A 29-year-old woman diagnosed with multiple cavernomas in 1996, the year in which she underwent surgery following a frontal haemorrhage of a right frontal cavernous angioma, with evacuation of the haematoma and resection of the vascular lesion. In the same year, a small left protuberancial angioma of 3 mm in diameter was detected. The patient remained asymptomatic until January 2004, when she was admitted for headache and vomiting associated with haemihypoaesthesia with paraesthesia of the right hemibody and right hemiparesis 4/5. Examination revealed a vertical nystagmus, right hemiparesis 4/5 and right hemihypoaesthesia with sensory extinction.
The cranial CT scan performed on admission showed a right posterior protruberancial haematoma of 20 mm in diameter, related to the known location of the cavernoma.

Brain MRI confirmed the presence of a protuberancial cavernous angioma with the presence of perilesional haemosiderin defining the growth and bleeding of the lesion, which deformed the IV ventricle in the rhomboid fossa at the level of the left facial colliculus without emerging into the ventricle or any other part of the surface of the brainstem.

Given the clinical and radiological progression of the lesion, it was decided to intervene surgically, using intraoperative monitoring consisting of stimulation of the surface of the rhomboid fossa, to determine the location of the nuclei of the left cranial nerves VII X and XII. Intraoperative auditory potentials were also performed. To stimulate the nuclei located in the rhomboid fossa, we used the NIM-Response® System (Medtronic, XOMED) nerve monitoring system, applying a monopolar stimulus at constant voltage with a rectangular monophasic pulse. The maximum intensity applied was 2 mA. The stimulus duration was 100 µseconds and the frequency was 10 Hz. Electromyographic reading was performed with electrodes placed 5 mm apart in a bipolar arrangement to cover the representative muscle area. Electrodes to monitor the facial nerve were placed on the orbicularis oculi muscles of the mouth and eye. To record the cranial nerve XII the electrodes were placed on the genioglossus muscle. The recording of the Xth pair was performed with vocal cord sensors located in the anaesthesia tube.
The patient was placed in prone position and a suboccipital craniectomy and excision of the posterior arch of C1 was performed. After dural and arachnoid opening and under microscopic control, asymmetry was observed at the level of the left facial colliculus. After anatomical identification of nuclei X and XII, their functionality was confirmed by intraoperative stimulation with positive electromyographic response. The facial nucleus was located only by stimulation and was found to be very lateral, on the border between the angle of the rhomboid fossa and the middle cerebellar peduncle. The nucleus was probably displaced laterally due to the mass effect of the haematoma, which was not visible on the surface. A pial incision was made, the haematoma was located and evacuated, as well as the cavernoma causing the haematoma. At the end of the resection, the monitoring of the nuclei involved was repeated, obtaining electromyographic response in all of them, although the intensity of the facial, especially of the orbicularis oris of the lips, had decreased. The auditory potentials were maintained with the same response as in the preoperative period.
Postoperatively, the patient presented transient X and XII paresis, which recovered completely after 48 hours and which we suspect was due to the block produced by the repeated electrical stimulus. She also presented an internuclear ophthalmoplegia with one-and-a-half syndrome, which has partially recovered, with a persistent nuclear paresis of the left VI pair, as well as a nuclear facial paresis which has progressively improved. No motor deficits were observed in the long pathways, although he is unstable when walking, although he can ambulate unaided, autonomously. The level of consciousness is good, with preserved higher cognitive functions.