Indications

Craniocervical junction (CCJ) anomalies refer to a group of diseases affecting a common area, which include the Chiari‐like malformation, elevation of the medulla at the CCJ, atlantooccipital overlapping, atlantoaxial instability, dorsal angulation of the dens, and atlantoaxial fibrous band (also referred to also as a “dural band,” a misnomer as dura is not a component of this tissue) [1–5]. Syringomyelia (i.e. an accumulation of fluid in the spinal cord) frequently develops as a component of these diseases, secondary to changes in cerebrospinal fluid flow through the CCJ [2, 6]. Both the CCJ themselves and their secondary syringomyelia, when present, are responsible for the clinical manifestations of these diseases; of note, clinical signs can occur in dogs without syringomyelia [3, 5, 7].

Particularly predisposed breeds include the Cavalier King Charles Spaniel and the Brussels Griffon, although small and toy breed dogs are generally over‐represented [7–9]. CCJ anomalies may also be present in cats [10, 11] (Figure 16.1). Clinical signs primarily include hyperalgesia and hyperesthesia along the cranium, CCJ, and cervical spine, although signs consistent with cerebellar/brainstem dysfunction and/or a cervical myelopathy may also develop [12]. There are few clinical descriptions of CCJ anomalies in cats. In addition to cranium and cervical pain and feline‐specific manifestations of these, signs of brainstem dysfunction are also described [10].

The CCJ is best evaluated diagnostically using magnetic resonance imaging. This allows for a more comprehensive evaluation of the area for surgical planning than the use of CT imaging or radiographs alone, though CT imaging may be helpful in defining the anatomic relationship between the skull, atlas, and axis. Diagnostic characteristics for each disorder are outlined in Figures 16.2, 16.3 and 16.4.

Medical and surgical management may be used to treat CCJ anomalies. The former is generally employed first, particularly in patients with mild signs and those for whom medical management has not yet been employed [12]. Reports of limited progression of signs in a subset of CCJ anomaly patients and the potential for maintenance of a good to excellent quality of life without surgery, suggest the need to first consider medical management in these groups [12]. This includes the modulation of neuropathic pain (using medications such as gabapentin, pregabalin, amantadine, topiramate, along with acupuncture), the inhibition of cerebrospinal fluid production (e.g. using furosemide, acetazolamide, omeprazole), and the use of corticosteroids for their effects on the pain pathway, CSF production modulation, their anti‐inflammatory effects [3, 13, 14]. Indications for surgery, in turn, generally include progressive disease despite medical management, neuropathic pain that is moderate to severe, pain that is not responsive to analgesics, and neurologic deficits attributable to these disorders (e.g. C1–C5 myelopathy secondary to an atlantoaxial fibrous band). Factors such as age and general health/anesthesia risk must also be considered.

Surgical Anatomy

The CCJ refers to the junction between the head and neck, including the occipital bone, foramen magnum, atlas, and axis, along with the ligaments and soft tissues that support this junction (Figure 16.5). A single “occipito‐atlas‐axis joint cavity” passes through this area [15].

The muscles encountered as part of the dorsal surgical approach to this area include the occipitalis, the semispinalis capitis (biventer cervicis) and the underlying rectus capitis dorsalis muscles, joined by the median fibrous raphe. The cleidocephalicus and sternocephalicus are encountered along the cervical spine superficially [15].

The foramen magnum can vary widely in its size; in patients with occipital dysplasia, the foramen magnum can be “keyhole‐shaped” (i.e. narrower dorsal extension of the foramen magnum overlying the cerebellum), circular rather than oval, and/or asymmetrically shaped [15–17]. This is commonly found among brachycephalic and toy/small breed dogs [17]. The shape and size of the foramen magnum should be evaluated prior to surgery and can be assessed with CT and/or MR imaging. Of note, occipital dysplasia is not a pathologic finding but rather individual variation of “normal” [16–18]. The dorsal atlantooccipital membrane is encountered between the craniodorsal lamina of the atlas and the dorsal aspects of the foramen magnum [15]. Identifying this and maintaining it as an intact structure during drilling can help protect underlying neural structures. This membrane is often abnormally thickened in patients with CCJ anomalies [4]. The dorsal atlantoaxial membrane is encountered on approach to the dorsal junction of the atlas and axis, in turn [15]. Additionally, the dorsal atlantoaxial ligament spans between the caudal lamina of the atlas and the craniomedial aspect of the lamina of the axis. The nuchal ligament joins the caudal spinous process of the axis with the spinous processes of the first thoracic vertebra (T1) [15].

