Clinical manifestations

Presentation and course

Basilar impression demonstrates a wide range and spectrum of clinical manifestations. Depending on the degree of impression, or specifically, invagination, clinical manifestation may range from asymptomatic with a normal physical exam to severe symptomatology requiring surgical intervention (35). In familial cases of primary basilar impression, this skeletal anomaly may manifest as a shortened neck accompanied by progressive stiffness and weakness of the extremities (46). As the contents of the brainstem invaginate into the foramen magnum, patients may show strabismus and nystagmus. Paralysis from cervical cord compression has also been noted (45). Respiratory depression, caused by compression of the brainstem, has also been described (53). In an 8-patient case series of basilar impression with associated occipitalization of the atlas, major symptoms at onset were nuchal pain (100%), vertigo (85%), hyposthenia (62%), paresthesia (37%), unsteady gait (25%), dysphagia (25%), dysarthria (25%), and diplopia (12%) (03). In this series, main neurologic signs included spastic tetraparesis (100%), hypesthesia (75%), sphincteral troubles (50%), and symptoms localized to dysfunction of cranial nerves X (25%), V (12%), and XII (12%). Furthermore, 75% of the patients studied had symptoms that were progressive, whereas 25% had symptoms that were intermittently progressive and remitting (03).

Multiple structural abnormalities and syndrome constellations have been associated with basilar impression. Bares, in 1975, classified this entity and its associated congenital anomalies into four clinicopathologic groups: (1) parenchymal abnormalities with pathogenetic significance, (2) skeletal abnormalities with pathogenetic significance, (3) skeletal abnormalities with dubious pathogenetic significance, and (4) distant anomalies (02). As the literature continues to expand, some of these anomaly clusters as well as others have been described. They include syringobulbia/syringomyelia (23), cervical arachnoidal cyst (19), hypoplasia of the paravertebral cervical musculature, the Klippel-Feil syndrome (46; 45), Chiari malformation (11; 38), Goldenhar syndrome (24), and SUNCT syndrome (52). Paget disease leading to secondary basilar impression has been described; in this case, the resultant basilar impression caused obstruction of cerebrospinal fluid through the basilar cisterns, resulting in ventricular enlargement associated with a syndrome resembling normal pressure hydrocephalus (gait disturbance, incontinence, and dementia) (50). Some cases suggest that basilar impression may predispose to an increased risk of vertebrobasilar vascular disease, and consequently, vertebrobasilar stroke (49; 29).

Prognosis and complications

Prognosis depends primarily on the degree of basilar impression and of symptomatology, which for secondary cases usually reflect the severity and progression of the underlying disorder. In general, prognosis depends on the surgical correctability of the underlying bony deformity. Surgical intervention aimed at decompression has been met with at least favorable immediate outcomes (see below).

Biological basis

Etiology and pathogenesis

From a clinical-etiologic perspective, basilar impression can be classified as either primary, in which there is no discernible underlying intrinsic abnormalities of bone, or as secondary due to the effects of bone softening caused by an underlying disease, although there are inherent nuances to such a classification scheme. As the literature on basilar impression evolves, the term “basilar invagination” has been used and deemed as synonymous in describing this entity. Although the term “platybasia” has also been used synonymously, it is not currently recognized as such. Rather, platybasia refers to a skull base with an abnormally obtuse angle between the plane of the clivus and the plane of the anterior fossa. It is a finding that carries no particular clinical significance, although it can occur in basilar impression (47). Furthermore, accepted diagnostic criteria for basilar impression include progressive deformity of the occipital bone in a setting of odontoid compression of the neuraxis; as such, odontoid compression of the neuraxis in the region of the foramen magnum without associated occipital bone deformity should be referred to as “odontoid invagination” (37). Attempts at morphometric characterization are occasionally encountered in the literature, as are attempts to group basilar impression with more commonly encountered, coexistent conditions such as Chiari I malformations (35).

Primary basilar impression is widely regarded as a familial, genetic, and developmental abnormality of bone development. As a primary disorder, it has generally demonstrated an autosomal dominant inheritance pattern, with incomplete penetrance, as first described by Bull and colleagues (06). Additionally, it has been observed in a small subset of patients (3%) with the 22q11.2 deletion syndrome (48). Primary impression is essentially a diagnosis of exclusion, in which the skeletal abnormalities, whether or not sufficient to cause symptoms, occur in the absence of intrinsic bone disease or of diseases that may affect the bone.

