Brainstem cavernous malformations (CMs) often require resection due to their aggressive natural history causing hemorrhage and progressive neurological deficits. The authors report a novel intraoperative neuromonitoring technique of direct brainstem somatosensory evoked potentials (SSEPs) for functional mapping intended to help guide surgery and subsequently prevent and minimize postoperative sensory deficits.
Between 2013 and 2019 at the Stanford University Hospital, intraoperative direct brainstem stimulation of primary somatosensory pathways was attempted in 11 patients with CMs. Stimulation identified nucleus fasciculus, nucleus cuneatus, medial lemniscus, or safe corridors for incisions. SSEPs were recorded from standard scalp subdermal electrodes. Stimulation intensities required to evoke potentials ranged from 0.3 to 3.0 mA or V.
There were a total of 1 midbrain, 6 pontine, and 4 medullary CMs—all with surrounding hemorrhage. In 7/11 cases, brainstem SSEPs were recorded and reproducible. In cases 1 and 11, peripheral median nerve and posterior tibial nerve stimulations did not produce reliable SSEPs but direct brainstem stimulation did. In 4/11 cases, stimulation around the areas of hemosiderin did not evoke reliable SSEPs. The direct brainstem SSEP technique allowed the surgeon to find safe corridors to incise the brainstem and resect the lesions.
Direct stimulation of brainstem sensory structures with successful recording of scalp SSEPs is feasible at low stimulation intensities. This innovative technique can help the neurosurgeon clarify distorted anatomy, identify safer incision sites from which to evacuate clots and CMs, and may help reduce postoperative neurological deficits. The technique needs further refinement, but could potentially be useful to map other brainstem lesions.
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