OBJECTIVE: To investigate the surgical method and efficacy of stage I posterior atlanto-occipital decompression combined with atlanto-axial repositioning and occipitocervical fixation and fusion for the treatment of Chari’s deformity combined with skull base depression. METHODS: Fifty-nine cases of Chari’s deformity combined with skull base depression were treated by posterior atlanto-occipital decompression combined with atlanto-axial repositioning and occipitocervical fixation and fusion. There were 31 male cases and 28 female cases; age: 17 C 62 years, mean 36.3 years. MRI and skull X-ray and CT were performed before surgery. Among them, 20 cases had Chiari malformation type 0 combined with skull base depression and 39 cases had Chiari malformation type I combined with skull base depression; according to the combination of spinal cord cavity or not, the corresponding suboccipital decompression method was adopted. Results: The efficacy of surgery was evaluated based on the changes in clinical symptoms, repositioning of the dentate process and changes in the spinal cord cavity before and after surgery. Among the 5 patients who underwent secondary surgery after original dentatectomy, there was no significant postoperative improvement; among the 5 patients who underwent secondary surgery after posterior decompression, 2 had significant improvement and 3 had no change; among the 49 cases who underwent the first surgery, 44 had improvement, 4 had no change, and 1 had aggravation. CONCLUSION: The traditional posterior suboccipital decompression is not an effective method applicable to the treatment of various atlanto-occipital deformities. When Chiari malformation is combined with skull base depression, an individualized treatment plan should be adopted according to the patient’s clinical symptoms, signs and imaging features. Suboccipital decompression is performed according to the combination of spinal cord cavity; then posterior atlantoaxial repositioning and stage I occipitocervical fusion are effective methods to treat Chiari malformation combined with skull base depression. The treatment of Chiari’s I malformation and syringomyelia was performed in 11 cases, Chiari’s I malformation and platybasia in 2 cases, Chiari’s I malformationand posterior occipital-cervical fusion 3 cases, Chiari’s I malformation and basilar invagination 3 cases. posterior decompression (PD) was performed on 15 patients, Anterior decompression (AD) was performed on 2 patients, Posterior decompression and occipital- Among 15 PD patients, post-operative syndromes were improved on 14, one recurrent Among 15 PD patients, post-operative syndromes were improved on 14, one recurrent patient’s post operative syndromes had no change. These include flattened skull base, skull base depression, atlanto-occipital fusion, and subungual herniation of the cerebellar tonsils. Because these deformities can occur separately or simultaneously, the indications and contraindications for each procedure should be taken into account when choosing surgical treatment, such as: should we choose anterior or posterior decompression surgery? Is it necessary to perform atlanto-occipital fusion at the same time as decompression? Improper selection may lead to aggravation or recurrence of the original neurological deficit, thus affecting the outcome of surgical treatment. In this paper, we retrospectively analyzed 19 cases of atlanto-occipital deformity treated in our hospital from April 2004 to November 2006, and discussed the selection and application of anterior decompression surgery, posterior decompression surgery and atlanto-occipital fusion surgery in the treatment of atlanto-occipital deformity. Data and methods 1. General data: 8 males and 11 females; age: 10 C 47 years old, average 30.3 years old. Duration of disease: 3 months-17 years, mean 3.3 years. There were 17 cases of asymmetric limb sensory disorders, 2 cases of headache and dizziness, 8 cases of limb weakness or muscle atrophy, and 4 cases of intermittent posterior neck and upper extremity pain, and the above symptoms could appear simultaneously in one case. 2. Imaging examination: MRI and cranial X-ray or CT examination were performed before surgery. Wakenheim’s line, Mc Rae’s line and Chamberlain’s line were measured in the atlanto-occipital junction area to determine the presence of bony structures with or without a flat skull base, skull base depression or atlanto-occipital fusion (Figure 1). MRI was used as the gold diagnosis for subungual herniation of the cerebellum (Chari malformation type I) and spinal cavity. In this group of cases: 11 cases of Chari malformation type I combined with spinal cord cavity; 2 cases of Chari malformation type I combined with flat skull base; 3 cases of Chari malformation type I combined with atlanto-occipital fusion; 3 cases of Chari malformation type I combined with skull base depression. 