Basilar impression …

Brian H Le MD

Developmental Malformations

Updated 01.16.2026
Released 12.28.2004
Expires For CME 01.16.2029

Basilar impression

Author: Brian H Le MD

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Editor: Harvey B Sarnat MD FRCPC MS

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Basilar impression

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Introduction

Overview

Basilar impression is a skeletal malformation characterized by translocation of the upper cervical spine and clivus into the foramen magnum, giving rise to various symptoms related to brainstem, upper spinal cord, and cerebellar compression. It is generally classified as primary (familial), in which the defect is unaccompanied by intrinsic abnormalities of bone, or as secondary, reflecting an intrinsic bone defect, as is the case in osteogenesis imperfecta. In this article, the author provides a review on basilar impression, with a discussion of additional experiences on diagnostic imaging criteria and surgical management.

Key points

	• Basilar impression is a skeletal malformation in which the upper cervical spine and clivus demonstrate translocation into the foramen magnum.
	• Depending on the degree of osseous invagination, clinical manifestations of basilar impression vary from asymptomatic to severe dysfunction.
	• Primary basilar impression, a diagnosis of exclusion, is generally considered a familial developmental abnormality showing mostly an autosomal dominant inheritance pattern with incomplete penetrance. Secondary basilar impression is attributed to metabolic or developmental abnormalities of bone that give rise to osseous softening, such as Paget disease and osteogenesis imperfecta.
	• Management of basilar impression rests primarily with surgery for cases where there is progressive spinal deformity, ataxia, nystagmus, apnea, or cranial nerve dysfunction, aiming to relieve compression of the neuraxis at the affected site and to prevent future additional compression. Treatment plans may be modified based on imaging characteristics of joint indices. Posterior decompression techniques have generally reported success.
	• Criteria for the radiographic diagnosis of basilar impression continue to evolve and be validated as imaging techniques are more readily accessible. The clivopalate angle, clivodens angle, clivoaxial angle, and foramen magnum angle as measured on midsagittal MRI images are of particular diagnostic value.

Historical note and terminology

Basilar impression, a skeletal malformation involving the relation between the cervical spine and the base of the skull, was first described as a postmortem anatomic entity by anatomists in 1790. The first ante-mortem cases in patients were described in Europe in the early 20th century (51; 26). Basilar impression emerged in the American literature in 1939, with the establishment of diagnostic criteria by Chamberlain, who defined a line, to be known as the Chamberlain line, drawn from the dorsal margin of the hard palate to the dorsal tip of the foramen magnum, caudal to which all parts of the atlas and axis should lie (11). In 1948, McGregor proposed a modification of the Chamberlain line by defining a line drawn from the upper surface of the posterior edge of the hard palate to the most caudal point of the occipital curve, to be known as the McGregor line (43). Subsequently, McRae in 1953 proposed another landmark below which the odontoid tip should lie. Known as the McRae line, this is the line drawn from the anterior border to the posterior border of the foramen magnum (44). In 1955, Bull and colleagues performed a comparative study of 120 patients, measuring the distance of the odontoid tip above the Chamberlain line and McGregor line. In addition, the investigators proposed a new parameter to diagnose basilar impression. Referred to as the Bull angle, this is the angle between the plane of the hard palate and the line formed by joining the midpoints of the anterior and posterior arches of the atlas in a lateral radiograph of the skull and cervical spine (07). This study established general means for these measurements, postulating that exceeding three standard deviations in any one of these criteria is sufficient to diagnose basilar impression. Studies have also described pathophysiologic correlations based on radiographic morphometric characteristics (19). Radiographic re-examination of this entity has proposed new diagnostic criteria (60; 61), and supplemental imaging parameters have been proposed (42; 20). Additionally, it has also been observed that craniocervical kyphosis is typically more pronounced in basilar impression than in normal subjects (06). Craniometric parameters continue to be explored radiographically, including the cephalic index, with diagnostic criteria showing good intraobserver reproducibility and interobserver agreement (15; 48; 08). The World Federation of Neurological Societies’ Spine Committee specifically reaffirmed the value of the more traditional parameters for diagnosis, specifically McRae line and Chamberlain line (62).

