Basilar Invagination and Platybasia

Basilar invagination and platybasia both refer to occipital anomalies described by Chamberlain in 1938.1 Unfortunately, Chamberlain and several other authors utilized both terms interchangeably, creating confusion over the years. Basilar invagination is a congenital form of occipital hypoplasia manifested by prolapse of the spinal column into the skull base ( Fig. 6.1 ). Platybasia is an anthropological term describing an abnormally obtuse (>140 degrees) angle between the anterior skull base and the clivus—the normal range of which lies between 120 and 140 degrees ( Fig. 6.2 ).1,2 Platybasia alone causes no neurological symptoms and may be associated with other conditions, such as Klippel-Feil syndrome and occipitalization of the atlas. It should be noted that basilar invagination is distinct from basilar impression—the latter is an acquired condition secondary to osseous disorders.3

Occipital hypoplasia may affect the basiocciput, the exoccipital bone, and the squamous occipital bone. Two extreme forms of invagination have been identified. Anterior (or ventral) invagination results when there is shortening of the basiocciput and clivus with concomitant platybasia and a shallow posterior fossa, whereas paramedian invagination is a consequence of hypoplasia of the exoccipital bones with normal clival development and is often accompanied by an element of posterior displacement.4 Paramedian invagination may be unilateral or bilateral; it may be associated with torticollis (when unilateral), compensatory downward curvature of the squamous occipital bone, and condylar hypoplasia. Usually features of both forms of basilar invagination are combined to produce a mixed clinical picture.3 Basilar invagination is occasionally associated with Down syndrome and skeletal dysplasia as well as segmentation defects. It should not, however, be confused with other proatlas segmentation anomalies that may include the anterior masses and cause cervicomedullary compression; these are much rarer, have a different embryological basis, and are treated differently.5

The radiological findings of basilar invagination were classically described using standard radiographs. Plain films have been supplanted by more sophisticated imaging techniques but the reference perspectives originally developed for them may still be applied to sagittal and coronal reconstructions of high-quality axial computed tomography (CT) scans.6 On coronal views, invagination is suggested by cephalad displacement of the atlantoaxial articulations—the tip of the dens should extend no more than 10 mm rostral to the bimastoid line.7 Invagination is more clearly appreciated on sagittal reconstructions: the tip of the odontoid should project no more than 2.5 mm above Chamberlain′s line (hard palate to opisthion). Wackenheim′s line, which courses down the posterior clivus, represents another classic means of diagnosing invagination. Posterior protrusion of the odontoid, with respect to Wackenheim′s clival line, is abnormal.1,8 The presence of these abnormal skeletal relationships mandates magnetic resonance imaging (MRI) of the CVJ to obtain a clear understanding of their relationship to the neural structures.

Symptoms of basilar invagination ensue when the mid-sagittal diameter of the foramen magnum is reduced to less than 20 mm (normal range, 35 6 4 mm).3 Most patients present with neck pain (80 to 85%), whereas myelopathy is by far the most common finding on clinical examination.2 A variant of a central cord syndrome related to venous stagnation in the lower cord may occur because these vessels, which normally drain cephalad toward the CVJ, are compressed. Additionally, myriad neuro-ophthalmological signs, such as internuclear ophthalmoplegia and downbeat nystagmus, are possible.3

Specific details of the surgical management of the pathologies described herein are discussed elsewhere in this textbook, yet the principles of treatment of basilar invagination lie in the assessment of reducibility and the degree and location of the neural compression at the CVJ. Low weight traction for 48 to 72 hours is an important first step of treatment. Reduction can be achieved in ~20 to 30% of patients with acquired cranial settling. Although no precise data define it, the overall reduction rate is clearly age-dependent, and ~80% of patients age 14 years or younger will be satisfactorily reduced with traction.3 If reduction is achieved, occipitocervical fusion will prevent recurrent invagination. Irreducible lesions with significant anterior neural compression usually require a decompression through the endonasal or transoral route followed by posterior occipitocervical fusion. Some patients may require additional rostral or caudal extensions for adequate anterior decompression.9 Certainly, any persistent symptomatic posterior compression with or without reduction should be treated with a decompression of the foramen magnum, posterior fossa, or upper cervical spine, as necessary.

Vertebralization of the Occiput (Occipital Vertebrae)

Other segmentation defects of the occiput occur and are collectively called “manifestations of occipital vertebrae.”3 They derive from the failure of the third occipital sclerotome and proatlas to fuse with the skull base during development. Rarely do they form completely independent spinal segments; most frequently, they are noted as osseous processes around the foramen magnum.10 Transverse fissures of the basiocciput, third occipital condyle, paracondylic process, epitransverse process, and bipartite atlantal facets are all part of this group. Technically, persistent ossiculum terminale is embryologically related, but this entity is grouped under anomalies of the dens for convenience and historical reasons ( Figs. 6.3, 6.4, and 6.5 ). Vertebralization of the occiput should not be confused with assimilation of the atlas (fusion of C1 sclerotome with the proatlas). The distinguishing feature between these two is the presence in the latter of a foramen through which the vertebral artery and suboccipital nerve traverse.3,10