3.Surgical methods: posterior suboccipital craniotomy and tonsillectomy: for cases with Chari malformation type I combined with spinal cord cavity and/or flat skull base and atlanto-occipital fusion, general anesthesia was applied in lateral position, and a posterior median suboccipital incision was made to reveal the occipital bone scales and the cervical 1 spinous process and posterior arch. The bone window covers the occipital scalene and the posterior arch of cervical 1, measuring approximately 2.0 X 2.0 cm. The spinous processes and laminae of the corresponding cervical segments should be occluded according to the degree of subungual herniation of the cerebellar tonsils, and the cervical 2 spinous processes and laminae are usually not occluded. Great care should be taken when occluding the laminae, where there is often abnormal bone development, deep bone embedding, or fusion of the atlanto-occipital bone. The bone should be removed with a thin biting forceps or abraded with a grinding drill. The thickened fibrous band and atlanto-occipital fascia are visible after bone removal, and the dura is cut in a “Y” shape. In patients with severe herniation of the cerebellar tonsils, the subperiosteal cerebellar tonsils are partially excised and the soft meninges are closed with 5.0 absorbable sutures, and the median foramen of the IV ventricle is patent with CSF flow after adequate decompression. In cases where partial herniation is not severe, the median foramen of the IV ventricle can be explored microscopically, and the surface of the cerebellar tonsils can be retracted by electrocautery to ensure patency of the median foramen. The dura is then repaired with an autologous fascial reduction suture (Figure 2). Anterior trans-oral pharyngeal foramen magnum osteotomy for decompression: In patients with skull base depression, compression of the medulla oblongata and cervical medulla can usually be seen on MRI from the bone of the anterior border of the foramen magnum ventralis and the posteriorly displaced dentate process of the cardinal spine. Surgery is performed using an anterior transoral pharyngeal approach for decompression. The cervical tracheotomy is performed before surgery, and then the soft and hard palate and posterior pharyngeal wall are exposed through the oral cavity. After dissecting and separating the mucosa, the anterior border of the foramen magnum, the anterior arch of cervical 1 and the dentate process were removed with an abrasive drill. The surrounding hyperplastic ligaments and fibrous tissue are also removed. No dural opening is required. Posterior suboccipital decompression and atlanto-occipital fusion: For patients with skull base depression combined with subxiphoid herniation or patients with recurrence of delayed cervical medullary compression after anterior decompression, posterior atlanto-occipital fusion is performed in conjunction with suboccipital decompression. The posterior median suboccipital approach is used to expose the occipital ramus above the incision and the cervical 3 spinous process below, with the width of both sides exposed to reach the lateral block of the cervical spine. After confirming the posterior atlantoaxial tuberosity and the pivot and cervical 3 spinous processes, the lateral blocks are revealed laterally along the spinous processes. After shaping the titanium plate to the physiological curvature of the atlanto-occipital region, it is fixed to the occipital ridge and the lateral blocks of cervical 2 and cervical 3 with titanium screws under C-arm X-ray monitoring and positioning, and four titanium screws are required on each side for the pedicle screws in cervical 2 and the lateral block screws in cervical 3. After satisfactory fixation, the occipital scalene and posterior atlantoaxial arch were then occluded and ground away with a grinding drill and biting forceps. The decompression window ranges from about 2.0 X 2.0 cm to achieve decompression. In general, patients with skull base depression and subungual herniation of the cerebellar tonsils are not obvious or mild and do not need to cut the dura. Among the 11 patients admitted to our clinic with subcerebellar tonsillar herniation combined with spinal cavity and 2 patients with subcerebellar tonsillar herniation combined with flat skull base, according to the degree of subcerebellar tonsillar herniation, electrocautery of the cerebellar tonsils was performed in 3 of them to open the median foramen, and submural partial resection of the cerebellar tonsils was performed in 9 of them. 12 patients showed