Chiari I Deformity: Beyond 5 mm below the Foramen Magnum.

Although originally described as a consecutive series of pathologic changes, Chiari syndrome represents a spectrum of disease divided into two subsets: development deformities of the paraxial mesoderm manifesting after birth (types 0-1.5) and true congenital malformations due to failure of neural tube closure present in utero (types 2-5). Heterogeneity among patients with a Chiari deformity and incomplete understanding of its pathophysiologic characteristics have led to inconsistency in radiologic reporting and difficulty in defining appropriate management strategies tailored to an individual patient's condition. The radiologist is tasked with going beyond the criteria for cerebellar tonsillar herniation to define an individual patient's disease state, determine candidacy for surgery, and assist in selecting the proper surgical approach. In addition, the radiologist must be able to identify conditions that result in cerebellar tonsillar herniation that are not related to Chiari deformity to avoid inappropriate surgery. Last, the radiologist must be able to interpret postoperative imaging examinations to assess for adequacy of treatment and complications. The authors summarize recent literature regarding the pathophysiologic basis of Chiari 1 and related deformities and detail the ideal morphologic and physiologic imaging assessment, focusing on Chiari 1 and related deformities (Chiari 0, 0.5, and 1.5). Also discussed are surgical techniques and "pearls" of postsurgical imaging, including complications that must be recognized. This review provides clarity to a commonly encountered but less understood condition to optimize outcomes for patients with Chiari 1 and related deformities. ©RSNA, 2024 Supplemental material is available for this article. See the invited commentary by Huisman in this issue.

Accuracy of sex estimation by morphometric evaluation of foramen magnum using computed tomography - a systematic review and meta-analysis.

The objective of this study is to determine whether the morphometric evaluation of the foramen magnum using computed tomography can be used as an accurate tool in estimating the sex of an individual. An extensive search of the databases, PubMed, ProQuest, Google Scholar, and Scopus, was carried out to procure articles that fulfilled the inclusion criteria. The AQUA tool was used to assess the quality of the included studies. A random effects model was used for the meta-analysis of the eligible studies using the STATA software, version 16, 2019 at 95% CI and p ≤ 0.05. Eleven eligible articles that measured the transverse and sagittal diameters of the foramen magnum using computed tomography were included in this study. The sagittal diameter of the foramen magnum was greater than the transverse diameter, and both the diameters were greater in males than in females. Meta-analysis revealed that both transverse and sagittal diameters were more reliable for male sex estimation. Since there is a dimensional variation between the male and female foramen magnum, it can be used for initial sex identification and also as an auxiliary to other advanced methods of sex estimation.

Spinal Cord Atrophy Predicts Progressive Disease in Relapsing Multiple Sclerosis.

OBJECTIVE: A major challenge in multiple sclerosis (MS) research is the understanding of silent progression and Progressive MS. Using a novel method to accurately capture upper cervical cord area from legacy brain MRI scans we aimed to study the role of spinal cord and brain atrophy for silent progression and conversion to secondary progressive disease (SPMS). METHODS: From a single-center observational study, all RRMS (n = 360) and SPMS (n = 47) patients and 80 matched controls were evaluated. RRMS patient subsets who converted to SPMS (n = 54) or silently progressed (n = 159), respectively, during the 12-year observation period were compared to clinically matched RRMS patients remaining RRMS (n = 54) or stable (n = 147), respectively. From brain MRI, we assessed the value of brain and spinal cord measures to predict silent progression and SPMS conversion. RESULTS: Patients who developed SPMS showed faster cord atrophy rates (-2.19%/yr) at least 4 years before conversion compared to their RRMS matches (-0.88%/yr, p < 0.001). Spinal cord atrophy rates decelerated after conversion (-1.63%/yr, p = 0.010) towards those of SPMS patients from study entry (-1.04%). Each 1% faster spinal cord atrophy rate was associated with 69% (p < 0.0001) and 53% (p < 0.0001) shorter time to silent progression and SPMS conversion, respectively. INTERPRETATION: Silent progression and conversion to secondary progressive disease are predominantly related to cervical cord atrophy. This atrophy is often present from the earliest disease stages and predicts the speed of silent progression and conversion to Progressive MS. Diagnosis of SPMS is rather a late recognition of this neurodegenerative process than a distinct disease phase. ANN NEUROL 2022;91:268-281.

