Basilarinvagination, also known as basilar depression, skull base entrapment, skull base entropion, or skull base pressure. It is a common clinical neurodevelopmental disorder in which the skull base bone tissue, atlanto-axial vertebrae and pivotal vertebrae centered on the occipital foramen are malformed, and the atlanto-axial vertebrae plunge into the cranial cavity and the pivotal dentate is higher than the normal level into the occipital foramen so that the occipital foramen is narrowed and the posterior cranial fossa becomes smaller, thus compressing the medulla cerebellaris and pulling the nerve roots to produce a series of symptoms.
Epidemiology
Authoritative and comprehensive incidence statistics have not been found. The disease is more common in males than in females, with a ratio of 3:2; it is most common in adolescents between the ages of 10 and 30, and in some patients with a later onset.
Etiology
The main cause of basilarinvagination is congenital bone dysplasia. Because the neural tube closes late in the atlanto-occipital region in embryogenesis, congenital malformations tend to occur in this region. Rarely, it can be secondary to other diseases.
Skull base depressions can be divided into two types.
① Congenital type: also called primary skull base depression, with atlanto-occipital fusion, flattening of the occipital bone, deformation of the foramen magnum, upward displacement of the foramen dentata and even into the foramen magnum, resulting in a reduction in the anterior and posterior diameters of the foramen magnum. At 2 to 3 weeks of embryonic development, the atlanto-occipital spine enters the foramen magnum to varying degrees due to local defects in embryonic segmentation, sometimes fusing with it, etc. In recent years, it has been found that the disease is related to genetic factors, i.e., several siblings in the same family can develop the disease.
Secondary type: also known as acquired skull base depression, less common, often secondary to osteitis, osteogenesis imperfecta, rickets osteochondrosis, rheumatoid arthritis or hyperparathyroidism, etc., resulting in softening of the skull base bone, softening of the skull base bone by the cervical spine compression and invagination, occipital foramen elevation sometimes up to the tip of the rock bone, and become funnel-shaped while the cervical spine is also set into the skull base in order to adapt to the posterior arch of the atlantoaxial vertebrae in the posterior occipital foramen may appear The posterior atlantoaxial arch does not fuse with the occipital bone.
Pathogenesis
Skull base depression leads to narrowing of the foramen magnum and a smaller posterior cranial fossa, which compresses the medulla oblongata, cerebellum and pulls on the nerve roots, resulting in a series of symptoms combined with compression of the vertebral artery and inadequate blood supply.
The pathological changes are as follows: foramen magnum malformation includes abnormal development of the lateral part of the base of the occipital bone and the condyle, resulting in inward depression of the skull base, shortening of the distance between the atlanto-occipital spine and the occipital bone, atlanto-occipital fusion, occipitalization of the atlanto-vertebrae, and sometimes combined with atlanto-axial deformity, vertebral plate fissure or absence, and abnormal curvature of the craniocervical migration.
The base of the occipital bone, the condyles and the scales of the occipital bone fall into the cranial cavity with the occipital foramen as the center, and the distance between the atlanto-axial vertebrae becomes shorter at the edge of the occipital foramen, and even fuses with the posterior arch of the atlanto-axial vertebrae, and the occipital condyles are dysplastic and asymmetrical, and the base of the occipital bone becomes shorter and straighter with an uneven skull base. The atlas is funneled into the skull and the dentate process of the cardinal vertebrae enters the occipital foramen at a higher than normal level and the occipital foramen is shortened anteriorly and posteriorly, thus narrowing the posterior cranial fossa and producing a series of neurological symptoms and signs.
In addition to the above bony changes, the local soft tissues may also become thickened and constricted, and the adhesions and thickening of the dural arachnoid ligament near the foramen magnum may become band-like, thus compressing the cerebellum, medulla oblongata, cerebral nerve, superior cervical medulla, cervical nerve and vertebral artery, etc., and producing symptoms in the late stage.
Skull base depression is often combined with cerebrospinal and other soft tissue malformations such as cerebellar tonsillar herniation, spinal cord cavitation and arachnoid adhesions.
Clinical manifestations
Most patients have slow progression of symptoms, with occasional remission. Some patients may be asymptomatic and only have deformity of the occipital foramen magnum area on X-ray.