The cervical vertebral venous system is also to be considered when performing surgery in this area. This valveless venous system is continuous with the cranial venous sinuses and contains external and internal components. When approaching the CCJ dorsally, the veins of the dorsal external vertebral venous plexus are first encountered within the epaxial musculature (i.e. intervertebral and interspinous veins). These traverse ventrally through the ligamentum flavum, to enter the vertebral canal [9, 15]. The interarcuate branches of the internal vertebral plexus are then encountered underlying the ligamentum flavum at the atlantooccipital and atlantoaxial junctions. The bilateral basilar sinuses are located at the lateral aspects of the cervical vertebral column [15].

The paired cervical spinal branches of the vertebral artery perforate the dura, passing into the subarachnoid space at each intervertebral foramen and splitting into dorsal and ventral radicular arteries. The dorsal radicular artery and dorsal spinal artery may be seen if a durotomy is performed; care must be taken to preserve these to maintain adequate irrigation of the spinal cord. The occipital branch of the common carotid courses along the nuchal crest bilaterally. The caudal meningeal arteries branch from these vessels, enter the cranial cavity at the mastoid foramen, and arborize along the dura of the dorsocaudal aspects of the caudotentorial cranial cavity [15].

Patient Preparation and Positioning

Prior to anesthesia for surgery, a comprehensive treatment plan must be made considering the CCJ, generating an individualized surgical plan for each patient. Additionally, as part of anesthetic planning, a guarded endotracheal tube must be used to allow adequate positioning of patients for surgery in this area.

Once anesthetized, the patient is first shaved from the level of the bregma cranially through to the mid‐cervical spine (approximately C4/C5) caudally, and laterally overlying the temporal bones, the dorsal half of the pinnae and mid‐lateral cervical spine. The tips of the pinnae are restrained ventrally bilaterally in dogs with erect ears, either using a penetrating towel clamp or suture to attach these to the lateral aspect of the head, or by tucking them under the head in patients with downward sloping ears. If a fat graft is to be used, then an area overlying the wing of the ilium should be prepped to harvest the adipose tissue. This is usually done at the start of the procedure (see below).

The patient is then placed on the surgical table in a “sphinxlike” position (i.e. in sternal recumbency with limbs flexed alongside the body), with the head above the level of the thoracolumbar spine. The CCJ must be fully flexed for the first portion of the surgery, with the hard palate perpendicular to the cervical spine (i.e. nose perpendicular to the surgical table). This can be accomplished using commercially available positioning system or pegboard, or using a vacuum system such as the “Hug‐U‐Vac Surgical Positioning System” (www.vetorsolutions.com). Of note, a surgical assistant must be able to release CCJ from its flexed position at a later point in the surgery. To accomplish this, it is helpful to have a separately movable positioning device for the head, from that of the neck and body (e.g. Hug‐U‐Vac surgical head positioner, surgical tape across the bridge of the nose to the body/surgical table bilaterally). Regardless of the method of restraint used, care must be taken to avoid jugular vein pressure throughout the surgery; this will avoid the negative effects of jugular vein compression on intracranial pressure (i.e. Queckenstedt’s maneuver) [19, 20]. Once desirable positioning is achieved, the area is aseptically prepped.

Surgical Technique

If a fat graft is to be used, this must be harvested first, using separate gloves and instruments [1]. The harvested tissue must be large enough to easily cover the surgery site, with accommodation for it shrinking by a third of its size during revascularization, and should be of a thickness of approximately 0.5–0.8 cm. The fat graft is wrapped in saline‐soaked gauze post‐harvesting and is set aside for later use. It is then rinsed again in saline prior to its use [21, 22].

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