Natural History of Chiari 1 Deformity.

PURPOSE: Chiari type 1 deformity (Ch1) is associated with bony deformity of the skull base and herniation of cerebellar tonsils more than 5 mm below the foramen magnum. Magnetic resonance imaging (MRI) is used for diagnosis and surgery is advised for symptomatic children. We present a case series using MRI including cerebrospinal fluid (CSF) flow, in children with Ch1 to demonstrate a variety of outcomes, both surgical and spontaneous: spontaneous resolution, spontaneous worsening, postsurgical improvement, and postsurgical deterioration. CASE: A 2-week-old female newborn underwent brain MRI demonstrating an ectopic neurohypophysis, under opercularization suggesting brain immaturity and a normal craniocervical junction (CCJ). Follow-up (F/U) MRI at 6 years of age showed interval spontaneous development of Ch1 with decreased CSF spaces at CCJ. CASE: A 6-year-old girl referred for imaging with short stature and growth hormone deficiency demonstrated incidental findings of Ch1 without syringomyelia. There was 15-mm protrusion of pointed cerebellar tonsils through the foramen magnum and a reduced CSF space at the craniocervical junction. No surgery was performed, and F/U MRI at the age of 7 years demonstrated spontaneous resolution of the tonsillar ectopia (cerebellar tonsils now 3 mm right and 6 mm left) and expansion of the CSF spaces at CCJ. CASE: A 7-year-old boy with headaches and staring spells underwent an MRI demonstrating 6-mm protrusion of pointed cerebellar tonsils and CSF space reduction at CCJ. No surgery was performed, and F/U imaging at the age of 9 years demonstrated spontaneous improvement in cerebellar tonsillar position and increased bidirectional CSF flow at CCJ. CASE: A 17-month-old boy underwent brain MRI for unsteady gait and poor vestibular response, which showed Ch1 and narrow CSF spaces at the foramen magnum and with reduced CSF flow. At the age of 3 years, after posterior fossa decompression, F/U MRI showed postsurgical improvement of the position of the cerebellar tonsils and increased CSF space at CCJ. CASE: A 4-month-old male infant with a history of 34-week prematurity, prior germinal matrix hemorrhage, and neonatal subdural hemorrhage was referred for MRI of the cervical and thoracic spine for evaluation of developmental delay and hypotonia with torticollis. Magnetic resonance imaging of the spine demonstrated mild protrusion of inferiorly pointed cerebellar tonsils up to the foramen magnum, with visible CSF and without retroflexion of the dens. Follow-up MRI scans demonstrated progressive worsening of the Chiari 1 deformity, even after multiple surgeries. CONCLUSIONS: It is important to be aware of a variety of different outcomes with Ch1, including spontaneous resolution, spontaneous worsening, improvement with surgery, and even deterioration after surgery. More research is required to determine objective criteria for predicting outcome, which include both anatomic measures and physiologic measures of CSF flow, so that better surgical decisions can be made and for evaluating patients who have undergone surgery.

Atlas invagination through the foramen magnum: expanding the spectrum of craniovertebral junction malformations.

PURPOSE: Malformations of the craniovertebral junction (CVJ) range from mild, asymptomatic conditions to severe forms of instability with basilar invagination. Rarely, there have been accounts of forms of so-called paramedian basilar invagination, with abnormal bone masses invading the lateral portion of the foramen magnum. All these entities have been comprehensively classified both from an anatomical and embryological standpoint. METHODS: Here, we report a case of a unique CVJ malformation which is not included in any existing classification framework and could represent a novel pathologic entity. We also provide an overview of the pertinent literature. RESULTS: The patient was a 14-year-old boy with a recent onset of spastic tetraparesis. Radiological studies documented a malformation of the atlas which invaginated through the foramen magnum, causing anterolateral medullary incarceration. Surgical treatment involved posterior decompression with resection of the abnormal bone and occipito-cervical fusion. CONCLUSION: Our report enriches the panorama of CVJ malformations, showing how anatomical knowledge and embryological insights constitute the basis for the correct assessment and treatment of these complex entities.

