Beverly K. Sturges University of California, Davis, CA, USA The caudal fossa, housing the brainstem and cerebellum, offers unique surgical challenges and rewards due to the location and the critical nature of the structures contained within it. As is true of all cranial surgery, an intimate knowledge of regional anatomy, both outside of the calvarium as well as within the calvarium, is essential for surgical success. This is often challenging due to major variations in anatomy between species as well as between breeds and individuals. Although this chapter focuses on the standard surgical approaches to the caudal fossa, some of what is presented is based on experience and/or anecdote from the author and others providing critique and mentorship over the years. The caudal–cranial fossa is separated from the cranial–cranial fossa, or supratentorial compartment, by the tentorium cerebelli dorsally; thus, the caudal fossa forms the infratentorial compartment. It is bounded rostrally by the dorsum sellae and ends caudally at the foramen magnum. The dorsal, lateral, and caudal aspects are formed by the occipital bone, being delineated from the temporal fossa (of the occipital bone) by the nuchal crest. The caudal fossa forms a compact and rigid compartment with limited compliance of the parenchymal structures protected within, which include the cerebellum, pons, medulla oblongata, nuclei of cranial nerves V–XII, and the ascending and descending tracts of the spinal cord. It is also where outflow of cerebrospinal fluid (CSF) from the third ventricle into the fourth ventricle and lateral apertures reside. Together the neural structures within the caudal fossa support most vital functions of the body, level of consciousness, and coordination and modulation of activity from the relay of information to and from all parts of the body. The contents of the caudal fossa are supplied by multiple pairs of arteries: the caudal cerebellar arteries arising from the basilar artery to supply the caudal part of the cerebellum and the rostral cerebellar arteries, supplying the rostral portion of the cerebellum, branch from the caudal communicating artery coming off the terminal internal carotid artery. Other branches from the basilar artery supply other parts of the brainstem. Since the arterial supply to the structures within the cauda fossa is located ventrally with close perforation into parenchyma, disruption of major arterial vessels is rare in most caudal fossa surgery. Venous structures are encountered frequently, though, and detailed knowledge of their location is essential for successful surgery in this region. The ventral venous sinus system contains the basilar sinus that lies on the floor of the occipital bone and eventually joins the internal ventral vertebral venous plexus. The dorsal venous sinus system of the brain consists of the dorsal sagittal sinus which, via the confluence of the sinuses (confluens sinuum), gives rise to the transverse sinuses that run ventrolaterally within the occipital bone. Along their course the transverse sinuses receive major veins from other parts of the brain before branching and exiting the skull ventrally to join the peripheral venous system. These vascular structures, by their proximity to the limits of occipital craniectomy, are given consideration whenever approaching the caudal fossa (Figure 24.1) [1]. Muscles originating from the cervical vertebrae compose most of the musculature of the craniodorsal cervical region. Superficially, the broad, flat superficial muscles of the cervicoauricular–occipital muscle complex are fused on dorsal midline and represent the initial layer in surgical dissection. The deeper paired dorsal muscles attaching to the caudal occipital bone provide essential support to the head and contribute to controlled movement of the occipital–atlanto–axial (OAA) joints. These include the semispinalis capitis (biventer and complexus), and rectus capitis dorsalis (major and minor) muscles. Ligamentous structures also provide support and flexibility of the head at the occipital–atlantal joint. The dorsal atlanto‐occipital (AO) membrane, extending between the dorsal edge of the foramen magnum and the arch of C1, is of particular importance when approaching this region [2]. Figure 24.1 Venous system of the caudal fossa. Venous sinuses of the caudal fossa, medial aspect. The dorsal system of venous sinuses begins with the dorsal sagittal sinus running caudally from within the falx cerebri to empty into the confluence of the sinuses (confluens sinuum) within the occipital bone and branches into paired transverse sinuses. These run laterally within the osseous canal, dividing into the sigmoid and temporal