Surgical navigation in ENT

Evidence of benefit of surgical navigation in ENT

In endoscopic endonasal surgery, navigation is intended to enhance efficacy and safety [ ]. The complications rate is already fairly low, at 0–4.3% depending on the report, but the associated morbidity is severe and may be life-threatening [ ]. Meta-analyses assessing the benefit of navigation systems are severely biaised by comparing procedures in heterogeneous pathologies, performed by surgeons of varying experience, and whose experience, moreover, varies over time [ , ]. Morbidity and mortality in endonasal surgery is poorly predictable, greatly depending on intraoperative conditions, and notably hemorrhage. The ideal design would be a large-scale prospective multicenter trial randomizing operated sides between navigation and no navigation, and classifying the surgeons, but this would not be ethically acceptable and has in fact never been attempted. Published studies mainly report, without precise quantification, shorter operative times [ ]; some also report reduced operator stress, but again without quantification.

In our opinion, the question cannot be answered if formulated in this way, as it fails to take account of the ultimate objective of surgery: complete lesion resection, sparing surrounding structures. However, the wider the resection, the greater the risk of lesion, and this is true for a simple sinonasal polyposis as for a pituitary adenoma or vestibular schwannoma. In our view, it is this general rule that navigation should seek to overturn, improving both safety and resection, in a non-zero-sum game. Unfortunately, there are almost no relevant studies [ ], perhaps partly because navigation has become so well-established that its interest no longer seems to need demonstrating.

General rules for using navigation systems

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  The basic purpose of navigation systems is not to replace the surgeon but to enhance his or her performance by adding an artificial sense to the visual and tactile informations. Like any machine, the navigator may provide faulty information, and the right way of using it is based on constant critical analysis of the information displayed.

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  System accuracy is liable to drift during the procedure and needs to be regularly checked against straightforward landmarks, especially at start of surgery, and further periodic checks should be made throughout ( Figure 3.1 ), as accuracy can vary within a given working volume according to a complex distribution function [ ]. When the displayed position differs markedly from that estimated by the operator, the discrepancy should be resolved by checking, for example, bordering landmarks. These checks enable not only safe operating, but also an estimation of the value and limitations of the equipment. They also encourage a certain humility and willingness to keep on learning anatomy, no matter how experienced you may be.

  

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  Long flexible instruments should not be used, as any excess pressure deforms their geometry and hence the calibration, with consequent loss of precision in proportion to instrument deformation. This is especially important in frontal sinus surgery, where bone dissection requires pressure on relatively long and fine instruments.

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  We do not feel that navigation should be reserved only for the most difficult cases. This would mean encountering inherently complex equipment for the first time just when the surgeon is most in need of it and single-mindedly concentrated on the surgical procedure. Despite what is regularly claimed, navigation systems do not, in our opinion, have specific indications: they should be deployed according to common sense. The only limitations are the set-up time and, for certain systems, the cost of consumables.

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  Navigation is also a valuable educational tool. Because it continuously correlates the location of the surgical instrument simultaneously in the operating field and on patient imaging, it is obviously an accelerator of learning curves, although few publications have focused on this. Large display screens, now common in operating theaters, disclose where the operator is working, and thus help all those involved in the surgical procedure (anesthetists, nurses, etc.) to enhance their participation.

Imaging protocols

The essential component of navigation is patient imaging, and certain principles need to be known to optimize image quality ( Table 3.1 ). CT scanners are recalibrated regularly and provide images with far more precise voxel coordinates than MRI. Multimodal MRI-CT is very useful in tumor resection, and for this the MRI scanner needs to be registered on the CT scanner and not vice-versa. Native axial slices should be used in DICOM format ¹

1. DICOM: Digital Imaging and Communication in Medicine, a standard defining the way medical images are encoded. Basically, a header contains the patient's identification and image parameters such as voxel size.

, rather than derived axial reconstructions, the accuracy of which is dependent on the manufacturer's particular algorithm.

Cone-beam CT generally shows excellent spatial resolution, but poorer density resolution than conventional CT (around 1,024 gray levels, compared to 4,096), although this is usually innocuous; however, the field of view is often restricted, which may hinder registration.

Navigation for sinus and anterior skull base surgery

Sphenoid sinus surgery

Diagnostic navigation now provides histologic data on sphenoid sinus lesions, where biopsy would not have been readily performed 20 years ago.

Navigation for therapeutic purposes also facilitates direct sphenoidotomy via the sphenoethmoidal recess, without posterior ethmoidectomy, thus enabling precisely targeted surgery. Navigation is also helpful in case of very large sphenoid sinus fungus balls, especially extending into the lateral sphenoidal recess, or to explore bone destruction to screen for active meningeal breach ( Figure 3.2 ).

Arterial pedicle hemostasis

Difficult hemorrhagic conditions, narrow nasal cavities, mucosal trauma and inflammation following nasal packing can make epitaxis haemostasis difficult to achieve for non-expert operators. Navigation provides very straightforward location of the sphenopalatine foramen, even under hemorrhage. Some navigation systems include a stereotactic location module to create an instrument guidance tunnel, greatly simplifying the procedure ( Figure 3.3 ). Likewise, the anterior ethmoidal artery can be easily identified ( Figure 3.4 ).

Location and closure of meningeal breaches

Certain osteo-meningeal breaches with chronic cerebrospinal fluid leakage can be identified on imaging, but exact location for closure is hard to determine. Navigation can specify their exact coordinates, enabling direct intraoperative approach ( Figure 3.5 ).