The foramen of Monro: a review of its anatomy, history, pathology, and surgery

R. Shane Tubbs 0 Peter Oakes 0 Ilavarasy S. Maran 0 Christian Salib 0 Marios Loukas 0 0 R. S. Tubbs Centre of Anatomy and Human Identification, University of Dundee , Dundee, UK 1 ) Pediatric Neurosurgery , Children's of Alabama, Birmingham, AL, USA Introduction The foramen of Monro lies at the junction between the paired lateral ventricles and the third ventricle of the brain. Methods A comprehensive review of the literature was performed focusing on the foramen of Monro. Conclusions A good understanding of the anatomy of the foramen of Monro is essential for the neurosurgeon, especially with the increasing use of intraventricular endoscopy. - The foramen of Monro is a short conduit between the paired lateral ventricles and the third ventricle of the brain. This deep structure becomes clinically significant when obstructed and leads to obstructive (non-communicating) hydrocephalus. Etiologies of obstruction at the foramen of Monro include infections, congenital atresia, vascular malformations, and neoplastic processes [7]. Herein, we will review the historical discovery of the foramen of Monro, its development, the microsurgical and surface anatomy of the foramen, pathology in the region, and neurosurgical treatments for such pathology. Alexander Monro secundus (17331817) was born into a lineage of Scottish physicians, with his father and son being Alexander Monro primus and tertius, respectively [2]. Together they held the chair of anatomy at the University of Edinburgh for 126 years [17]. Monro secundus recorded detailed descriptions and illustrations regarding the communication between the lateral and third ventricles of the brain as well as describing changes seen in hydrocephalus. Though many credit Monro secundus with the discovery of the interventricular foramen, he stated in his Observations on the structure and function of the nervous system (1783), These cavities have been described by Galen, and by many succeeding authors of eminence, as all communicating with each other [10]. Monro secundus is also known for helping to establish the Monro-Kellie hypothesis [9]. The hypothesis states that the sum of volumes of brain parenchyma, CSF, and intracranial blood is constant. These two contributions make him the most well-known of the Monro lineage. The brains ventricular system develops from cavities within the neural tube [8]. While the five brain vesicles develop, the cavity in the forebrain divides into the two lateral ventricles and the third ventricle. The lateral ventricles, which form as outpouchings of the rostral third ventricle, are both interconnected with the third ventricle via the foramen of Monro. During the early stages of hemispheric development, differential growth patterns persist for a considerable period [14]. Formation of the choroidal fissure depends on related growth patterns of surrounding structures. Particularly important is the relatively slow growth of the foramen of Monro, the secondary fusion between the lateral diencephalon and medial hemisphere walls encompassing the upper brain stem by the forward growth of the temporal lobe and its pole toward the apex of the orbit, and the massive expansion of the two great cerebral commissures, the fornix and corpus callosum. At this point, the choroidal fissure is seen as a caudal extension of the interventricular foramen, which arches above the thalamus and a few millimeters from the median plane [14]. Near the caudal end of the thalamus, the foramen of Monro diverges ventrolaterally, with its curve reaching to the tip of the inferior horn of the lateral ventricle. The upper portion of this arch will be overhung by the corpus callosum and throughout its convexity; it is bordered by the fornix and its derivatives [8]. Microsurgical anatomy of the foramen of Monro The microsurgical anatomy of the foramen of Monro has been elegantly illustrated and described by Rhoton et al. [18]. The foramen of Monro is located on each side at the junction of the roof and anterior wall of the third ventricle (Figs. 1, 2, 3, 4, 5, and 6). The foramen is bounded anteriorly by the junction of the column and body of the fornix and the anterior pole of the thalamus posteriorly. The shape and size of the foramina correlate with the size of the ventricles: if the ventricles are small, each foramen will be a crescent-shaped opening anteriorly bounded by the concave curve of the fornix and posteriorly by the convex anterior tubercle of the thalamus. As the ventricles enlarge, the foramen on each side becomes more rounded. Structures passing through the foramen include the choroid plexus, the distal branches of the medial posterior Fig. 2 Midsagittal T1-weighted MRI of the brain noting the right foramen of Monro (arrow) Superficial landmarks for the foramen of Monro Superficial relationships of a deep landmark are helpful in planning deep operative approaches [13]. At the cranial surface, the foramen of Monro is roughly 2 cm superior to the pterion, just behind the lower third of the coronal suture. At the cerebral surface, the foramen is located deep to the central Fig. 3 Superior view of an axial section through a cadaveric brain noting the right foramen of Monro (tip of needle) portion of the pars opercularis. At the insular level, it is located deep to the central part of the second short insular gyrus. The thalamus is located at the center of the brain with the foramen of Monro at one end and the pineal gland at the other. Together, the surface landmarks of the foramen of Monro and pineal gland estimate the deep position of both the thalamus and third ventricle. The foramen of Monro approximates the anterosuperior thalamic margin, and the pineal gland defines its posterior edge. The foramen of Monro becomes clinically significant when it is obstructed causing obstructive (non-communicating) hydrocephalus [8]. Stenosis of the foramen of Monro has been attributed to infectious origins (particularly TORCH infections) causing inflammation and scarring in the region, congenital atresia, vascular malformations, and neoplastic processes [7]. Some of the masses most frequently encountered within the Fig. 4 Right sided foramen of Monro (arrow) as seen through the third ventricle of a cadaver brain. For reference, note the choroid plexus (CP) Fig. 5 Glass model of the ventricular system noting the left and right foramina of Monro (arrows) foramen of Monro include colloid cysts, subependymal giant cell tumors (SGCT) associated with tuberous sclerosis complex (TSC), and subependymal nodules and hamartomas [4]. Colloid cysts are the most common masses of the foramen of Monro [4]. They have an incidence of 0.22 % of all intracranial tumors [5]. This well-defined round cyst occurs in adult patients and may be from several millimeters to 3 cm in size and also attached to the anterosuperior aspect of the third ventricle [4, 12]. They possess no intrinsic pathological properties and exert symptoms by acting as inert masses. (...truncated)