sinuses ventrally. The temporal sinuses exit via the retroarticular foramen to the maxillary veins and, then, the jugular veins. The basilar sinus branches off the sigmoid sinus and runs through the condyloid canal (condyloid vein) before joining the internal ventral vertebral venous sinus as it exits the foramen magnum. The condyloid vein may be ruptured if drilling too ventral in the occipital bone. The ventral interbasilar sinus connects the right and left basilar sinuses at the foramen magnum. It lies on the floor and sides of the occipital bone, is irregularly defined, and often has many fingerlike connections to the lateral and ventral meninges of the caudal brainstem. Occasionally there is also a dorsal interbasilar sinus with similar meningeal connections. When removing masses alongside the brainstem, especially when they are invading meninges, hemorrhage may become profuse from rupture of these sinuses. The occipital emissary veins, fed by the transverse and sigmoid sinuses, exit ventrolaterally on the caudal aspect of the skull via the mastoid foramen. During lateral muscle elevation, these may be encountered causing hemorrhage. Anatomic variations in intracranial dural venous sinus anatomy are common in dogs and should be taken into consideration during surgical planning. The clinical presentation of small animals with caudal fossa disease depends on the location and acuity of the lesion as well as the impact of the pathology on cerebrospinal fluid flow dynamics. Chronic, progressive lesions of the caudal fossa generally cause cranial nerve dysfunction (CNs V, VII, VIII, IX, X, XII), cerebellar and vestibular signs (ataxia, tremor, motor dyscoordination), alterations in mental state, sensorimotor deficits in the limbs, and potentially cardiac and respiratory abnormalities. If CSF outflow from the fourth ventricle is severely compromised due to obstruction or swelling, clinical signs may be more consistent with developing intracranial hypertension including marked alteration of consciousness, abnormal posturing, altered respiration, etc. The surgical indications for caudal fossa surgery primarily include the following: Dogs and cats undergoing caudal fossa surgery require the same preoperative assessment as other cranial surgical patients (refer to Chapter X). The presence of cranial nerve deficits resulting in dysphagia, loss of gag reflex, and/or laryngeal nerve dysfunction increase the risk for aspiration and require specific precautions at the time of extubation and in the early postoperative period. Typically, a baseline assessment including CBC, serum biochemical profile, urinalysis, and imaging assessment of the thoracic and abdominal structures is done prior to surgery. Baseline blood gases prior to premedication is recommended for assessment of ventilatory function prior to surgery and is useful for postoperative comparison. Premedication for craniotomy should be directed at the individual patient’s level of anxiety, baseline neurological status, and other comorbidities. Dogs and cats with caudal fossa pathology may be especially sensitive to the respiratory depressant effects of benzodiazepines and opioids and may be at increased risk of excessive sedation and aspiration. Sedatives should be titrated to effect using small doses of medication. In patients that are particularly neurologically impaired, withholding sedation until the patient is instrumented for monitoring, and in a setting allowing immediate airway management, is desirable. The choice of anesthetic agents, hemodynamic, and ventilatory management and physiologic monitoring are similar for other cranial surgeries in small animals. Overarching goals include the maintenance of hemodynamic stability and cerebral perfusion pressure while avoiding increases in intracranial pressure. Patients are positioned in sternal recumbency with the head flexed as close to perpendicular to the longitudinal axis of the vertebral column without affecting patency of the endotracheal tube (Figure 24.2). The level of the brain should be maintained higher than the level of the heart and direct pressure on jugular veins is strictly avoided since this can interfere with venous outflow and increase intracranial pressure. In some instances, it is useful to tilt the head away from the side of the lesion if it is lateralized. Once positioned, the patient is secured firmly in place using a combination of sandbags, tape, inflatable sandbags, or other positioning devices. A mouth gag is used to prevent compression of the tongue during surgery, which can lead to lingual swelling and postoperative airway obstruction; in more severe situations it can result in ischemia and necrosis of the