Basilar impression is divided into two basic types: type A (sometimes also referred to as type I) is accompanied by atlantoaxial instability and is usually associated with ligament failure, whereas type B (also referred by some entities as type II) has atlantoaxial stability and is commonly associated with developmental malformation of the skull base (35; 62).

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 (37). 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 (51). 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 (49). Respiratory depression, caused by compression of the brainstem, has also been described (59). 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 (25), cervical arachnoidal cyst (21), hypoplasia of the paravertebral cervical musculature, the Klippel-Feil syndrome (51; 49), Chiari malformation (13; 41), Goldenhar syndrome (26), and SUNCT syndrome (58). 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) (55). Some cases suggest that basilar impression may predispose to an increased risk of vertebrobasilar vascular disease, and consequently, vertebrobasilar stroke (54; 31). Basilar impression occurring simultaneously with Chiari 1 malformation has also been reported with some frequency and poses additional challenges (38).

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.

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 (52). 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” (40). 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 (37).

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 (07). Additionally, it has been observed in a small subset of patients (3%) with the 22q11.2 deletion syndrome (53). 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 (29). Secondary basilar impression also occurs in Paget disease, in which hearing loss is reportedly common (18). Other causes of secondary basilar impression share the common etiopathogenic denominator of bone weakness and softening and include hyperparathyroidism, osteoporosis, osteogenesis imperfecta (34), osteomalacia secondary to nutritional deficiencies (10), and tuberculosis infection affecting the craniovertebral junction (39). Smeets and colleagues reported a case of basilar impression due to calcium pyrophosphate dihydrate (CPPD) deposition at the craniocervical junction, causing vertebral artery dissection and resultant posterior fossa hemorrhage (57).

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 (28). 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 (14).

In a report of institutional and personal experience, Goel categorized patient cases into two groups based on clinical and radiographic parameters (22). 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 (22).

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 (51; 13). 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, fourth 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 (18). Symptomatology has also been attributed to aqueductal stenosis secondary to compression (56). 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 (32).

Diagnostic workup

Basilar impression is essentially a radiographic diagnosis that is rendered by meeting specific criteria. The World Federation of Neurological Societies’ Spine Committee published consensus recommendations for diagnosis and classifications based on radiographic findings (62). Of note, radiographic assessments, regardless of the imaging modality used, should be primarily based on images at the midsagittal plane.

Among the historical reference range parameters used to diagnose basilar impression are those involving measurements relative to Chamberlain line, McGregor line, Bull angle, and McRae line (previously discussed in Historical note and terminology) (07). Several studies have suggested that the modern gold standard for evaluating basilar impression is MRI (34; 15). 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 (60). In a separate analysis, they also concluded that the clivodens angle, 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 (61). The clivopalate angle, as determined from a sagittal MRI view, is of particular value according to studies by Ma and colleagues (42). The analysis of Botelho and colleagues suggest that basilar impression is essentially a more pronounced form of craniocervical kyphosis (06). 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 (15).

The foramen magnum angle, which is formed by the Chamberlain line and McRae line, has been demonstrated to be substantially larger in basilar impression (approximately 22%) when compared to unaffected patients (approximately 6%) (35). It has been deemed to be an especially useful parameter in assessing type B basilar impression (47; 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 (10). Neurophysiologic exams may be indicated in the assessment of somatosensory evoked potentials to document possible conduction blocks (21). However, the diagnosis is ultimately rendered by radiographic studies (19; 37; 62).

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 (27), transoral atlantoaxial reduction (65), and combined approaches that also involve posterior fusion (36). 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 (33; 63). Additionally, more substantial case series have been published, advocating for early intervention with occipitocervical bracing (45).

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 (17). 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 (55).

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 (30). 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 (22).

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 (23). 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 (22). A case series documented the outcomes of 15 patients who underwent endoscopic image-guided transcervical odontoidectomy (ETO) (12).

The literature continues to evolve with studies and meta-analyses characterizing the effectiveness of posterior fossa decompressive procedures. Case series, in-depth analyses of the literature and meta-analyses regarding posterior decompression techniques have generally reported positive outcomes (16; 09; 65; 36). An individualized, comprehensive, and integrated management approach that involves multiple modalities, including physiotherapy, is generally recognized as important to maintaining or improving quality of life (50). Based on review of literature and experiences over the past decade, the World Federation of Neurological Societies’ Spine Committee issued recommendations based principally on the extent of compression and atlantoaxial dislocation, with the objectives of optimizing stability and, as necessary, facilitating decompression (05; 62).