Far Lateral Approach: "Trans-tumor Approach" on Huge Dumbbell-Shape Neurofibroma of Anterior Foramen Magnum Without Craniectomy-Anatomical Consideration and New Trend.

The foramen magnum approach is always challenging because of the relationships between vital neurovascular structures in this area. Several approaches have been described, among them, the far lateral approach remains a cornerstone for the resection of anterior or anterolateral processes of the foramen magnum. This approach displays two main steps: the first is cervical, whereas the second is cranial.We report the case of a 63 year-old woman admitted for a progressive quadriplegia with swallowing disorders revealing a process of the anterior and anterolateral part of the foramen magnum. A cervical step of a far lateral approach without opening the foramen magnum achieved a near total resection of the process via a trans-tumor corridor and confirmed a dumbbell shape neurofibroma. The postoperative period showed a resolution of swallowing disorders and a progressive improvement of muscular strength. At 8 months follow-up, she was asymptomatic and able to walk with a normal balance. The surgical technique and anatomical correlation of this trans-tumor approach are discussed.

New minimally-invasive approach in adult for Chiari I malformation.

BACKGROUND: Chiari I malformation is defined by tonsillar herniation through the foramen magnum. There is no consensus on the treatment of Chiari malformation. A simple follow-up is recommended for asymptomatic cases. The classic approach is the midline sub-occipital craniotomy. METHODS: For four years, we operated on six patients with Chiari malformation I using our endoscopic minimally invasive sub-occipital approach. We compared the results with six other patients operated by the classical sub-occipital approach. RESULTS: Patients operated by endoscopic approach had shorter hospital stays, and wounds healed faster and smoother. Mid-term results were similar in the two groups. CONCLUSION: This paper proposes a new endoscopic Minimally invasive paramedian sub-occipital approach for Chiari malformation I. Although the number of cases is limited, the results look promising. We need to gather more cases to have significant numbers to perform a global comparison between the two approaches and assess the advantages and disadvantages of each technique.

How I do it - The "drum skin" duraplasty technique after foramen magnum decompression for Chiari malformations.

BACKGROUND: Chiari malformations are a spectrum of posterior cranial fossa anomalies characterized by herniation of the cerebellar tonsils through the foramen magnum. Surgery is the treatment of choice for selected patients with good postoperative outcomes. METHODS: We describe foramen magnum decompression (FMD) with dural opening and a "drum skin" duraplasty technique. CONCLUSIONS: In our experience, FMD with "drum skin" duraplasty is a safe and effective procedure, reducing the risk of CSF leakage and arachnoidal adhesions and yielding better long-term clinical and radiological outcomes than other traditional techniques.

Cranio-cervical junction malformation causing cord compression in infant with achondroplasia: a bigger picture.

INTRODUCTION: Achondroplasia is a genetic disorder known for short stature and skeletal abnormalities. CASE REPORT: We present a case of narrowing of the foramen magnum from a large opisthion extending to the spinal canal. CONCLUSION: Foramen magnum stenosis and cervicomedullary stenosis are potentially life threatening neurological manifestations of achondroplasia.

Centrally mediated obstructive apnoea and restenosis of the foramen magnum in an infant with achondroplasia.

Achondroplasia is associated with foramen magnum stenosis. We report a male infant with achondroplasia and centrally mediated obstructive apnoea who underwent two foramen magnum decompression due to bone regrowth. He presented at six weeks of age with breath holding and apnoeic episodes associated with significant desaturation, requiring non-invasive ventilation. Craniospinal imaging revealed a narrow foramen magnum without signal change in the spinal cord. Sleep studies showed obstructive, but not central, apnoea. Respiratory abnormalities persisted and reimaging at two months showed development of significant signal changes at the cervicomedullary junction (CMJ). He underwent emergency foramen magnum decompression with initial clinical improvement. Ten days later he relapsed with further apnoeic episodes requiring respiratory support. After extensive re-investigations including CT and MRI, incomplete initial decompression and foramen magnum restenosis were considered and confirmed with a CT head scan 15 weeks after the initial operation. Repeat decompression of bone and removal of thickened dural bands resulted in complete resolution of the apnoeic episodes. Obstructive sleep apnoea can be centrally mediated and further decompression of foramen magnum stenosis should be considered, especially if significant respiratory compromise persists or recurs.