Unilateral or bilateral transverse fissures of the basiocciput ( Fig. 6.3 ) are transverse clefts in the region of the clivus resulting from incomplete assimilation of occipital somites. These anomalies may be detected by CT as radiolucent clefts in the clivus, occasionally extending into the hypoglossal canal, in which case the term “bipartite hypoglossal canal” is applied. The third occipital condyle is an osseous protuberance coursing from the basiocciput along the anterior margin of the foramen magnum, typically articulating with the anterior arch of C1 or the dens. If multiple, these ossicles are called basilar processes. This anomalous articulation is commonly associated with basilar invagination, and it often further contributes to neural compression and limitation of movement at the occipital-C1 junction.11

A plethora of exotic atlanto-occipital calcifications, pseudoarticulations, and fusions have been described, and new case reports appear regularly.12,13 Usually these malformations involve the persistence of bony growths protruding lateral to the occipital condyles to the transverse process of C1. Although these osseous masses have been termed “paramastoid,” “paraoccipital,” or “anomalies of the styloid process,” they occur because of the persistence of the transverse processes of the proatlas and, thus, have no embryological relation to the styloid or mastoid processes.12 They may receive different names according to size, the mildest form of which is a paracondylic tubercle, a small protuberance of the skull base ( Fig. 6.4B ). Paracondylic processes articulate or are fused with the C1 transverse process ( Fig. 6.4C ). When an independent ossicle lies between and articulates with a tubercle and the transverse process of C1, it is called a paracondylic “mass” ( Fig. 6.4D ). These entities can even lead to complete fusion via a laterally placed osseous trabecula ( Fig. 6.4E ). Epitransverse processes are similar to paracondylic processes except that they are rostral extensions of the C1 transverse processes and articulate with either a paracondylic process or mass ( Fig. 6.4F ). It is evident that the different names describe only nuances of the same problem and can create some confusion. Nonetheless, all of these entities are usually asymptomatic, although such abnormal articulations or fusions may rarely be associated with headache and limitation of motion, necessitating surgical removal.14

Bipartite superior atlantal facets are defined by the presence of a small cleft in the superior articulating facets of the atlas. A corresponding division of the occipital condyles may occur in conjunction with these clefts. This anomaly results from the failure of the proatlas to fuse with the lateral part of the C1 sclerotome. The normal process leads to the formation of the C1 lateral mass; when there is incomplete fusion, the vestiges of the superior aspect of the C1 sclerotome give rise to these supernumerary articular facets, with a typically larger anterior surface ( Fig. 6.5 ).15

Occipital Condylar Hypoplasia

The occipital condyles are derived from the proatlas. Flattened hypoplastic occipital condyles result in superior migration of the atlas and axis relative to the skull base.15 Hypoplastic occipital condyles may occur unilaterally or bilaterally, and they often accompany paramedian basilar invagination. Both disorders result from hypoplasia of the occipital somites, which form the lateral aspect of the foramen magnum.3 Although this disorder may occur in isolation, it has been associated with Morquio disease, Conradi syndrome, and spondyloepiphyseal dysplasia. Radiological confirmation of occipital condylar hypoplasia is confirmed by an abnormal increase in the Schmidt-Fischer angle. Measured using an open mouth odontoid view or coronal CT reconstructions, the Schmidt-Fischer angle is defined as the angle at the intersection of two lines passing through the occipitoatlantal joints (normal range, 124–127 degrees) ( Fig. 6.6 ).8

Assimilation of the Atlas

Atlas assimilation, also called occipitalization of the atlas, is one of the most common anomalies of the CVJ, affecting 0.14 to 2.76% of individuals. Atlas assimilation is defined by congenital osseous fusion between the skull base and the atlas; relative movement alone is not enough to make this diagnosis. This malformation is caused by persistence of continuity between the hypochordal arch of the atlas and the basal plate of occipital sclerotomes.3 Assimilation of the atlas was classically thought to primarily involve the anterior arch of C1 and the foramen magnum with the cortex and/or medullary component of the bones being in continuity; however, at times these may be joined only by a thin bony plate.16 A recent compilation of cases studied with CT scans by Gholve and colleagues revealed that there are a variety of sites that may fuse alone or in a combined fashion. The condylar fusions are usually asymmetrical, with one side having a cephalad horizontal facet and the opposite side with an inferior oblique facet. Often a remnant of the posterior arch of C1 can be identified along the posterior aspect of the foramen magnum.12,17 Assimilation of the atlas may also be associated with other conditions, such as Klippel-Feil syndrome, basilar invagination, and Chiari malformations.16 Rarely, cervicomedullary compression may ensue from congenital canal stenosis or canal compromise, secondary to a thickened posterior dural band.

Atlas assimilation associated with posterior displacement of the dens producing myelopathy is the most common presentation. Neurological symptoms typically arise when the atlantodental interval is ≥4 mm.2,5,16 Many patients suffer from neck pain. Myriad other symptoms may affect adults and children, including ataxia, tinnitus, nystagmus, dizziness, and bulbar dysfunction. Acute onset often occurs when atlantoaxial instability is associated with the congenital anomaly.3,16,17

Atlas assimilation per se requires no treatment; when any of the previously mentioned symptoms develop, surgical treatment aimed at decompression and stabilization is necessary. The treatment of these specific conditions (atlantoaxial instability, basilar invagination, dorsal compression, and Chiari malformation) is addressed in separate sections of this chapter.