24 Monro Foraminoplasty
24.1 Introduction
Monro foraminoplasty is a neuroendoscopic procedure employed to restore cerebrospinal fluid (CSF) flow through the foramen of Monro in patients with unilateral or bilateral hydrocephalus. Monro foraminoplasties consist of perforation through parenchyma, scar tissue, or membranous occlusions at the foramen of Monro with subsequent dilatation in patients with complete foraminal obstruction or simple dilatations of the foramen of Monro in patients with patent but stenotic apertures. Originally described by Dott,1 unilateral hydrocephalus is a condition where one lateral ventricle becomes dilated due to obstruction of the cavity or its outlet. In bilateral hydrocephalus, both lateral ventricles are enlarged as a result of obstruction. The etiology of obstruction in unilateral or bilateral hydrocephalus can be due to a variety of causes. In rare idiopathic cases, either congenital atresia or membranous occlusion of the foramen of Monro occurs (Fig. 24.1). However, more commonly, pathologic occlusion of the foramen of Monro can be a sequelae of neoplastic, infectious, inflammatory, traumatic, vascular, or hemorrhagic events.2,3,4 “Functional” occlusions that come about as a result of iatrogenic procedures such as shunting or endoscopic third ventriculostomy (ETV) have also been described.4,5,6,7 Clinically, hydrocephalus due to foraminal obstruction presents most commonly with headaches and occasionally with near-syncopal events elicited by straining.8 Radiographically, unilateral or bilateral foraminal obstruction is suggested by computed tomographic (CT) and magnetic resonance imaging (MRI) that depicts unilateral or bilateral ventricular enlargement (Fig. 24.2 and Fig. 24.3) in concurrence with smaller caliber third and fourth ventricles. Suspected foraminal obstruction can be further investigated via more refined MRI CSF flow studies or through ventricular cisternograms via contrast administration through external ventricular drains (EVDs) or proximal catheters of ventricular shunts. Through the evolution of neuroendoscopy, techniques including SPF, Monro foraminoplasties, and foraminal stenting have been developed and have been proven to be as efficacious.14,15
Before the advent of neuroendoscopy, CSF diversion through shunting,8,9,10,11 stereotactic septostomy,12 or open microscopic procedures, such as microsurgical reconstruction of the foramen of Monro11 and micro-surgical septum pellucidum fenestration (SPF),13 was performed to treat patients with lateral ventricular hydrocephalus. Through the evolution of neuroendoscopy, techniques including SPF, Monro foraminoplasties, and foraminal stenting have been developed for this less-invasive approach and have been proven to be as efficacious.14,15 Whether performed alone or in combination with other endoscopic techniques, Monro foraminoplasty serves as a valuable tool in the neuroendoscopist’s armamentarium. This chapter focuses on the indications, operative techniques, complications, and outcomes of Monro foraminoplasties.
24.2 Indications/Contraindications
Monro foraminoplasties are indicated as a primary procedure in the treatment of unilateral or bilateral hydrocephalus caused by foraminal stenosis or obstruction. They may also be employed in conjunction with other neuroendoscopic procedures, such as SPF, in cases of bilateral foraminal blockage (Fig. 24.4). In patients with bilateral lateral ventricular dilatation due to stenosis or obstruction of both foramina, the treatment plan is further developed according to whether the septum pellucidum has been naturally fenestrated due to chronic hydrocephalus. If this is the case, only one foraminoplasty may need to be performed as communication between the lateral ventricles already exists. If the septum pellucidum is not naturally fenestrated, then, in addition to a foraminoplasty, an endoscopic SPF may be considered. Bilateral foraminoplasties have been reported, but performing this procedure remains controversial due to potential damage to bilateral fornixes.16 If concern exists that the foraminoplasty will restenose, a foraminal stent may be placed immediately after the foraminoplasty during the same operation.10,17,18
Monro foraminoplasties are contraindicated in cases where the fornix cannot be visualized due to extensive scarring.19 Also, any vascular structures or lesions causing foraminal obstruction, such as dilated cerebral veins associated with arteriovenous malformations, should preclude attempting a foraminoplasty.
24.3 Operative Techniques
24.3.1 Preoperative Preparation
Prior to the operation, the nuances of the operative plan should be carefully considered, and a thoughtful salvage plan should be formulated. For example, in cases of unilateral hydrocephalus, a simple foraminoplasty should be planned on the side of the ventricular dilatation. If the foraminoplasty cannot be completed or is inadequate (fails to maintain patency immediately after dilatation), an attempt to fenestrate the septum pellucidum from the same bur hole could be made. However, this may be technically difficult because of the more medial location of the foraminoplasty bur hole in comparison to the conventional more lateral, frontally placed bur hole that is ideal for SPF. Thus, before performing a foraminoplasty, one may plan rescue strategies in case difficulty is encountered. For example, a more lateral bur hole to the standard Kocher’s point (2.5 to 3 cm from midline and slightly anterior to the coronal suture) where the foraminoplasty is attempted may be prepared into the operative field. This additional access point to the ventricle from a more lateral trajectory may serve as a salvage option if an SPF cannot be performed safely through the Kocher’s point trajectory from which the foraminoplasty was attempted. In addition, if the neurosurgeon is adept in utilizing the flexible endoscope, both procedures, a Monro formainoplasty and SPF, can be performed through the same standard Kocher’s point bur hole.
In cases of bilateral hydrocephalus, particularly in cases where naturally occurring fenestrations in the septum pellucidum do not exist, right-sided approaches are favored to avoid more eloquent left hemispheric areas. The surgeon may want to prepare the operative field so that an SPF could be made if naturally occurring fenestrations do not already exist and if the fenestration cannot be made from the preexisting bur hole. In cases of bilateral ventricular dilatation, injury to the contralateral ventricular wall during SPF would be less likely due to the enlarged contralateral ventricle.
Bilateral foraminoplasties have been reported with details of access though a more medially placed bur hole where a unilateral foraminoplasty followed by SPF and foraminoplasty of the contralateral ventricle is performed.16 However, this approach—as well as bilateral foraminoplasties through separate bur holes—remains controversial due to increased risk of potential injury to both fornixes.
Finally, in the preoperative stage, consideration may also be given to preparing the operative field for possible ventricular shunting. Preoperative preparation for stenting or shunting may be beneficial in cases where restenosis may be a concern.
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