Abrupt foramen magnum syndrome due to shunt malfunction in a previously asymptomatic chiari I malformation: the hidden predictable risk in long-lasting shunted patients.

Cerebrospinal fluid overdrainage in long-lasting shunted hydrocephalus is a well recognized complication. It may lead to different clinical syndromes which may occur over time, namely cranioencephalic disproportion and hindbrain herniation. Among the latter, Chiari malformation type 1 classically refers to one of the mildest form. When symptomatic, the patients generally exhibit a chronic, slowly progressive disease course. Although well documented in Chiari type II malformation and hydrocephalus, ventriculo-peritoneal shunt malfunction causing acute neurological deterioration in acquired Chiari malformation type 1 has been described only in three cases. All these patients underwent urgent shunt revision and suboccipital decompression in two of them, achieving good clinical improvement. We report on a 20-year-old woman harboring a previously asymptomatic Chiari malformation type 1, who experienced an acute foramen magnum syndrome with ventriculo-peritoneal shunt malfunction. We performed an endoscopic third-ventriculo-cisternostomy and definite removal of the shunt, obtaining the complete resolution of symptoms and shunt independence at 2 years follow-up. The phisiopathogenetic mechanisms and surgical management are discussed.

Radiologic Evaluation of the Fossa Navicularis on Dry Skull: A Comparative CBCT Study.

Fossa navicularis (FN) is defined as bony depression that is not always present and is located anterior to the foramen magnum and pharyngeal tubercle on the inferior aspect of the basilar part of the occipital bone. It has been reported that FN can create an infection spread path from the pharynx to the intracranial structures. Therefore, the diagnosis of this variation is important. Although cone beam computed tomography (CBCT) diagnostic accuracy has mostly been verified in detection and quantification particularly on human skulls, there is no study comparing morphometric measurements between direct measurement on the skull and CBCT measurement. The main object of this study is to evaluate the presence of FN on dry bones and CBCT images of the same dry skull and to examine the morphometric and morphological features of this formation. Thirty-two random craniums that were made available for this study that did not have any fractures or deformities of the cranial base were examined. The sagittal diameter, transverse diameter, and depth of the FN was measured both directly on dry skulls and radiologically on CBCT images of dry skull. In addition, the shape of FN (SFN) was determined. FN was detected in 10 (31.25%) of 32 craniums examined with both modalities. It was determined that sagittal diameter of the FN, transverse diameter of the FN, depth of the FN, and the shape of FN did not show a statistically significant difference between the 2 measurements. Unlike the literature, FN was investigated on dry bones both directly and in CBCT images in this study. In contrast to previously thought the FN may be smaller according to this findings, and this small variation can be detected with CBCT images. According to this findings, it can be said that morphometric evaluations on CBCT are accurate and reliable, and CBCT is a safe method for clinical diagnosis and treatment.

Morphology of the Occipital Bones and Foramen Magnum Resulting From Premature Minor Suture Fusion in Crouzon Syndrome.

To identify skull-base growth patterns in Crouzon syndrome, we hypothesized premature minor suture fusion restricts occipital bone development, secondarily limiting foramen magnum expansion.Skull-base suture closure degree and cephalometric measurements were retrospectively studied using preoperative computed tomography (CT) scans and multiple linear regression analysis.Evaluation of multi-institutional CT images and 3D reconstructions from Wake Forest's Craniofacial Imaging Database (WFCID).Sixty preoperative patients with Crouzon syndrome under 12 years-old were selected from WFCID. The control group included 60 age- and sex-matched patients without craniosynostosis or prior craniofacial surgery.None.2D and 3D cephalometric measurements.3D volumetric evaluation of the basioccipital, exo-occipital, and supraoccipital bones revealed decreased growth in Crouzon syndrome, attributed solely to premature minor suture fusion. Spheno-occipital (ß = -398.75; P < .05) and petrous-occipital (ß = -727.5; P < .001) suture fusion reduced growth of the basioccipital bone; lambdoid suture (ß = -14 723.1; P < .001) and occipitomastoid synchondrosis (ß = -16 419.3; P < .001) fusion reduced growth of the supraoccipital bone; and petrous-occipital suture (ß = -673.3; P < .001), anterior intraoccipital synchondrosis (ß = -368.47; P < .05), and posterior intraoccipital synchondrosis (ß = -6261.42; P < .01) fusion reduced growth of the exo-occipital bone. Foramen magnum morphology is restricted in Crouzon syndrome but not directly caused by early suture fusion.Premature minor suture fusion restricts the volume of developing occipital bones providing a plausible mechanism for observed foramen magnum anomalies.