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.

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Contributors

All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.

Author

Brian H Le MD

Dr. Le of Novant Health Presbyterian Medical Center has no relevant financial relationships to disclose.
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Editor

Harvey B Sarnat MD FRCPC MS

Dr. Sarnat of the University of Calgary has no relevant financial relationships to disclose.
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Patient Profile

Age range of presentation: 2 to 44 years
Sex preponderance: male = female
Heredity: Heredity typical

Heredity may be a factor, especially in cases of secondary basilar impression

Autosomal dominant, with variable penetrance
Population groups selectively affected: None selectively affected
Occupation groups selectively affected: None selectively affected

ICD & OMIM codes

ICD-10: Other congenital malformations of skull and face bones: Q75
ICD-11: Structural developmental anomalies of cranium: LB70
OMIM: Basilar impression, primary: %109500

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Basilar impression

Associated Disorders

Achondroplastic dwarfism
Brain stem dysfunction
Cerebellar ataxia
Cervical arachnoid cyst
Chiari malformation
- Cranial nerve palsies
- Familial aqueductal stenosis
- Goldenhar syndrome
- Hydrocephalus
- Hyperparathyroidism
- Klippel-Feil syndrome
- Metabolic disease of bone
- Osteogenesis imperfecta
- Osteomalacia
- Osteoporosis
- Paget disease
- Rickets
- Spinal cord compression
- SUNCT syndrome
- Syringobulbia
- Syringomyelia
- Tricho-rhino-phalangeal syndrome type I

Differential Diagnosis

transient ischemic attack
spinal cord compression
syringomyelia
multiple sclerosis
4th ventricular obstruction
- chronic degenerative diseases
- Ménière disease
- acoustic neuroma
- infectious tumors
- postinfectious tumors
- toxic tumors
- metabolic tumors
- structural CNS lesions

Also Relevant

Activity-related headache
Chiari malformation
Klippel-Feil syndrome
Sleep and neuromuscular and spinal cord disorders
Syringomyelia

Clinical Categories

Developmental Malformations

	• Osteogenesis imperfecta: underlying condition that may cause secondary basilar impression
	• Paget disease: underlying condition that may cause secondary basilar impression
	• Chiari malformation: associated with and may be observed concurrently with basilar impression
	• Syringobulbia/syringomyelia: associated with and may be observed concurrently with basilar impression

Basilar impression …

Basilar impression

Cite this article

Introduction

Overview

Key points

Historical note and terminology

Clinical manifestations

Presentation and course

Prognosis and complications

Biological basis

Etiology and pathogenesis

Epidemiology

Prevention

Differential diagnosis

Associated or underlying disorders

Diagnostic workup

Management

Outcomes

Special considerations

Pregnancy

Anesthesia

References

Contributors

Author

Editor

Patient Profile

ICD & OMIM codes

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Questions or Comment?

Also in This Category

Reducing body myopathy

Joubert syndrome

Atlantoaxial dislocation

Cerebro-oculo-facio-skeletal syndrome

Oral-facial-digital syndromes

22q11.2 deletion syndrome

Meckel-Gruber syndrome

Norrie disease

Basilar impression

Cite this article

Introduction

Overview

Key points

Historical note and terminology

Clinical manifestations

Presentation and course

Prognosis and complications

Biological basis

Etiology and pathogenesis

Epidemiology

Prevention

Differential diagnosis

Associated or underlying disorders

Diagnostic workup

Management

Outcomes

Special considerations

Pregnancy

Anesthesia

References

Contributors

Author

Editor

Patient Profile

ICD & OMIM codes

This is an article preview. Start a Free Account to access the full version.

Questions or Comment?

Also in This Category

Reducing body myopathy

Joubert syndrome

Atlantoaxial dislocation

Cerebro-oculo-facio-skeletal syndrome

Oral-facial-digital syndromes

22q11.2 deletion syndrome

Meckel-Gruber syndrome

Norrie disease

This is an article preview.
Start a Free Account
to access the full version.