A Practical Approach to Neurophysiologic Intraoperative Monitoring, Second Edition by Husain MD Aatif M

Author:Husain MD, Aatif M. [Husain MD, Aatif M.]
Language: eng
Format: epub
Publisher: Springer Publishing Company
Published: 2014-12-10T16:00:00+00:00

FIGURE 16.2 Typical patterns of CNN displacement by brainstem tumors with different locations are shown schematically.

Source : From Ref. (14). Morota N, Deletis V, Lee M, Epstein FJ. Functional anatomic relationship between brainstem tumors and cranial motor nuclei. Neurosurgery 1996;39:787–794.

SYMPTOMS

Because of the complex neuroanatomy of the brainstem, lesions in this area can produce a variety of mixed neurologic symptoms. Patients may experience sensory, motor, cerebellar and CN symptoms. In addition, it is not uncommon for patients to exhibit a combination of these symptoms.

Brainstem lesions causing motor symptoms commonly produce bilateral motor deficits, often associated with sensory symptoms, CN deficits and problems with equilibrium. More focal brainstem lesions usually cause CN deficits on the ipsilateral side and a contralateral hemiparesis. The level of the brainstem lesion will determine the CN affected.

Lesions of the spinothalamic tract in the dorsolateral medulla and pons produce sensory disturbances involving the opposite side of the body. When a lesion involves the spinothalamic tract in the medulla, the pinal trigeminal nucleus is also involved, causing impairment of sensation on the ipsilateral side of the face. This produces a crossed sensory deficit affecting the ipsilateral face and contralateral limbs. If the spinothalamic lesion is above the spinal trigeminal nucleus, then the symptoms affect the contralateral face, limbs and trunk. Lesions involving the medial lemniscus can cause touch and proprioception deficits on the opposite side of the body. However, a single lesion in the upper brainstem where the spinothalamic tract and medial lemniscus run together can cause loss of all superficial and deep sensation in the contralateral side of the body.

Patients are typically referred for operative intervention when their symptoms suggest brainstem involvement, and brain imaging studies identify space occupying lesions in or adjacent to the brainstem. Depending on the location of the lesion, the clinical state of the patient and the brain imaging characteristics of the lesion, patients may undergo initial conservative nonsurgical management, but later require surgical intervention due to worsening symptoms. In most instances, surgical intervention is required when there is evidence of brainstem compression or ischemia.

SURGICAL CONSIDERATION

When preparing to do the NIOM technical setup, it is important to have an understanding of the planned surgical procedure and patient positioning on the surgical bed. The following description of the surgical approaches to treating vascular malformations serves as a useful guide for the general surgical approach strategies used to access brainstem lesions.

For surgical procedures targeting midline medullary or pontine lesions, a midline suboccipital approach is used, operating through the floor of the fourth ventricle. The patient is placed in the prone position and a suboccipital craniotomy is performed with removal of the foramen magnum. The cerebellar tonsils are retracted, and the inferior midline vermis is sectioned to facilitate exposure. If a medullary or pontine lesion is located laterally or anteriorly and presents to one of these pial surfaces but not to the fourth ventricular surface, the far lateral suboccipital approach is employed. The patient is placed in the lateral or supine position, with the head turned. Midbrain or pontomesencephalic lesions are exposed via a subtemporal or subtemporal transpetrosal approach.

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