Patients may have headache, vertigo, tinnitus, diplopia, vomiting, etc. Patients may have head and neck deviation, cheek asymmetry, thick and short neck, low posterior hairline, limited neck movement and fixed in a special angular position. Patients often complain of cervical ankylosis with progressive lower limb weakness and difficulty walking as the first symptom. The onset of the disease is usually insidious and gradually aggravated, or it may suddenly develop or worsen after head trauma, i.e., numbness and weakness of the limbs appear after minor head trauma or excessive head tilting or neck flexion, and even tetraplegia and dyspnea occur repeatedly and the whole condition is progressively aggravated.
The neurological symptoms and signs are mainly manifested as foramen occipitalis syndrome, the main clinical manifestations of which are.
1, upper cervical nerve root stimulation symptoms are mainly due to skull base deformity bone stimulation and compression of atlanto-occipital fascial ligament and dura mater to make it occur hyperplasia, hypertrophy or formation of fibrous fascia compression of the upper cervical nerve root patients often complain of chronic pain in the occipital area neck activity is limited sensory loss, one or bilateral upper limbs numbness, pain muscle atrophy, forced head position, etc.
2, posterior group of cerebral nerve disorder symptoms often due to brainstem displacement, pulling or arachnoid adhesion to the posterior group of cerebral nerve involvement and swallowing difficulties choking and coughing hoarseness tongue muscle atrophy, slurred speech, reduced gag reflex and other symptoms of ball palsy, as well as facial hypesthesia hearing loss, reduced corneal reflex and other symptoms.
3, the medulla oblongata and the upper cervical medulla compression signs are mainly due to the cerebellar tonsillar herniation, local pathological tissue compression of the medulla oblongata and the upper cervical medulla and secondary spinal cord cavitation caused by the patient manifested as weakness of the limbs, sensory impairment, positive cone bundle sign urinary retention swallowing and breathing difficulties, finger fine motor impairment, loss of position sense; sometimes there is unilateral or bilateral segmental pain and temperature sensation disappearance in the cervicothoracic segment of the spinal cord, while tactile and deep sensation exists this Separate sensory disorders are the characteristic manifestations of spinal cavernous disease.
4.Cerebellar dysfunction is common with nystagmus, mostly horizontal nystagmus, but also vertical or rotational nystagmus. In the late stage, cerebellar ataxia may appear as unstable walking and slurred speech, and the finger-nose test is not allowed, and the heel-knee-shin test is not stable.
5, intracranial pressure increase symptoms early patients generally do not have intracranial pressure increase once the emergence of serious conditions, and most of them are late. The symptoms are caused by the occurrence of obstructive hydrocephalus. Individual patients who appear earlier may be combined with intracranial tumors or arachnoid cysts because the patients show severe headache, nausea and vomiting, optic disc edema, or even occipital foramen hernia, impaired consciousness, respiratory and circulatory impairment or sudden respiratory arrest and death.
6, vertebral artery blood supply disorder manifested as episodes of vertigo visual impairment nausea and vomiting ataxia, facial sensory disorders, tetraplegia and ball palsy and other clinical symptoms.
X-ray plain film and MRI show obvious deformity of the skull base (see the section of auxiliary examination for the manifestation of radiological examination)
Complications
Primary cases are often combined with other malformations such as flattened skull base, atresia of the middle cerebral aqueduct, submicrocephaly, hydrocephalus, medulla oblongata and spinal cord cavernosa. Secondary cases include osteochondrosis rickets, osteogenesis imperfecta and rheumatoid arthritis.
Diagnosis – According to the age of onset, the disease progresses slowly, and the clinical manifestation is the syndrome of occipital foramen magnum area and the characteristic head appearance can be diagnosed by X-ray examination. The clinical application of CT scan and MRI has made a breakthrough in the diagnosis of this disease, especially MRI can help the early diagnosis of this disease, which can show the lower herniated cerebellar tonsils and the combined spinal cord cavitation clearly, which cannot be done by conventional X-ray examination.
Treatment of skull base depression instruments
Differential diagnosis This disease needs to be differentiated from the following diseases.
1, spinal cord cavernous disease spinal cord cavernous disease often coexists with skull base depression, its clinical features are cervicothoracic segment of the spinal cord distribution area is separated sensory impairment hand small muscles more atrophy or even deformity. If the symptoms continue to worsen and there is damage to intracranial structures, the possibility of skull base depression should be considered CT and MRI can help diagnose.
2.Superior cervical medullary tumor may manifest as neck and occipital pain, diaphragm and intercostal muscle paralysis, and progressive increase in symptoms of stiff limb palsy. X-ray or myelogram can help to make a differential diagnosis.
