Background

Prior to the development of modern imaging and operative techniques, a diagnosis of VBSC was typically made post mortem. The most common causes of VBSC at the CVJ include inflammatory conditions, such as rheumatoid arthritis; trauma; and tumors. In some cases, VBSC is associated with CM-I. Efforts to surgically treat VBSC associated with CM-I began with posterior decompression with or without fusion and advanced to include direct ventral decompression. These techniques were pioneered by surgeons, such as Kanavel, who, in 1917, used the transoral route to remove a bullet lodged between the atlas and clivus. Advances in neuroimaging and surgical technique (eg, transnasal endoscopy) have broadened the armamentarium for approaches to these usually complex problems. Factors that influence the specific surgical treatment include

• 1.
  

  Patient age

  
  
• 2.
  

  Symptomatology

  
  
• 3.
  

  Etiology of the pathologic process

  
  
• 4.
  

  Reducibility of the compressive lesion.

Ventral Brainstem Compression in Chiari Malformation, Type I

Although CM-I has been recognized as a common entity, identified in 1% to 5% of all patients undergoing head and cervical MRIs, the identification of VBSC is less common. In a review of 364 symptomatic adults with CM-I, however, Milhorat and colleagues identified an abnormally retroflexed odontoid process in 26% and basilar invagination (BI) in 12%. In their study of 38 patients, Grabb and colleagues quantified the extent of VBSC in 40 pediatric and young adult patients through measurement of a line perpendicular to the basion-C2 line (B-C2), termed the pB-C2 ( Fig. 1 ). The group found that all patients with a pB-C2 measurement of less than 9 mm were treated successfully with posterior fossa decompression alone, despite the presence of VBSC on subjective evaluation. A subset of patients with pB-C2 measurements greater than 9 mm required occipitocervical stabilization with or without ventral decompression in addition to posterior fossa decompression. These investigators also noted that pB-C2 did not increase with age. Ladner and colleagues described ventral canal encroachment in a pediatric population as a pB-C2 greater than 3 mm. In their series, only 1.3% of patients demonstrated a pB-C2 greater than 9 mm and none required ventral decompression.

Sketch of a sagittal view of the craniocervical junction showing the B-C2 line, drawn from the basion to the inferoposterior aspect of the C2 body, and a line perpendicular to this line, pB-C2, drawn through the odontoid tip to the ventral cervicomedullary dura. The distance of pB-C2 is then measured in millimeters between the thick and the thin arrows .

( From Grabb PA, Mapstone TB, Oakes WJ. Ventral brain stem compression in pediatric and young adult patients with Chiari I malformations. Neurosurgery 1999;44(3):522 [discussion: 527–8]; with permission.)

Incidence of Ventral Brainstem Compression in Basilar Invagination and Related Disorders

BI, or basilar impression, is an uncommon condition in the general population but it may be identified in as many 25% to 35% patients with CM-I. BI is most commonly a developmental anomaly of the CVJ in which the odontoid process prolapses into or through the foramen magnum. It is associated with several underlying congenital, metabolic, and inflammatory conditions ( Box 1 ). BI often coexists with osseous anomalies of the CVJ, including atlanto-occipital assimilation; incomplete ring of C1; and hypoplasia of the basiocciput, occipital condyles, and atlas.

Clinical Examination

In the context of CM-I, patients with VBSC may present with the classic signs and symptoms of reversible occipitocervical headache or neck pain. The presentation may be insidious or rapid, and false localizing signs may be present. There are numerous studies discussing the spectrum of symptoms encountered in CM-I but also VBSC. These symptoms can range from a mild basilar headache to progressive cranial neuropathies and wheelchair dependence. As recently described by Menezes, however, this population is more likely to also demonstrate long tract signs, myelopathy, brainstem dysfunction, and lower cranial nerve abnormalities. In their series, 3 of 84 patients (3.6%) suffered facial pain and another 6 (7.1%) experienced facial hypalgesia. Urinary frequency and incontinence were present in 14 of 84 patients (16.7%). Box 2 shows typical clinical signs/symptoms of VBSC associated with CVJ anomalies.

Radiographic Findings

Both pediatric and adult patients who present with signs/symptoms consistent with CM-I or VBSC should be evaluated with MRI. CM-I is diagnosed when the cerebellar tonsils herniate greater than or equal to 5 mm below the level of McRae line (drawn from the basion to the opisthion). When CM-I is identified, imaging of the entire spinal cord should be reviewed for the presence of syringomyelia. BI has been assessed through several measures, including the relationship of the odontoid tip to Chamberlain line (drawn from the posterior hard palate to the opisthion), McRae line, Wackenheim clival canal line (drawn along the sagittal plane of the clivus and extending into the cervical canal), the length of the clivus (measured from the top of the dorsum sellae to the basion), the anteroposterior width of the foramen magnum, and the basal angle ( Fig. 2 ).

Commonly assessed craniovertebral metrics.

( From Herring JA. Tachdjian’s pediatric orthopaedics. 5th edition. Elsevier; 2013; with permission.)

In the presence of VBSC, additional imaging should include flexion/extension occipitocervical radiographs to evaluate for pathologic motion. Noncontrast CT better defines the osseous anatomy and should be evaluated if posterior instrumentation is planned. In addition, the vascular imaging, either with MRI or CT, should be strongly considered prior to surgical intervention, because the course of the vertebral arteries is commonly anomalous in the context of VBSC.

As described by Grabb and colleagues, using a midsagittal MRI, the B-C2 can be drawn from the basion to the inferoposterior aspect of the C2 body. The distance of the longest pB-C2 from the B-C2 line through the odontoid to the ventral dura, may serve as an objective measurement of encroachment by the odontoid and any investing tissues into the foramen magnum or rostral spinal canal (see Fig. 2 ).