In secondary basilar impression, the skeletal abnormalities seen are generally attributed to metabolic or developmental abnormalities of bone that give rise to bone softening. This phenomenon was first postulated by Hurwitz and Shepherd in 1966, in which families with metabolic bone diseases, particularly vitamin D-resistant rickets, were studied (27). Secondary basilar impression also occurs in Paget disease, in which hearing loss is reportedly common (16). Other causes of secondary basilar impression share the common etiopathogenic denominator of bone weakness and softening and include hyperparathyroidism, osteoporosis, osteogenesis imperfecta (32), osteomalacia secondary to nutritional deficiencies (08), and tuberculosis infection affecting the craniovertebral junction (36).

Despite this division of primary and secondary classifications, it is important to note that etiologic factors of bone softening that occur in early childhood such as malabsorption may disappear in adult life. However, the structural evidence of basilar impression persists. Thus, in the absence of detectable bone or other metabolic diseases, basilar impression in such patients may be classified as primary, whereas in reality it reflects a secondary cause (26). Further confounding factors for the etiologic classification of basilar impression into primary and secondary manifest is the observation that adults classified as having primary basilar impression are four times more likely to have a history of birth injury (due to prolonged labor, high birth weight, forceps, or breech delivery) than a control group (12).

In a report of institutional and personal experience, Goel categorized patient cases into two groups based on clinical and radiographic parameters (20). The common pathogenesis and clinical features of patients in group A appeared to be related to mechanical instability, whereas the pathogenesis of those categorized into group B reflected embryonic dysgenesis. Such clinico-etiologic division may provide a basis for rational approach to surgical patient management (20).

Three nonexclusive mechanisms of pathogenesis have been proposed for the structural and symptomatologic manifestations of basilar impression. The first is expression of faulty genetic control and abnormalities of migration, postulating that a cranio-cervical growth collision in embryonic development gives rise to abnormalities seen in primary basilar impression, simultaneously accounting for the common association of this entity with the Chiari malformation (46; 11). Secondly, reduction in volume of the posterior fossa results in direct compression caused by the surrounding bone structures. Indeed, skeletal derangement of the craniovertebral junction may cause compression of the spinal cord, brain stem, cerebellum, 4th ventricle, and may even give rise to cerebellar herniation. Osseous compression may subsequently lead to reactive arachnoiditis and obstruction of CSF flow. Those cases involving softening of bone have been attributed to a combined effect of softening at the base of the skull, increased weight of the cranium, and upward thrust of the cervical spine (16). Symptomatology has also been attributed to aqueductal stenosis secondary to compression (51). A third pathogenic mechanism is vasculature compromise of the brainstem and cervical region. It has been observed that head-turning can result in signs compatible with vertebral artery obstruction and insufficiency (30).

Epidemiology

As basilar impression is characterized by a broad spectrum of associated anomalies and severity; the true incidence is not known; prevalence is estimated at 1% of the general population (43). Patients with primary basilar impression may be completely asymptomatic (06), and those with basilar impression resulting from underlying abnormalities of bone may not exhibit any notable signs or symptoms. No definitive patterns as to sex, ethnic, or racial predilection have been established, and cases have been reported from all over the world. The onset of symptoms usually manifest beginning with the second decade of life and onward, being more frequent in older than younger individuals because the diagnostic bone deformity may occur long before the onset of symptoms (32).

Prevention

Not applicable. Although primary basilar impression is generally considered a congenital developmental abnormality, some factors causing birth injury such as prolonged labor, forceps, and breech delivery have been suggested as potential contributing etiologic factors (12). Cases of secondary basilar impression reflect the evolving severity of other underlying diseases. Awareness of the possibility of basilar impression patients with upper cervical, brainstem, and cranial nerve symptoms, as well as in patients with signs and symptoms suggestive of a chronic degenerative disease is critical as viable surgical treatment options may be available.

Differential diagnosis

Associated or underlying disorders

Diagnostic workup

Basilar impression is essentially a radiographic diagnosis that is rendered by meeting specific criteria. Various reference ranges using Chamberlain’s line, McGregor’s line, Bull’s angle, and McRae’s line have been proposed (06). Although these anatomic parameters were initially defined using lateral skull x-ray, with the development of sophisticated radiographic procedures, CT scan and MRI have emerged as providing more diagnostic information. Several studies have suggested that the modern gold standard for evaluating basilar impression is MRI (32; 13). MRI generally permits artifact-free imaging of the posterior fossa and of the craniocervical region in three planes and, thus, facilitates a better assessment of the neural structures directly involved in basilar impression. Furthermore, functional MRI has been reported to be superior to static MRI in evaluating patients with rheumatoid arthritis with basilar impression, as it provides more clinically useful data regarding the nature and extent of cord impingement into the foramen magnum, cord compression, as well as slippage and angulation of the vertebrae (01). Xu and Gong proposed that clivus height measurement, as assessed by CT, can reproducibly establish a diagnosis of basilar impression with good intra- and interobserver agreement (54). In a separate analysis, they also concluded that the clivodens angle (CDA), which is the angle formed at the intersection of a line that runs along the long axis of the clivus and one that runs along the long axis of the dens, can be utilized for diagnosing basilar impression (55). The clivopalate angle, as determined from a sagittal MRI view, is of particular value according to studies by Ma and colleagues (39). The analysis of Botelho and colleagues suggest that basilar impression is essentially a more pronounced form of craniocervical kyphosis (05). de Brito Silva and colleagues demonstrated that craniometric parameters on MRI studies show excellent intraobserver reproducibility and interobserver agreement in the diagnosis of basilar impression (13).