3.Primary lateral sclerosis mainly manifests as positive cone bundle sign on both sides, i.e. tetraplegia, if the lesion spreads to the cortical medullary bundle, swallowing difficulty and hoarseness may also appear, but there is no sensory impairment. craniocervical X-ray examination is mostly normal.
4, progressive spinal muscular atrophy because the lesion often starts from the lower cervical segment and the upper thoracic segment of the anterior horn cells of the spinal cord, generally the earliest manifestation of the two fingers weakness, holding objects unstable, small muscle atrophy and muscle fiber tremor in the hands, and gradually developed to the forearm, arm and shoulder, generally no sensory impairment cranial base X-ray examination is normal.
5, cervical spondylosis mainly manifests as upper limb muscle atrophy as well as long bundle sign, often with nerve root pain, segmental sensory disorders evident at the lesion level are rare, there may be symptoms of insufficient blood supply to the vertebral artery but lack of cerebral nerve involvement and cerebellar symptoms generally no increased intracranial pressure manifestation cervical spine X-ray examination can be diagnosed.
6, spinal syphilis in the presence of proliferative cervical medullary dural spondylitis can appear upper limb sensory disorders, atrophy, as well as weakness and lower limb cone fasciculation signs. There is a lack of manifestation of intracranial structural damage. Myelography reveals a history of syphilis in patients with subarachnoid obstruction, a short history of spinal cord syndrome, and positive blood and cerebrospinal fluid Walden and Kang reactions. Craniocervical X-ray can clarify the diagnosis.
7.Other diseases need to be differentiated from posterior cranial fossa tumor, cervical disc herniation and amyotrophic lateral fascicular sclerosis.
Editorial examination
Laboratory tests
Blood, urine and stool routine examination are normal; cerebrospinal fluid examination is generally not abnormal.
Other auxiliary examinations
Radiological examinations include cranial-cervical frontal and lateral radiographs, body level radiographs, anterior-posterior open-ended radiographs (examination of atlas and cardinal spine), skull base radiographs (examination of slope and dentate) and other cranio-cerebral plain radiographs centered on occipital foramen area. The diagnosis is usually confirmed by lateral cranial-cervical radiographs, which is the easiest way to diagnose skull base depressions. If necessary, CT scan and sagittal reconstruction can be used to observe the deformity of occipital foramen more clearly.
1.Cranial plain film using cranial plain film to diagnose skull base depression requires various measurements, due to the large local normal anatomical variation of occipital foramen magnum area although there are more measurement methods, but there is not an ideal method to diagnose this disease very reliably, so at least 2 kinds of obvious abnormal measurement results according to the following methods are needed to make the diagnosis.
(1) Chamberlain’s line (Chamberlain’s line): also known as the palato-occipital line. A line from the posterior edge of the hard palate to the posterior superior edge of the foramen magnum on a lateral cephalometric film is the Chamberlain’s line. If the dentate is below 3 mm of this line in a normal person, the skull base is depressed.
(2) McGregor’s line (McGregor’s line): also known as the basal line. The line from the posterior edge of the hard palate to the lowest point of the occipital scales, i.e. McGregor’s line, the normal dentate process should not be higher than this line 6mm, if it is more than this, it is skull base depression.
(3) Bull angle: the angle between the plane of hard palate and the plane of atlantoaxial vertebrae, normal is less than 13°, more than 13° is the skull base depression.
(4) Basal angle: The angle formed by the two lines from the nasal root to the center of the pterygoid saddle and the center of the pterygoid saddle to the anterior margin of the foramen magnum is normally 109° to 148°, with an average of 132.3°. This angle increases in skull base depression.
(5) Klaus’s index: the vertical distance from the apex of the dentate to the line between the saddle node and the intraoccipital ridge. Normal is 40-41mm, if it is less than 30mm, it is considered as skull base depression.
(6)The line of the diastasis (fishgold line): the distance from the tip of the dentate process to the line of the diastasis on the anterior-posterior cranial tomogram, normal is 5~15mm, if the apex of the dentate process is close to this line or even more than this line, it is skull base depression.
(7) Double mastoid line: in the orthopantomogram, the line between the two mastoids is normal when this line is passing through the atlanto-occipital joint, and the dentate can reach this line or exceed this line by 1~2mm when the skull base depression is abnormal.