significant improvement of the original neurological deficit after surgery; 1 case was treated with suboccipital decompression at an outside hospital for 1 year. The MRI showed that the bone window of the original surgery was 4 X 4 cm in size and the cerebellum was severely collapsed; intraoperatively, the cerebellar tonsils were herniated into the spinal canal about 2 cm, and the brainstem was deformed by compression; the cerebellar tonsils herniated into the spinal canal were removed and the dura was repaired in the second surgery, and the symptoms did not improve significantly after the surgery. In two cases of subcerebellar tonsillar herniation combined with flat skull base and three cases of subcerebellar tonsillar herniation combined with atlanto-occipital fusion, all patients underwent posterior suboccipital decompression, and electrocautery of the cerebellar tonsils was performed to open the median foramen of the IV ventricle. Among the three patients admitted with submicrocephalic tonsillar herniation combined with skull base depression, two of them underwent transoral anterior osteotomy and decompression, and one of them had recurrence of cone bundle compression symptoms six months after the operation. The other case underwent trans-oral anterior osteotomy and decompression, and the postoperative MRI showed satisfactory osteotomy and decompression, and there was no recurrence of symptoms in 4 months of follow-up. In another case, a patient with skull base depression but subcerebellar tonsillar herniation was not obvious, and a direct posterior suboccipital decompression atlanto-occipital fusion was performed. 3 days after the operation, the patient’s muscle strength of both lower limbs increased significantly, and the symptoms of limb numbness improved significantly. Discussion Atlanto-occipital deformity is a disease caused by congenital abnormalities of the base of the occipital bone and the first and second cervical vertebrae, which is combined with abnormalities of the surrounding soft tissues and the nervous system in addition to skeletal abnormalities at the craniocervical junction. These include flattened skull base, skull base depression, atlanto-occipital fusion, and subungual herniation of the cerebellar tonsils (Arnold-Chiari’s malformation). These anomalies may occur separately or together. The clinical symptoms and the degree of malformation are inconsistent. Most of them are young and middle-aged, with a slow, progressive worsening of the disease. The symptoms are mainly motor disorders of the limbs, limb paralysis, sensory disorders, and even urinary and fecal incontinence due to compression of the pyramidal fasciculus and craniocervical nerve roots caused by compression of the medulla oblongata and high cervical medulla. In addition, the cervical and thoracic spinal cord cavity and the corresponding neurological symptoms can be complicated. For patients with significant and progressive worsening of symptoms, surgery is the only effective treatment option. The aim of surgical treatment is to relieve the compression of the spinal cord by the inferior occipital border and the atlantoaxial spine. However, because the atlanto-occipital joint is located in the craniocervical junction area, it supports various movements of the head such as forward flexion, dorsiflexion and lateral rotation. Therefore, improper surgical decompression will not only fail to relieve or improve the symptoms of spinal cord compression, but will also further increase the instability of the atlanto-occipital joint, leading to increased spinal cord compression and even serious complications such as tetraplegia and/or fecal incontinence. Regardless of which surgical approach is chosen, careful preoperative evaluation is required to strictly grasp the indications for each surgical approach. The most commonly used surgical methods for atlanto-occipital deformity are the anterior decompression represented by transoral pharyngotomy and the posterior decompression represented by suboccipital craniotomy of the posterior border of the occipital bone and the posterior arch of neck 1. Due to the complex pathological structure of atlanto-occipital deformities, a single anterior decompression or posterior decompression is not an effective method for all kinds of atlanto-occipital deformities, and sometimes a combination of both surgical methods or a combination of atlanto-occipital fusion is required to achieve good treatment results. The use of preoperative MRI and CT and X-ray examinations can accurately determine the possibility of coexisting skull base depression, flattened skull base, or subungual herniation of the cerebellar tonsils. When the dentary does not exceed the Chamberlan’s line (the line between the superior margin of the hard palate and the posterior margin of the greater occipital foramen) by more than 3 mm and the Wakenheim’s line is smooth and straight, skull base depression can be excluded. For subxiphoid tonsillar herniation deformity that does not combine with skull base depression, suboccipital craniotomy with posterior decompression is feasible; when decompression is performed, the bone window of the occipital squamous and posterior cervical arch should not be too large, usually around 2.0X2.0 cm, and the resection of the posterior cervical arch should not exceed the vertebral artery notch, otherwise it may cause postoperative cerebellar collapse and spinal cord compression symptoms cannot be effectively relieved. Whether the cerebellar tonsils are resected submurally or not should be determined according to the degree of intraoperative herniation. In this group of cases, in 6 cases, only electrocautery of the cerebellar tonsils was performed to retract them and ensure the patency of the median foramen of the IV ventricle without further cerebellar tonsillectomy. In 10 cases, submural resection of the cerebellar tonsils was performed under the surgical microscope to relieve the brainstem from compression and allow for the opening of the median foramen of the IV ventricle. In one case, it was the second operation, because the bone window was too large during the first operation to decompress the cerebellum, causing cerebellar collapse, which caused the cerebellar tonsils to dehiscence to the level of cervical 2. After surgical removal of the herniated tonsils, the patient’s spinal cord compression symptoms did not recover significantly. In patients with atlanto-occipital deformity combined with cervical and/or thoracic spinal cord hollowing, specialized treatment of the spinal cord hollowing is generally not necessary, as long as the surgical decompression is adequate and the median foramen of the IV compartment is open, the spinal cord hollowing can improve within a period of time after surgery. When the skull base depression is combined with or without submicrocephalic tonsillar herniation, assessment of the bony structure of the craniocervical junction area prior to surgery is important. When the skull base depression is not combined with submicrocephalic tonsillar herniation, the compression of the brainstem and spinal cord mainly comes from the anterior border of the foramen magnum ventralis and the posteriorly displaced dentate process of the cardinal vertebrae, and decompression via the oropharyngeal anterior approach can relieve the compression and relieve the symptoms in the short term, but the long-term effect is rather uncertain. Because of the presence of atlanto-occipital joint abnormalities in the skull base depression itself, surgical removal of the anterior border of the foramen magnum and dentate may further aggravate the instability of the atlanto-occipital joint. Most of the literature advocates a transoral anterior decompression followed by a second-stage posterior atlanto-occipital fusion. In this group of cases, trans-oral anterior decompression was used in two patients with skull base depression, and the postoperative MRI showed that the ventral brainstem compression was released and the clinical symptoms were significantly relieved. However, one patient had recurrence of conus fasciculus compression symptoms six months after surgery, and MRI review showed skull base depression and ventral brainstem compression, and posterior suboccipital decompression and atlanto-occipital fusion were performed for the second operation. Another patient, less than six months after surgery, has no recurrence of symptoms at the present follow-up. In another case, a patient with skull base depression was operated directly with posterior decompression and atlanto-occipital fusion, and the patient recovered well after the operation, with no recurrence of symptoms at one year follow-up. In conclusion, atlanto-occipital deformity is a pathological process in which one and/or multiple deformities coexist. The traditional posterior suboccipital decompression is not an effective method applicable to the treatment of various atlanto-occipital deformities. Preoperative evaluation of the bony and neurological structures of the atlanto-occipital junction region is important in selecting the appropriate surgical approach. In patients with skull base depression and brainstem compression, whether oropharyngeal anterior decompression or posterior suboccipital decompression is performed, stage I or II atlanto-occipital fusion should be considered to ensure the stability of the atlanto-occipital joint after surgery and to consolidate the treatment effect.