The hindbrain is not malformed but deformed. Accordingly, “Chiari type 1 deformity,” not “Chiari type 1 malformation” is the correct term to characterize primary tonsillar herniation.

Defined as cerebellar tonsillar herniation ≥ 5 mm below the foramen magnum ¹⁾.

Chiari malformation Type 1.5 (CM 1.5) was defined as the association of Chiari malformation Type I (CM I) and brainstem herniation.

Although CM 1.5 patients presented with brainstem herniation and more severe tonsillar herniation, other clinical and imaging features and surgical outcomes were similar with CM I patients. Liu et al. think CM 1.5 is just a subtype of CM I, rather than a unique type of Chiari malformations ²⁾.

Taylor et al. identifies two subtypes, crowded and spacious, that can be distinguished by MRI appearance without volumetric analysis. Earlier age at surgery and presence of syringomyelia are more common in the crowded subtype. The presence of the spacious subtype suggests that crowdedness alone cannot explain the pathogenesis of Chiari I malformation in many patients, supporting the need for further investigation ³⁾.

Chiari type 1 deformity is commonly seen in pediatric neurology, neuroradiology, and neurosurgery and may have various clinical presentations depending on patient age. In addition, Chiari type 1 deformity is increasingly found by neuroimaging studies as an incidental finding in asymptomatic children ⁴⁾.

In the past, it was estimated that the condition occurs in about one in every 1,000 births. However, the increased use of diagnostic imaging has shown that CM may be much more common. Complicating this estimation is the fact that some children who are born with the condition may not show symptoms until adolescence or adulthood, if at all. CMs are more prevalent in certain groups, including people of Celtic descent.

A statistically significant (P = .03) female predominance of the malformation was observed, with a female: male ratio of approximately 3:2.

Associated skeletal anomalies were seen in 24% of patients.

Syringomyelia was detected in 40% of patients, most commonly between the C-4 and C-6 levels. Of the 25 patients who presented with spinal symptoms, 23 (92%) proved to have a syrinx at MR imaging. When the syrinx extended into the medulla (n = 3), however, brainstem symptoms predominated. Patients with objective brain stem or cerebellar syndrome had the largest mean tonsillar herniations. Patients with tonsillar herniations greater than 12 mm were invariably symptomatic, but approximately 30% of patients with tonsils herniating 5-10 mm below the foramen magnum were asymptomatic at MR imaging. “Incidental” Chiari I malformations are thus much more common than previously recognized, and careful clinical assessment remains the cornerstone for proper diagnosis and management ⁵⁾.

The innate bony dysontogenesis in patients with CMI contributes to tonsilar ectopia and exacerbates CSF flow obstruction. A pressure gradient that existed between syringomyelia(SM) and SAS supports the perivascular space theory that is used to explain SM formation. Our findings demonstrate that phase-contrast magnetic resonance imaging (PCMR) maybe a useful tool for predicting patient prognosis ⁶⁾.

In adult CIM, most tonsillar herniations are asymmetrical and most syringomyelia is eccentrical. The dominant side of tonsillar herniation determines the side of syrinx deviation, which in turn determines the main side of clinical presentations and the convex side of scoliosis. The results suggest that the more the descended tonsil tilts to one side, the more the syrinx tilts to the same side ⁷⁾.

Chiari malformation Type I (CM-I) related to syndromic craniosynostosis in pediatric patients has been well-studied. The surgical management consists of cranial vault remodeling with or without posterior fossa decompression. There were also cases, in whom CM-I was diagnosed prior to the craniosynostosis in early childhood.

A 16-year-old boy who admitted with symptoms related to CM-I. With careful examination and further genetic investigations, a diagnosis of Crouzon syndrome was made, of which the patient and his family was unaware before. The patient underwent surgery for posterior fossa decompression and followed-up for Crouzon’s syndrome.

This is the only case report indicating a late adolescent diagnosis of Crouzon syndrome through clinical symptoms of an associated CM-I ⁸⁾.