Retrospective evaluation of the morpometric and morphological anatomy of the occipital condyle, hypoglossal canal and foramen magnum in Turkish populaton with CBCT.

PURPOSE: The aim of this study is to morphometrically and morphologically examine the occipital condyle, which is an important anatomical region in terms of surgery and forensic medicine, and its surrounding structures, to evaluate the change in mean values according to gender and age, and to evaluate the correlation of the measurements obtained. METHODS: 180 (90 men, 90 women) CBCT images selected from the archive of Ankara University Faculty of Dentistry. Occipital Condyle length and width, Hypoglossal Canal-Basion distance, Hypoglossal Canal-Opistion distance, Hypoglossal Canal-Occipital Condyle anterior and posterior border distance, Occipital Condyle thickness, Hypoglossal Canal length, the widest diameter of Hypoglossal Canal, the narrowest diameter of the Hypoglossal Canal, the length of the Jugular Tubercle, the width of the Jugular Tubercle, the anterior intercondylar distance, the posterior intercondylar distance, and the Foramen Magnum index were measured. At the same time, the presence of septum or spicule in the hypoglossal canal and protrusion of the occipital condyle were evaluated. The relationship of age, gender, anterior and posterior intercondylar distance, and foramen magnum index measurements with all measurements were examined. RESULTS: In our study, all measurements were repeated 1 month after the first measurements to evaluate the intra-observer agreement, and the agreement between the obtained measurements and the first measurements was evaluated by calculating the intraclass correlation coefficient and 95% confidence intervals. Men's measurements were found to be significantly higher than women's measurements. When the coefficients of concordance in all measurements were examined, it was observed that there was a perfect concordance. CONCLUSION: When the results of the study are evaluated, it is seen that the values ​​obtained are generally close to the studies related to CT. Considering this, an idea can be gained as to whether CBCT, which has a lower dose and less cost, can be used as an alternative to CT in studies to be conducted with more comprehensive and different methods in skull base surgical planning.

Endovascular treatment of medullary bridging vein-draining dural arteriovenous fistulas: foramen magnum vs. craniocervical junction lesions.

PURPOSE: Dural arteriovenous fistulas (AVFs) draining to medullary bridging vein (MBV) are located at foramen magnum (FM) and craniocervical junction (CCJ). Such fistulas are rare but pose a challenge to endovascular management. This study was undertaken to assess clinical manifestations, angiographic features, and outcomes of endovascular treatment in patients with MBV dural AVFs. METHODS: A number of our patients (N = 22) were diagnosed with MBV dural AVF and treated by endovascular means. There were 9 FM lesions and 13 CCJ lesions. We reviewed clinical records and imaging studies to define clinical characteristics, vascular anatomic details, and treatment outcomes, comparing FM- and CCJ-level subsets. RESULTS: Subjects ranged from 37 to 74 years of age (mean, 57.7 years) with male predominance (2.7:1). They presented with intracranial hemorrhage (11/22, 50%), myelopathy (8/22, 36%), or nonspecific symptoms (3/22, 14%). In 17 patients (77.3%), the shunts showed complete or near-complete occlusion following endovascular treatment (FM, 100%; CCJ, 61.5%). However, seven patients experienced ischemic events (FM, 11.1%; CCJ, 46.2%) and one patient sustained a hemorrhagic complication. No hemorrhages recurred during follow-up monitoring, and myelopathic symptoms abated. CONCLUSION: MBV dural AVFs are highly aggressive lesions for which proper diagnosis and treatment are of utmost importance. Although transarterial embolization proved highly successful in FM lesions, shunt occlusion was less frequent in the CCJ subset, with greater risk of ischemic complications.