tongue. The head is shaved and prepped from the eyebrows to the mid‐cervical region and a surgical prepping solution (3MTM DuraPrepTM) is applied to the shaved skin and allowed to dry before placing an iodophor‐impregnated plastic incision drape (3MTM IobanTM) over the area. This improves the adhesion of the iodophor‐impregnated plastic incision drape to the skin while the combination of the two provide long‐lasting antimicrobial persistence. Both bony and soft tissue anatomy of the caudal fossa is uniquely specific to every patient and must be carefully evaluated preoperatively since it heavily influences surgical positioning, approach, instrumentation, and overall success of the procedure. This is especially applicable in dogs with occipital dysplasia/malformation, dolicocephalic breeds with an occipital bone that is rostrally offset from a prominent external occipital protuberance, and large dogs with heavy muscling of the head and neck. Figure 24.2 Positioning for caudal fossa surgery. Proper positioning is important for accessing the caudal fossa. The patient’s head is strongly flexed, as close to 90° as possible without compromising the endotracheal tube. Jugular compression and lingual compression are avoided to prevent complications from venous drainage from the brain and postoperative airway obstruction respectively. A small sandbag placed between the manubrium and the mandibles is useful in larger dogs to wedge the head against before securing into position. A mouth gag is needed to prevent the tongue from swelling. This can also be done simply by using a roll of tape around the endotracheal tube in the mouth and pulling the tongue out laterally behind the tape roll (inset). A small U‐shaped inflatable sandbag is very helpful for holding the head and anesthesia‐related equipment in non‐dislodgeable orientation. The midline approach through the occipital bone provides a good view of the cerebellar vermis, the medial portion of the cerebellar hemispheres, and the caudodorsal part of the brainstem. With retraction of the cerebellar vermis, the obex and fourth ventricle are also visualized. It is often combined with C1 partial or complete laminotomy to increase access to the area when pathology affects the cervicomedullary junction (C1 spinal cord segment and caudal brainstem). In small animals, it is the most straightforward approach and is frequently used to decompress Chiari‐like malformations (Figure 24.3), remove caudal cerebellar convexity masses, smaller, midline fourth ventricular masses and tumors affecting the cervicomedullary junction (C1), and caudal brainstem (Figure 24.4). Figure 24.3 Midline occipital craniectomy to decompress caudal occipital malformation. Mid‐sagittal T2W images from a six‐month‐old Ocelot kitten. (a) Severe malformation of the caudal occipital bone (white arrow) reducing the volume of the caudal fossa is resulting in marked cerebellar compression, foraminal herniation (arrowhead), and obstructive hydrocephalus with involvement of olfactory bulb, and syrinx formation (yellow arrow). Neurologically, this kitten was weakly ambulatory and markedly ataxic with positional vertical nystagmus. (b) Immediate postop image using midline occipital craniectomy affords decompression of the cerebellar vermis (white arrow), resolution of reduction in obstruction of CSF outflow, and partial resolution of syrinx. The kitten became neurologically normal within a few days. A dorsal midline skin incision is made from 2 cm rostral to the external occipital protuberance to approximately the level of C3 vertebrae. Using an electrocautery pencil, subcutaneous fat tissue and superficial cervical musculature are incised together on midline following the dorsal median raphe (Figure 24.5). Gelpi or other self‐retaining retractors are placed to retract tissue at the cranial and caudal aspect of the surgical site and repositioned as needed. From here, the spinous process of C2 and the external occipital protuberance are palpable landmarks that may be used to envision midline as dissection and elevation of deeper musculature is performed to eventually expose the caudal occipital bone, the arch of the C1 and the rostral portion of the spinous process of C2. First, the paired semispinalis capitis biventer cervicis muscles are separated on midline and retracted revealing the deeper paired rectus capitis dorsalis muscles (Figure 24.6
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Surgery of the Caudal Fossa
Anatomy
Indications for Surgery
Preoperative Assessment and Anesthetic Management
Surgical Positioning
Surgical Approach(es) to the Caudal Fossa
Midline Occipital Approach [3–5]
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