A population-based genealogical resource with linked medical data was used to define the observed familial clustering of Chiari malformation Type I (CM-I). METHODS All patients with CM-I were identified from the 2 largest health care providers in Utah; those patients with linked genealogical data were used to test hypotheses regarding familial clustering. Relative risks (RRs) in first-, second-, and third-degree relatives were estimated using internal cohort-specific CM-I rates; the Genealogical Index of Familiality (GIF) test was used to test for an excess of relationships between all patients with CM-I compared with the expected distribution of relationships for matched control sets randomly selected from the resource. Pedigrees with significantly more patients with CM-I than expected (p < 0.05) based on internal rates were identified. RESULTS A total of 2871 patients with CM-I with at least 3 generations of genealogical data were identified. Significantly increased RRs were observed for first- and third-degree relatives (RR 4.54, p < 0.001, and RR 1.36, p < 0.001, respectively); the RR for second-degree relatives was elevated, but not significantly (RR 1.20, p = 0.13). Significant excess pairwise relatedness was observed among the patients with CM-I (p < 0.001), and borderline significant excess pairwise relatedness was observed when all relationships closer than first cousins were ignored (p = 0.051). Multiple extended high-risk CM-I pedigrees with closely and distantly related members were identified. CONCLUSIONS This population-based description of the familial clustering of 2871 patients with CM-I provided strong evidence for a genetic contribution to a predisposition to CM-I ⁹⁾.

The pathophysiology of CMI is poorly understood and it remains unknown how ICP alterations relate to symptoms and radiological findings.

There is some evidence of impaired intracranial compliance as an important pathophysiological mechanism ¹⁰⁾.

Magnetic resonance imaging measurement of transcranial CSF flow and blood flow may lead to a better understanding of the pathophysiology of Chiari malformations and may prove to be an important diagnostic tool for guiding for the treatment of patients with Chiari I malformation ¹¹⁾.

The pathogenesis of a Chiari I malformation of the cerebellar tonsils is grouped into 4 general mechanisms. ¹²⁾.

It appears that the pathogenesis of Chiari malformation with or without associated basilar invagination and/or syringomyelia is primarily related to atlantoaxial instability. The data suggest that the surgical treatment in these cases should be directed toward atlantoaxial stabilization and segmental arthrodesis. Except in cases in which there is assimilation of the atlas, inclusion of the occipital bone is neither indicated nor provides optimum stability. Foramen magnum decompression is not necessary and may be counter-effective in the long run ¹³⁾. It occurs in children and adults. Clinical symptoms mainly develop from alterations in CSF flow at the foramen magnum and the common subsequent development of syringomyelia.

Patients with Chiari malformation type 1 (CMI) often present with elevated pulsatile and static intracranial pressure (ICP).

Several lines of evidence suggest common pathophysiological mechanisms in Chiari malformation Type I (CMI) and idiopathic intracranial hypertension (IIH). It has been hypothesized that tonsillar ectopy, a typical finding in CMI, is the result of elevated intracranial pressure (ICP) combined with a developmentally small posterior cranial fossa (PCF).

The study of Frič and Eide showed comparable and elevated pulsatile intracranial pressure, indicative of impaired intracranial compliance, in both CMI and IIH cohorts, while static ICP was higher in the IIH cohort. The data did not support the hypothesis that reduced PCFV combined with increased ICP causes tonsillar ectopy in CMI. Even though impaired intracranial compliance seems to be a common pathophysiological mechanism behind both conditions, the mechanisms explaining the different clinical and radiological presentations of CMI and IIH remain undefined¹⁴⁾.

Chiari malformation Type I was incidentally detected on MR images in 11 of 22 patients. The remaining 11 patients had minimal clinical signs at presentation that were not regarded as necessitating immediate surgical treatment. Seventeen patients (77.3%) showed progressive improvement in their symptoms or remained asymptomatic at the last follow-up whereas 5 patients (22.7%) experienced worsening, which was mild in 2 cases and required surgical correction in the remaining 3 cases. On MR imaging a mild reduction in tonsillar herniation was appreciated in 4 patients (18.18%), with complete spontaneous resolution in 1 of these. In 16 patients, tonsillar herniation remained stable during follow-up.

Data confirm the common impression that in both asymptomatic and slightly symptomatic patients with CM-I, a conservative approach to treatment should be adopted with periodic clinical and radiological examinations ¹⁵⁾.

Although the most common presentation is occipital headache, the association of audio-vestibular symptoms is not rare.

The headache is commonly aggravated by Valsalva and sensory and motor deficits.

In a series of 71 patients, pain was the commonest symptom (69% of patients); other symptoms included weakness (56%), numbness (52%), and unsteadiness (40%). The presenting physical signs consisted of a foramen magnum syndrome (22%), central cord syndrome (65%), or a cerebellar syndrome (11%) ¹⁶⁾.

The appearance of audio-vestibular manifestations in CM-I makes it common to refer these patients to neurotologists. Unsteadiness, vertiginous syndromes and sensorineural hearing loss are frequent. Nystagmus, especially horizontal and down-beating, is not rare. It is important for neurotologists to familiarise themselves with CM-I symptoms to be able to consider it in differential diagnosis ¹⁷⁾.