New insights in craniovertebral junction MR changes leading to stenosis in children with achondroplasia.

PURPOSE: To characterize natural history and early changes of craniovertebral junction stenosis in achondroplasia correlating with clinical and radiological outcome. METHODS: Retrospective measures on craniovertebral junction were performed blindly, on sagittal T2-weighted images, in 21 patients with achondroplasia referred from 2008 to 2020. Clinical and polysomnography data were retrospectively collected. Each patient was paired for age and gender with four controls. Wilcoxon means comparison or Student's t-tests were applied. RESULTS: Twenty-one patients (11 females, from 0.1 to 39 years of age) were analyzed and paired with 84 controls. A craniovertebral junction stenosis was found in 11/21 patients (52.4%), all before the age of 2 years. Despite a significant reduction of the foramen magnum diameter (mean ± SD: patients 13.6 ± 6.2 mm, controls 28.5 ± 4.7 mm, p < .001), craniovertebral junction stenosis resulted from the narrowing of C2 dens-opisthion antero-posterior diameter (8.7 ± 3.9 mm vs 24.6 ± 5.1 mm, p < .001). Other significant changes were opisthion anterior placement (-0.4 ± 2.8 mm vs 9.4 ± 2.3 mm, p < .001), posterior tilt of C2 (46.2 ± 13.7° vs 31.6 ± 7.9°, p < .001) and of C1 (15.1 ± 4.3° vs 11.9 ± 5.0°, p = 0.01), and dens thickening (9.4 ± 2.2 mm vs 8.5 ± 2.1 mm, p = 0.03), allowing to define three distinguishable early craniovertebral junction patterns in achondroplasia. All children with C2-opisthion antero-posterior diameter of more than 6 mm had a better clinical and radiological outcome. CONCLUSION: Craniovertebral junction in achondroplasia results from narrowing between C2 dens and opisthion related to anterior placement of opisthion, thickening of C2 dens, and posterior tilt of C1-C2. A threshold of 6 mm for dens-opisthion sagittal diameter seems to correlate with clinical and radiological outcome.

Severe Basilar impression in osteogenesis imperfecta treated with halo gravity traction, occipitocervicothoracic fusion, foramen magnum and upper cervical decompression and expansive duroplasty: a technical note.

Osteogenesis imperfecta (OI) is a rare bone disease due to an abnormal synthesis of 1-type collagen. OI is frequently associated with basilar impression (BI), defined by the elevation of the clivus and floor of the posterior fossa with subsequent migration of the upper cervical spine and the odontoid peg into the base of the skull. Bone intrinsic fragility leading to fractures and deformity, brainstem compression and impaired CSF circulation at cranio-vertebral junction (CVJ) makes the management of these conditions particularly challenging. Different surgical strategies, including posterior fossa decompression with or without instrumentation, transoral or endonasal decompression with posterior occipito-cervical fusion, or halo gravity traction with posterior instrumentation have been reported, but evidence about best modalities treatment is still debated. In this technical note, we present a case of a 16-years-old patient, diagnosed with OI and BI, treated with halo traction, occipito-cervico-thoracic fixation, foramen magnum and upper cervical decompression, and expansive duroplasty. We focus on technical aspects, preoperative work up and postoperative follow up. We also discuss advantages and limitations of this strategy compared to other surgical techniques.

The foramen magnum in scaphocephaly.

