The brainstem and deep nuclei contain tissue that sub-serves vital functions including control of the heartbeat, blood pressure and respiration amongst many others. Lesions in these areas may result in progressive and profound disability.
The Body's Control Centre
The Midbrain, Pons and Medulla.
The brainstem serves multiple vital functions including the regulation of conciousness, the rhythm of breathing and the heartbeat. Almost all the information transmitted to and from the rest of the body pass via the brainstem and it serves as an important relay station along the route. It consists of three parts- the midbrain, the pons and the medulla oblongata. Ten pairs of cranial nerves emerge from the brainstem controlling, amongst other functions, eye movements, facial sensation, hearing, balance, taste and swallowing.
The midbrain is directly connected to the cerebral hemispheres by two promontories its superior aspect- the cerebal peduncles. Through the peduncles flow the nerve fibres transmitting informtion to and from the higher centres.
The pons was so named by Italian anatomist Costanzo Varolio because when viewed in axial cross section (e.g. looking up from the feet as one would on a CT or axial MR scan) it's shape resembles a Venetian bridge (Latin: pons). Amongst many other things the pons is an important relay station to the cerebllum which controls muscle co-ordination and attaches to the back of the brainstem via three cerebellar peduncles.
The medulla oblongata is the lowest part that becomes continuous with the spinal cord. Within it are centres regulating automatic function such as breathing and the heart beat. It is the site ofoigino nerves which control swallowing and the operation of th breathing muscles as well as transmitting the ascending and descending tracts connecting the rest of the central nervous system to the periphery.
The Deep Nuceli
Deep nuclei are collections of nerve cell bodies adjacent to the brainstem.
The basal ganglia are a group of subcortical nuceli that contribute to a wide variety of functions including voluntary movements, emotional reactions and many others. Not all of their functions are completely understood. They are intimately related to the limbic system (white matter structures important in laying down memory and modulating emotion), the corticospinal tracts(initiating volunary movements) and the thalamii and spinothalamic tracts (relaying sensation to the higher centres).
Although anatomically not part of the brainstem, these deep nucei are equally as susceptible to injury by small amounts of bleeding from a therapeutic perspective pose similar challenges. There are deep nuceli in the cerebellum as well and lesions here may affect not only co-ordination of movement but also mood, emotion and speech.
Treatment
Approaching lesions in these areas surgically is difficult given the density and importance of functional neurological tissue surrounding the cavernoma. A symptomatic lesion may produce progressively worsening, profound disability in a stepwise fashion resulting from repeated bleeding events. International consensus allows for deferring intervention for one and perhaps two such events before the risks of intervention are likely to balance the risk from the cavernoma left untreated.
Microsurgical Removal
The brainstem is accessible by a variety of craniotomies with the optimal determined by the anatomy related to the individual cavernoma. Neurophysiological mapping and monitoring of brainstem function is utilised to maximise safety and image guidance is necessary to determine the optimal trajectory of approach given the narrow surgical corridors imposed.
The risks of surgery to a deep brain cavernoma are informed by its precise location within the brainstem, whether it presents to a pial surface and whether it respects the midline. The presence of an associated developmental venous anomaly (DVA) can complicate its removal as every effort must be made to preserve the DVA so it may continue to provide venous drainage from the brainstem.
The image to the right shows the characteristic "berry"-like appearance of a cavernoma located , in this case, in the pons of the brainstem. The golden colour of the surrounding tissue is caused by haemoglobin iron deposited there as blood is resorbed into the body. The iron cannot be dissolved but is very toxic to nerve cells so is cocooned in a protein creating a compound called haemosiderin.
If the cavernoma does not present through the surface of the brainstem it may still be accessible to removal by a carefully planned approach that exploits knowledge of anatomical structures gathered around the cavernom to minimise any disturbance. The potential complications of any such approach would be discussed with you in detail by your prospective surgeon.
Stereotactic Radiosurgery
Given the technical challenges reaching cavernomas in these locations radiosurgery has been investigated as an alternative treatment to surgical removal. Early attempts at treatment were dogged by complications possibly because the radiation doses prescribed were more appropriate to the more metabolically active cancers that had previously been the radiosurgeons therpeutic target.
The topic has been revisited for cavernomas deemed "inoperable" using lower radiation prescriptions. This has significantly reduced the complications of treatment although it has not abolished them completely. Also work remains to be done establish that radiosurgery is reliably effective in reducing the long-term risk of bleeding. To date here have been no adequately powered, randomised comparative studies with surgical removal and for that reason neither approach should be presented as equivalent to the other for a given cavernoma.