The foramen magnum angle, which is formed by Chamberlain’s line and McRae’s line, has been demonstrated to be substantially larger in basilar impression (approximately 22%) when compared to unaffected patients (approximately 6%) (33). It has been deemed to be an especially useful parameter in assessing type B basilar impression (44; 04).

In the diagnostic workup, general laboratory data are usually obtained to exclude metabolic causes that may contribute to the patient’s symptoms. These include electrolytes, particularly calcium and phosphorous, and alkaline phosphatase. Additional laboratory studies, including liver and thyroid function tests, urinary electrolyte studies, and creatine phosphokinase may be indicated based on clinical scenario (08). Neurophysiologic exams may be indicated in the assessment of somatosensory evoked potentials to document possible conduction blocks (19). However, the diagnosis is ultimately rendered by radiographic studies (17; 35).

Management

The primary mode of management is surgery for cases that are clinically indicated. Various techniques and strategies have been employed, including endoscopic transnasal decompression (25), transoral atlantoaxial reduction (58), and combined approaches that also involve posterior fusion (34). There are frequent contributions to the literature that describe experience with clinical outcomes of traction-type treatments aimed at achieving acceptable alignment in individual cases (31; 56). Additionally, more substantial case series have been published, advocating for early intervention with occipitocervical bracing (42).

Surgical intervention is generally indicated in patients with progressive spinal deformity and in those with ataxia, nystagmus, apnea, and sensory, motor, or cranial nerve dysfunction (15). The surgical approach to each patient must be individualized, with the ultimate aim being 2-fold: (1) to relieve compression of the neuraxis at the affected site, and (2) to prevent additional compression in the future (50).

Ibrahim and Crockard, in a retrospective review of 20 patients having undergone surgical intervention for basilar impression over a 21-year period, documented sustained long-term benefits in 60% of the patients, with 15% showing no improvement and 25% experiencing recurrence of brainstem compression symptoms or dying (28). Some spinal changes characteristically associated with basilar impression, such as short neck, torticollis, and cervical spondylotic changes, are potentially reversible after decompression and stabilization of the craniovertebral junction (20).

A method of treatment that involves distraction of the atlantoaxial joint using specifically designed spiked spacers has been described, with report of experience in 11 cases (21). Follow-up, ranging from 8 to 40 months, demonstrated neurologic improvement with atlantoaxial stabilization and bone fusion. Additionally, the division of patients with basilar impression into groups A and B, as proposed, appears to provide a basis for surgical management (20). A case series documented the outcomes of 15 patients who underwent endoscopic image-guided transcervical odontoidectomy (ETO) (10).

The literature continues to evolve with studies and metaanalyses characterizing the effectiveness of posterior fossa decompressive procedures. Case series, in-depth analyses of the literature and metaanalyses regarding posterior decompression techniques have generally reported positive outcomes (14; 07; 58; 34).

Outcomes

In general, outcomes depend on the surgical correctability of the underlying bony deformity. Surgical intervention aimed at decompression has been met with favorable outcomes. Modifications of previously described techniques continue to be attempted and studied and have been met with some degree of success.

Special considerations

Pregnancy

No information is available regarding the risk to the fetus or the mother during pregnancy.

Anesthesia

Of primary consideration for patients undergoing general anesthesia is special attention to movements of the neck. In a case report, a 9-year-old patient underwent endotracheal intubation for an appendectomy procedure and in the postoperative period demonstrated slurred speech and ataxia, for which radiographic studies revealed infarction of the right cerebellar hemisphere. As later follow-up revealed basilar impression, the conclusion was made that the associated cervical hyperextension of the neck during intubation, coupled with muscle relaxation, resulted in trauma to the vertebral arteries (57). This case illustrates that airway management must be carefully executed in patients with basilar impression; additionally, the possibility of basilar impression must be considered in the patient without obvious risk factors for cerebral vascular disease, but in whom vertebral artery strokes occur following intubation.