The operating microscope was introduced to neurosurgerical practice in the 1960s after its earlier adoption in surgery to the ear. The development of light sources and mechanisms to allow these sometimes unwieldy instruments to be manoeuvred and images to be stable ushered in the era of microneurosurgery.
Micro-neurosurgery become more than just the reliance on an instrument to magnify tissues but rather a philosophy of working rapidly driven by complementary technologies such as bipolar diathermy, specifically developed instruments, improved anaesthetic techniques and an increasing appreciation of the microscopic anatomy in the nervous system. The natural planes of cleavage between tissues have been relentlessly studied to develop approaches to the most sensitive of areas with a minimum of his to the healthy tissue. Training in these techniques forms the bedrock of of neurosurgical training and the surgeons personal development continues through their career both through practice and the training of others.
Seeing Clearly
The ancient Assyrians may have used ground rock crystals as magnifying lenses and the physics of optics has evolved to complex reflecting systems used today that provide crystal clear images, providing depth of field without chromatic aberration. Assistants can see the same field as the operator and ever improving light sources minimise the potential for heat injury to tissue all the while affording superb illumination. Well-maintained optical systems from many years ago can still facilitate the highest quality microsurgery .
The Modern Operating Microscope
A modern operating microscope combines hight quality conventional optics with digital imaging and robotic positioning technology. The microscope has evolved to provide more than a clear image. Augmented reality allows the injection of angiographic images demonstrating blood flow in realtime in the brane providing early warning if that blood fo is compromised by surgical manoeuvre. Indocyanine green videoangiography requires only an intravenous injection to provide instant reassurance that blood flow has been preserved, vital in surgery to brain aneurysms and vascular malformations. These images can then be processed to provide an overview of tissue perfusion. In the near future laser speckle imaging will allow still more detailed evaluation of capillary blood flow.
An oral drug can be taken that renders cells sensitive to light and will be preferentially taken up by rapidly dividing cancer cells. When illuminated under a blue light delivered through the microscope this drug- called protoporphyrin IX (Trade name Gliolan®) in the tumour glows an intense red, while the normal brain tissue appears blue. This facilitates the detection of residual malignant brain tumour tissue with the aim of removing as much of the disease at operation as possible.
Endoscopes
Endoscopes are tubes through which surgical instruments may be introduced and when attached to a high-definition camera and light source permit surgical access to remote, small areas within the nervous system. Different viewing angles permit surgeons to look around corners in restricted areas. They are most often employed within the fluid chambers normally present within the cranial cavities to facilitate treatment of disorders affecting the cerebrospinal fluid pathways. They are used to treat pituitary tumours through the nose and increasingly to treat tumours within the cranial cavity in appropriately selected cases. Increasingly they are sued to treat some degenerative spinal disorders with reduced disturbance of the paravertebral musculature.
As with any tool however it is not suited to every job. Even when not the main means of accomplishing ones goal the endoscope can still provide complimentary information by allowing the surgeon to inspect inaccessible areas at the margins of a surgical field e.g. checking for any remaining tumour tissue. The QEVO micorvisualisation tool attaches quickly to Zeiss pentero operating microscopes without any additional light source or optics being required, providing a 45 degree view quickly and this can be an invaluable aid to neurovascular surgery in particular.
An oral drug can be taken that renders cells sensitive to light and will be preferentially taken up by rapidly dividing cancer cells. When illuminated under a blue light delivered through the microscope this drug- called protoporphyrin IX (Trade name Gliolan®) in the tumour glows an intense red, while the normal brain tissue appears blue. This facilitates the detection of residual malignant brain tumour tissue with the aim of removing as much of the disease at operation as possible.
Exoscopes
An exoscope is essentially a high definition camera and light source with a long focal length that may sit above an operative field, usually at the end of an articulating arm which permits many degrees of movements and provides the surgeons with highly detailed while maintaining constant focus at various depths of field. The image may be rendered in three dimensions with the aid of appropriate eye wear as is the case for endoscopic images as well. Otherwise these 2 dimensional monocular images lack the depth of field enjoyed under the surgical binocular microscope. Endoscopes are especially useful when deploying minimally invasive tubular retraction ports for subcortical surgery.
Finding The Way
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Modern Microsurgery benefits from technology that can relate surgical instruments in the operating field to imaging in real time. This is called neuronavigation or image guidance. In effect the neurosurgeon has the equivalent of satellite navigation technology in motor vehicles. A scan taken before surgery is loaded into the system and then a computer model is created by scanning the surface of the patient as they lie on the operating table which is then related to the preoperative imaging.
A camera or electromagnetic sensor overlooking the operating field detects reflective elements on surgical instruments allowing the surgeon to determine precisely which element of the preoperative scans simply by pointing to them. This allows precise trajectories to be planned allowing surgeons to reach the desired area with a minimum of disturbance to surrounding healthy tissue. It is possible to merge images from catheter angiography too and inject the resulting information into a head-up display on the operating microscope.
A limitation of image guidance is that tissues shift position as surgery progresses. Real-time images can be obtained using ultrasound which also provides useful information about blood flow within tissues in selected cases.
Checking The Results
The holy grail of image-guided surgery would be to have real-time continuously updated images as the procedure progresses. The closest modern neurosurgical technology comes to that is with intraoperative MRI and CT imaging. In the intraoperative MR system used at the Wellington Hospital a patient may be transferred directly into the mRI scanner without coming off the operating table. When the scan is complete the patient can be re-registered to the image guidance system in an instant. This technology helps minimise the chance of residual disease being missed during tumour or cavernoma resections.
