Basilar impression is an acquired or congenital indentation into the skull of the occipital bone in the area of the craniovertebral junction. The decrease in the volume of the cranium and cranial (towards the skull) displacement of the spine that occur during basilar impression are the cause of the development of hydrocephalus, cerebellar disorders, disorders of the cranial nerves, radicular symptoms and signs of damage to the upper cervical spinal cord. Disease is diagnosed during radiography of the craniovertebral junction, CT and skull MRI and cervical spine. Pathology is treated only surgically. However, in the presence of mild non-progressive clinical changes, such treatment is not required.
Q75 Other congenital anomalies [malformations] of the bones of the skull and face
Basilar impression is a funnel-shaped depression of the slope of the occipital bone in combination with the deepening of the edges of the large occipital foramen into the cranial box. These changes lead to a displacement of the spine closer to the brain and a decrease in the volume of the posterior cranial fossa. With basilar impression, the dentoid process of the second cervical vertebra is located at the level of the occipital foramen or even above it — in the cranial cavity.
In neurology, basilar impression refers to anomalies of the craniovertebral junction, which also includes platybasia, atlant assimilation, atlantoaxial dislocation and Chiari anomaly. Often, congenital basilar impression is combined with other craniovertebral anomalies and malformations of the spine. The most common is a combined defect, in which there is a basilar impression and congenital platybasia.
Congenital (primary) basilar impression is formed during intrauterine development. It is associated with both hereditary predisposition and negative effects on the fetus during pregnancy. Factors that can cause malformations, including basilar impression, include diseases of the mother and infections she suffered during pregnancy (chlamydia, rubella, measles, cytomegalovirus infection, etc.), radioactive radiation, contact with harmful chemicals.
Acquired (secondary) basilar impression develops as a result of a violation of the normal bone structure of the occipital bone. Its cause may be osteoporosis, which in turn is associated with hypoparathyroidism, osteomalacia, severe rickets, Paget’s disease. Acquired basilar impression is possible as a result of destructive changes in the occipital bone when a tumor of the posterior cranial fossa grows into it or as a result of an inflammatory process (syphilis, tuberculosis, actinomycosis, osteomyelitis).
Basilar impression is characterized by the appearance of the first clinical symptoms before the age of 30, most often in the period from 15 to 20 years. Clinical manifestations occur as a result of a decrease in the volume of the posterior cranial fossa, leading to increased intracranial pressure, hydrocephalus and compression of anatomical formations located in it: cerebellum, brain stem, cranial nerve roots. In addition, the upper cervical segments of the spinal cord are also compressed, which leads to the appearance of radicular and spinal symptoms.
The general neurological symptoms, which are accompanied by basilar impersia, are reduced to headaches in the occipital region, often having a paroxysmal character and depending on the position of the head. Against the background of headache, vegetative disorders are possible: a feeling of cold or heat in the body, tachycardia, hyperhidrosis, changes in blood pressure, etc. In some cases, unilateral pain in the occipital region is observed, which can gradually take on a bilateral character. Occasionally, pain from the back of the head spreads anteriorly into one or both eyes. It can occur by the type of occipital neuralgia, increase with head turns, coughing, laughing and straining.
Cerebellar symptoms, which manifest basilar impression, consist in the appearance of ataxia — a violation of the stability of the body position and shakiness when walking. Brain stem damage is manifested by nystagmus, choking when eating (dysphagia), hoarseness of voice, hearing loss (hearing loss), dizziness, facial skin sensitivity disorder. Stem symptoms are most pronounced in cases where basilar impression is combined with Atlantean assimilation.
Basilar impression is often accompanied by pain in the neck and upper extremities, as well as paresthesia (numbness, a feeling of “crawling goosebumps”) in the back of the head and neck. These symptoms are caused by compression of the upper roots of the cervical spine. In the case of disease, compression myelopathy in the upper cervical region rarely leads to the development of spastic tetraparesis. Basically, with basilar impression, moderate dissociated sensitivity disorders are observed, requiring the exclusion of the initial manifestations of syringomyelia.
When examining patients who have a basilar impression, a short neck and an understated border of hair growth often attract attention. There is shakiness in the Romberg pose, a miss when performing coordination tests (knee-heel and toe-nasal), a decrease in sensitivity in the area of cervical dermatomes and on the face.
The patient’s complaints, the history of their occurrence and development (anamnesis of the disease), the results of a neurological examination often lead the neurologist to think about the presence of pathology in the posterior cranial fossa and craniovertebral junction. At the same time, in order to identify a basilar impression or other anomaly in this area, an X-ray examination is first performed. A better visualization of the bone structures of this area than on an X-ray, as well as data on other malformations, with which congenital basilar impression is often combined, can be obtained using CT scans of the skull and spine.
Targeted radiography of the skull in the area of the craniovertebral junction and radiography of the spine in the cervical region are carried out in direct (facial) and lateral (profile) projections. Basilar impression is diagnosed by increasing the angle between the plate of the main bone and the slope more than 130 °, as well as by the upward displacement of the process of the second cervical vertebra. The location of the process is determined in relation to the conditional lines: Chamberlain lines (between the posterior edge of the hard palate and the posterior edge of the foramen magnum) and de la Petit lines (between the apices of the right and left mastoid processes). Normally, the process of the second cervical vertebra is located below these lines. Basilar impression is characterized by a displacement of the process 6-30 mm above the Chamberlain line and 10-15 mm above the de la Petit line.
Ophthalmoscopy in patients with basilar impression may reveal stagnant optic nerve discs, while threshold audiometry may reveal a sensorineural type of hearing loss. To identify the condition of the soft tissue structures of the posterior cranial fossa and spinal cord, to assess the degree of their compression during basilar impression, it is necessary to conduct an MRI of the brain and an MRI spine.
Basilar impression requires differentiation from other anomalies of the craniovertebral junction, syringomyelia, Friedreich’s ataxia, cerebellar tumor, Pierre-Marie ataxia, as well as from the cortical type of ataxia.
In cases where the basilar impression is an accidental radiological finding and does not cause symptoms of compression of brain structures, treatment is not required. Indications for surgical intervention in basilar impression may be a steady progression of symptoms; clinical signs of compression of the spinal cord, brain stem or cerebellum; pronounced cerebrospinal fluid disorders with increasing occlusive syndrome. In order to decompress during the operation, the large occipital opening is expanded.