Temporal Lobe | Vagal Nerve | Frontal Lobe
The human brain has a right and left temporal lobe, wherein one mirrors the other. Mesial temporal lobe anatomy involves specific structures frequently implicated as a cause for seizures in patients with temporal lobe epilepsy. The structures include the hippocampus, the parahippocampus, and the amygdala. Many or all of these will be removed during surgery to stop seizures arising in this area. The most common reason for which seizure surgery is performed in the temporal lobe is for scarring in the hippocampus, or hippocampal sclerosis. Removal of the hippocampus in this situation will lead to stopping seizures in the majority of the patients.
The anatomy of this region is depicted in the following drawing.
Specific structures the surgeon looks for during surgery include arteries and veins supplying the region, the third and fourth cranial nerves (control eye movements), the brainstem, and the base of the skull. These structures are identified and protected during surgery to prevent post-operative neurological symptoms.
Blood supply to the hippocampus involves an arterial supply (shown in red) from the posterior cerebral artery and venous drainage (shown in blue) into the basal vein of Rosenthal as depicted in the cadaver specimen to the left. Other arteries sometimes seen during surgery include the middle cerebral artery system, the anterior choroidal artery, and the internal carotid artery.
Function of the Temporal Lobe
The hippocampus plays a role in memory processing; however, the exact memory circuit in the human brain is poorly understood. The hippocampus on the language-dominant side (usually the left side of the brain in most people) has a much more important role in memory than the non-dominant side. This hippocampus functions in verbal or language memory and short-term memory. Clinicians use several tests pre-operatively to determine the risk of impairing memory if the dominant hippocampus needs to be removed.
The dominant temporal lobe (left) often has a region specialized in language skills. This area of the brain is known classically as Wernicke's area and involves language comprehension. Its exact location is variable (see sketch) and, if surgery is mandated in the posterior lateral temporal lobe, this language site needs to be identified and protected.
Other functions in the temporal lobe include a portion of the visual fiber pathway known as Meyer's loop. These fibers project from the lateral geniculate nucleus of the thalamus to the visual cortex. They follow a variable course within the deep white matter of the temporal lobe and are sometimes injured unavoidably during temporal lobe surgery. This leads to a defect in peripheral vision with a black spot in the opposite upper peripheral field of vision ("pie in the sky" deficit).
The vagus nerve is a mixed cranial nerve containing approximately 80% sensory fibers. Efferent fibers innervate the larynx and provide parasympathetic control to the heart, lungs, and abdominal viscera. The right vagus nerve innervates the sinoatrial node of the heart while the left innervates the atrioventricular node. In the dog model, stimulation of the right vagus nerve caused more cardiac slowing then the left. For this reason, the left human vagus nerve is implanted. Adverse cardiac complications have not been seen, perhaps because vagus nerve stimulation of the nerve can be done on a segment away from the origin of the cardiac branches (see diagrams).
The nerve runs from the lower brainstem through the base of the skull to travel in the neck with the carotid artery and jugular vein. It then penetrates the chest to travel to the heart and lungs. It continues on to the abdomen where it breaks up into a network of nerves to the abdominal organs.
For implantation of the vagal nerve stimulator (VNS), the nerve is identified in the neck between the carotid artery and jugular vein. Here the nerve is a good size and the electrode is spiraled around it easily.
The frontal lobe is the largest in the brain extending from the anterior skull back to behind the ear. Within it's borders lies a region that moves the opposite side of the body (primary motor cortex), a part that allows for coordinated eye movements to the opposite side (frontal eye fields), a region concerned with initiation and coordination of smooth motor movements (supplementary motor area), and if on the dominant side a motor language area (Broca's area).
Dominant side refers simplistically to the side of the brain where language function is. Language is characterized by different areas within one side of the brain each serving a different function and connected by white matter tracts (see diagram). While this is a simplistic view, it serves us well to try to understand how language is organized. Most humans have language on the left side of the brain. If a person is left-handed or has had a problem with the left side of the brain early in life, the chance of having language on the right side of the brain goes up. Regardless of which side of the brain language is on, it classically is organized into Broca's area in the inferior frontal lobe just in front of the face motor area and Wernicke's area in the posterior temporal/parietal area. These surface language areas are important to identify and avoid during surgical removal of seizure zones.
The frontal lobe therefore has different important or eloquent areas depending on cerebral dominance (which side is dominant). This is important to know pre-operatively as the risks of surgery may vary right side to left side. Tests available to determine this include the intracarotid sodium amobarbital test (WADA), neuropsychological testing, and functional magnetic resonance imaging. Of these, the WADA test is most reliable but also carries a slight risk due to the invasive nature of the test.
The general organization of the frontal lobe independent of side includes the formation of gyri (the gray and white matter convolutions of the brain) and sulci (the natural planes in the plane containing the blood vessels). The surfaces of the frontal lobe include the orbital (basal) surface, the lateral surface, and the mesial surface (see diagrams). Within the deep white matter of the frontal lobe is the anterior part of the lateral ventricle which contains cerebrospinal fluid. The posterior part of each frontal lobe known as the precentral gyrus contains the primary motor cortex. Here motor function is arranged according to the part of the body moved (somatotopic representation). The arrangement of cortex is known as the homunculus (see diagram).
Depending on where the seizure starts, each patient may have variable types of seizures. It becomes important to determine side of dominance, side of seizure onset, and which region of the frontal lobe is affected. Once these have been determined, surgery to further map the frontal lobe may be necessary (link to subdural grids) or direct resection of frontal lobe cortex may be possible.