Interhemispheric

The interhemispheric approach follows the falx between the medial surfaces of the hemispheres. Anteriorly it reaches the genu and body of the corpus callosum, the pericallosal arteries and, by a callosotomy, the lateral and third ventricles; posteriorly it reaches the splenium, the atrium and the pineal region. Apuzzo and colleagues described the transcallosal interforniceal route to the third ventricle and its consequences in 1982^[1]. The ventricular anatomy relevant to these approaches was described in detail by Rhoton^[2].

• Colloid cysts and other tumours of the third ventricle and the foramen of Monro.
• Intraventricular tumours such as central neurocytoma and subependymoma.
• Falcine and parasagittal meningiomas.
• Distal anterior cerebral (pericallosal) artery aneurysms.
• Medial frontal and parietal lesions and corpus callosotomy for epilepsy.

Bridging veins drain the medial frontal and parietal cortex into the superior sagittal sinus and are most numerous around the coronal suture and behind it. The callosomarginal and pericallosal arteries run on the corpus callosum and must be separated before the callosotomy. In the lateral ventricle, the foramen of Monro is identified by following the choroid plexus, the septal vein and the thalamostriate vein forward. The third ventricle can be entered through the foramen of Monro (transforaminal), between the fornices (interforniceal), or through the choroidal fissure (transchoroidal)^[2].

• Supine with the head neutral and slightly flexed, or lateral with the side of approach down so that gravity retracts the hemisphere.
• Craniotomy crossing or reaching the sagittal sinus, about two-thirds in front of and one-third behind the coronal suture, planned on venography to fit between bridging veins.
• Open the dura toward the sinus; dissect the interhemispheric fissure down to the corpus callosum, separating the cingulate gyri and the pericallosal arteries.
• Make a callosotomy of about 2 cm in the body of the corpus callosum.
• Identify the anatomy of the lateral ventricle, then enter the third ventricle through the most appropriate corridor.

• Venous infarction from sacrifice of bridging veins: plan the craniotomy on venous imaging and preserve the large veins.
• Memory loss from injury to the fornices, particularly if both are damaged.
• Disconnection syndromes after long callosotomies; keep the callosotomy short and in the body.
• Injury to the pericallosal arteries, causing leg weakness.
• Hydrocephalus, ventriculitis and intraventricular haemorrhage.