Cavernous Malformations of
the Brainstem and Deep Nucleii

The brainstem and deep nucleei 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 disability

The Brainstem

The brainstem serves multiple vital functions for the individual for example it serves as the control centre for conciousness and the rhythm of breathing and the heartbeat are controlled from here. Almost all the information transmitted to and from the rest of the body pass via the brain stem 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 fibress 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). 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

Finally before the brainstem turns into the spinal cord is found the medullar oblongata usually referred to as simply the medulla. Within the medulla are groups of cell nucleii controlling involuntary activities such as breathing, the heartbeat and the muscle tone in the walls of blood vessels.

It is not possible to describe in great detail here all the detailed anatomy of the brainstem but it hopefully clear that there is a great deal of important brain tissue gathered together in close proximity here. Furthermore because there are multiple connections with every part of the nervous system an injury in this region may have far-reaching consequences and is the reason that bleeding from cavernomas in the brainstem are more likely to produce profound and lasting symptoms and disabilities than their lobar cousins.

Approximately 20% of cavernous malformations are located in the brain stem. It is approximately four times more likely that a brainstem cavernoma will present with symptomatic bleeding compared to lobar cavernomas and the estimated risk of a first ever haemorrhage is approximately 8% per five years. It is difficut to avoid producing symtoms from even small amounts of bleeding in such a functionally rich enviroment. For the same reason microsurgery to remove a brainstem cavernoma is more likely to result in worsed symtoms even after complete removal of the malformation. A useful rule of thumb is to imagine even a successful operation to remve the cavernoma as having similar consequences to the next symtpmatic haemorrhage it could cause. If decided upon treating a brainstem cavernoma then one hopes that risk from subsequent haemorrhages will be removed.

Surgery is most feasible when the cavernoma presents itself to the surface of the brainstem. If it does not there are limited options to gain access to the interior of the brainstem surgically but this requires the surgeon to traverse functioning nervous tissue as gently as possible. It is almost impossible to produce no disturbance of function but with very careful planning and modern microsurgical techniques some brainstem cavernous malformations may be removed and the patient recover to their pre-operative level of function over some time. The risks of such a procedure are dictated by the location and the planned route to the target. Your surgeon will wish to discuss the possible implications for you as an individual before proceeding. Happily these are almost always planned procedures and there should be plenty of time to address any questions you might have.

The Deep Nucleii

The basal ganglia are a group of subcortical nucelii 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 nucleei are equally as susceptible to injury by small amounts of bleeding from a therapeutic perspective pose similar challenges.

There are deep nucelii in the cerebellum as well and lesions here may affect not only co-ordination of movement but also mood, emotion and speech

Radiosurgery for Brainstem Cavernoma

Given the difficulties in reaching cavernomas in these locations radiosurgery has been investigated as an alternative treatment to surgical removal. Early attempts at treatment were dogged by complications possible becuase 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 controversy still exists as to it efficacy.

The radiosurgery does not, in general, cause the cavernoma to disappear. It may shrink slightly or not alter in appearence. Some series suggest that the rate of repeated haemorrhage is reduced after radiosurgical treatment although the pre-treatment calculated rates of haemorrhage are often higher thanrates obbserved in population studies. As the number of "events" (rebleeding) in absolute terms is relatively small it is difficult to be very confident at this stage that radiosurgery significantly imporves on the untreated course of the disease. It is an area in need of further research.