(8) Boogard’s angle: the angle formed by the line connecting the anterior and posterior margins of the occipital foramen and the slope of the occipital bone is normally 119.5°~136° and increases in the case of skull base depression
(9) Height index of external auditory foramen: the distance between the center point of external auditory foramen or the midpoint of the line connecting the external auditory foramen on both sides and the line connecting the anterior and posterior edges of the occipital foramen in the lateral cranial view is the height index of the external auditory foramen, which is normally 13～25mm, with an average of 17.64mm.
2.CT scan mainly shows the changes of brain tissue and ventricles, sometimes it is feasible to perform ventriculography CT scan after intracerebroventricular injection of non-ionic water-soluble contrast agent, which can observe whether the ventricular size of the middle cerebral aqueduct is open and the changes of the fourth ventricle and brainstem, and can outline the location of the lower edge of the cerebellar tonsils.
MRI is one of the best means to diagnose this disease, especially in the sagittal position, it can clearly show the changes of the fourth ventricle and brainstem of the midbrain canal and the degree of subcerebellar tonsillar herniation and the cervical medulla compression to facilitate the decision of surgical treatment plan.
The skull base depression often leads to a reduction in the effective space of the posterior cranial fossa and the upper cervical spinal canal, so the aim of treatment is to give enough space for decompression surgery. For asymptomatic patients found incidentally, treatment is generally not required. Patients should be advised to prevent cranial trauma and excessive strenuous head flexion and extension, and cervical spinal manipulation can aggravate the condition and should be contraindicated.
However, it must be noted that patients with mild symptoms should not be operated even if the deformity is found on imaging.
(1) Patients with compression of the medulla oblongata and superior cervical medulla.
②Patients with cerebellar symptoms and cervical nerve symptoms, which are progressively aggravated.
(3) Those with cervical nerve root involvement and spinal cord cavity
(iv) Those with cerebrospinal fluid circulation disorders or increased intracranial pressure.
⑤ Those with posterior cranial fossa tumors or arachnoid cysts.
The main surgical method is suboccipital decompression. The posterior edge of the foramen magnum and the adjacent posterior atlantoaxial arch of the occipital scales, the spinous process of the 23rd cervical vertebra and the vertebral plate are removed during surgery. The traditional surgical approach is to occlude the depressed bone and cut the dura to fully decompress the dura after releasing the compression of the bone to gradually relax and relieve its tension to achieve the desired effect of surgical decompression.
The purpose of surgery is to release the neural tissue compression to restore the cerebrospinal fluid circulation pathway and to fix the unstable atlanto-occipital and cervical joints if necessary. Because the surgery is performed in the medulla oblongata and the upper cervical medulla, the space is quite small and the surgical risk is much greater than the general suboccipital decompression, and the operation is also difficult.
In some patients, the compression of the medulla oblongata mainly comes from the posteriorly displaced dentate process of the cardinal spine at the anterior edge of the occipital foramen on the ventral side, mainly manifesting as damage to the cone bundle, which can be clearly seen in the sagittal position on MRI examination as the compression comes from the ventral side, so that only posterior occipital decompression has no significant effect, and decompression can be performed through the cervical or oropharyngeal anterior approach, removing the anterior arch of the atlantoaxial spine and dentate process at the anterior edge of the occipital foramen without opening the dura during surgery to prevent cerebrospinal fluid For patients with poor stability of the atlantoaxial region, bone graft fusion is required after the anterior approach.
In conclusion, the surgical treatment of skull base depression should follow the following principles: anterior decompression of the medulla oblongata and cervical medulla should be performed for anterior decompression of the medulla oblongata and posterior decompression should be performed for posterior decompression of the medulla oblongata, and bone graft fusion should be considered for all patients with craniocervical instability.
Prevention
It is generally believed that the shorter the history of the disease, the younger the age, the better the surgical result, and vice versa, the worse the outcome. In recent years, the literature reports that the surgical cure and improvement rate is 67%, and the morbidity and mortality rate is 0% to 7.1%, and the exacerbation rate is 0% to 8.1%, and those with more than 1 year of postoperative follow-up can participate in the workforce up to 600% can take care of themselves.
Some reports classify the long-term effect of surgery into 4 grades A: good health condition after surgery, can work all day accounting for 68.1%; B: good health condition but sometimes have mild numbness or weakness, occasional dizziness can only do light work or half day work accounting for 21%; C: postoperative condition improved, can walk on their own, life part or can not take care of themselves accounting for 7%; D: postoperative aggravation and death accounting for 3.5%
Prevention
The main thing is to pay attention to women’s pregnancy and perinatal health care to prevent diseases and not to use drugs arbitrarily to prevent the birth of congenital malformations.