorbital rim. After dissecting the orbicularis muscle and the orbital septum (palpebral ligament), the four incisions are elongated into the orbit behind the globe securely in contact with bone. Elastic drainage tubes are inserted to release extraconal and intraconal hematomas from the orbit. Illustration reproduced by kind permission of the AO Foundation, Davos, Switzerland.

Figure 7

Diagrammatic representation of VEP/ERG recording. Illustration reproduced by kind permission of the AO Foundation, Davos, Switzerland.