PURPOSE: The foramen magnum (FM) presents various alterations in craniosynostoses, such as brachycephaly or Crouzon syndrome. However, to date, no study has been devoted to its morphology and morphometry in scaphocephaly, which is the most common of cranial deformities resulting from premature fusion of cranial sutures. METHODS: We assessed the morphology and morphometry of FM using preoperative thin-cut CT scans of 107 children with non-syndromic sagittal craniosynostosis aged 1-12 months (mean age 5.38 months). A series of sagittal and transverse dimensions were taken and the FM area was calculated in each case. Obtained data were compared to the age-matched control group of 101 normocephalic children. RESULTS: Dolichotrematous type of FM was dominant in the scaphocephaly group and observed in 63/107 cases (58.9%). The mean FM area in the scaphocephaly group was 519.64 mm2 and was significantly smaller compared to the control group (p = 0.0011). The transverse diameter and anterior sagittal diameter were also significantly smaller (p = 0.0112 and p = 0.0003, respectively). CONCLUSION: The area of FM in scaphocephaly is smaller compared to normal individuals. This is associated with a significant reduction of the width of FM in children with sagittal craniosynostosis. FM in scaphocephaly is larger than in other reported series of children with brachycephaly or Crouzon syndrome.

The (extended) achondroplasia foramen magnum score has good observer reliability.

BACKGROUND: Achondroplasia is the most common skeletal dysplasia. A significant complication is foramen magnum stenosis. When severe, compression of the spinal cord may result in sleep apnea, sudden respiratory arrest and death. To avoid complications, surgical decompression of the craniocervical junction is offered in at-risk cases. However, practice varies among centres. To standardize magnetic resonance (MR) reporting, the achondroplasia foramen magnum score was recently developed. The reliability of the score has not been assessed. OBJECTIVE: To assess the interobserver reliability of the achondroplasia foramen magnum score. MATERIALS AND METHODS: Base of skull imaging of children with achondroplasia under the care of Sheffield Children's Hospital was retrospectively and independently reviewed by four observers using the achondroplasia foramen magnum score. Two-way random-effects intraclass coefficient (ICC) was used to assess inter- and intra-observer reliability. RESULTS: Forty-nine eligible cases and five controls were included. Of these, 10 were scored normal, 17 had a median score of 1 (mild narrowing), 11 had a median score of 2 (effacement of cerebral spinal fluid), 10 had a score of 3 (compression of cord) and 6 had a median score of 4 (cord myelopathic change). Interobserver ICC was 0.72 (95% confidence interval = 0.62-0.81). Intra-observer ICC ranged from 0.60 to 0.86. Reasons for reader disagreement included flow void artefact, subtle T2 cord signal and myelopathic T2 cord change disproportionate to canal narrowing. CONCLUSION: The achondroplasia foramen magnum score has good interobserver reliability. Imaging features leading to interobserver disagreement have been identified. Further research is required to prospectively validate the score against clinical outcomes.

Posterior fossa morphometry in 170 South Asian children and adults with Chiari malformation and its correlation with tonsillar descent.

AIM: Patients with Chiari I malformations (CM1) have posterior fossa hypoplasia with crowding of the neural structures. We aimed to study the posterior fossa measurements to analyse the posterior fossa morphology, presence of basilar invagination (BI) and correlated the measurements with the degree of tonsillar descent. METHODS: We retrospectively reviewed data in170 patients who underwent foramen magnum decompression (FMD) for CM1. The posterior fossa measurements were compared with 30 controls. Patients were divided into those with and without BI. The variables measured included clival length, posterior fossa height, supraocciput length, posterior fossa diameter, foramen magnum width, Boogard's angle, clival angle, clival slope and the newly introduced foramen magnum (FM) angle. RESULTS: The average clivus length and posterior fossa height were significantly shorter with a significant increase in the Boogard's and FM angle in the patient groups. Tonsillar descent showed a negative correlation with posterior fossa height (r = -0.498, p ≤ 0.001) and clivus length (r = -0.325, p ≤ 0.001) and a positive correlation with Boogard's angle (r = 0.469, p ≤ 0.001) and FM angle (r = 0.330, p ≤ 0.001). Patients with BI had statistically significant reduced posterior fossa height (p ≤ 0.001) and increased extent of tonsillar herniation (p = 0.001) compared to patients without BI. CONCLUSION: Patients with CM1 have significantly shorter clival length and posterior fossa height with smaller posterior fossa in support of published data. The presence of BI shortens the posterior fossa height and worsens the extent of tonsillar herniation. An increased Boogard's angle and FM angle result in a more horizontally placed suboccipital bone compared to a slanting bone in normal persons.