Feinberg et al present a case of Chiari malformation manifesting as isolated trismus, describe the typical symptoms associated with Chiari malformation, and discuss the potential anatomical causes for this unique presentation. A 3-year-old boy presented with inability to open his jaw for 6 weeks with associated significant weight loss. The results of medical and radiological evaluation were negative except for Chiari malformation type 1 with cerebellar tonsils 12 mm below the level of the foramen magnum. The patient underwent Chiari decompression surgery. Postoperatively, his ability to open his mouth was significantly improved, allowing resumption of a regular diet. Postoperative MRI revealed almost complete resolution of the syringobulbia. To the best of the authors’ knowledge, this is the first reported case of isolated trismus from Chiari malformation with syringobulbia ¹⁸⁾.

Along with tonsillar herniation, imaging studies have documented additional abnormalities, including smaller and overcrowded posterior cranial fossa ^{19) 20) 21) 22) 23)}.

MRI Findings After Surgery for Chiari Malformation Type I is important when evaluating postoperative changes ²⁴⁾.

Sagittal MRI overestimates the degree of tonsillar ectopia. Misdiagnosis may occur if sagittal imaging alone is used. The cerebellar tonsils are paramedian structures, and this should be kept in mind when interpreting midline sagittal MRI.

An accurate and reliable selection of patients based on clinical and neuroimaging findings is paramount for the success of neurosurgical treatment²⁵⁾.

see Posterior fossa decompresion for Chiari type 1 deformity.

Efforts to guide preoperative counseling and improve outcomes research are impeded by reliance on small, single-center studies.

Approximately 1 in 8 pediatric CM-I patients experienced a surgical complication, whereas medical complications were rare. Although complex chronic conditions (CCC) were common in pediatric CM-I patients, only hydrocephalus was independently associated with increased risk of surgical events. These results may inform patient counseling and guide future research efforts ²⁶⁾.

CM-I in children is not a radiologically static entity but rather is a dynamic one. Radiological changes were seen throughout the 7 years of follow-up. A reduction in tonsillar herniation was substantially more common than an increase. Radiological changes did not correlate with neurological examination finding changes, symptom development, or the need for future surgery. Follow-up imaging of asymptomatic children with CM-I did not alter treatment for any patient. It would be reasonable to follow these children with clinical examinations but without regular surveillance MRI ²⁷⁾.

Outcome assessment for the management of Chiari malformation type 1 is difficult because of the lack of a reliable and specific surgical outcome assessment scale. Such a scale could reliably correlate postoperative outcomes with preoperative symptoms.

Chicago Chiari Outcome Scale (CCOS)

Outcome is poor in approximately 3 in 10 patients ²⁸⁾.

The degree of tonsillar herniation has not been a reliable predictor of either symptom severity ²⁹⁾ or surgical outcome ³⁰⁾.

Arnautovic et al. identified 145 operative series of patients with CM-I, primarily from the United States and Europe, and divided patient ages into 1 of 3 categories: adult (> 18 years of age; 27% of the cases), pediatric (≤ 18 years of age; 30%), or unknown (43%). Most series (76%) were published in the previous 21 years. The median number of patients in the series was 31. The mean duration of the studies was 10 years, and the mean follow-up time was 43 months. The peak ages of presentation in the pediatric studies were 8 years, followed by 9 years, and in the adult series, 41 years, followed by 46 years. The incidence of syringomyelia was 65%. Most of the studies (99%) reported the use of posterior fossa/foramen magnum decompression. In 92%, the dura was opened, and in 65% of these cases, the arachnoid was opened and dissected; tonsillar resection was performed in 27% of these patients. Postoperatively, syringomyelia improved or resolved in 78% of the patients. Most series (80%) reported postoperative neurological outcomes as follows: 75% improved, 17% showed no change, and 9% experienced worsening. Postoperative headaches improved or resolved in 81% of the patients, with a statistical difference in favor of the pediatric series. Postoperative complications were reported for 41% of the series, most commonly with CSF leak, pseudomeningocele, aseptic meningitis, wound infection, meningitis, and neurological deficit, with a mean complication rate of 4.5%. Complications were reported for 37% of pediatric, 20% of adult, and 43% of combined series. Mortality was reported for 11% of the series. No difference in mortality rates was seen between the pediatric and adult series ³¹⁾.

Patients treated for Chiari I malformation (CM-I) with posterior fossa decompression (PFD) may occasionally and unpredictably develop postoperative hydrocephalus. The clinical risk factors predictive of this type of Chiari-related hydrocephalus (CRH) are unknown.

Younger patients, those with extensive intraoperative blood loss, and those found during surgery to have a fourth ventricular web were at higher risk for the development of CRH. Clinicians should be alert to evidence of CRH in this patient population after PFD surgery ³²⁾.

There is currently no consensus on the safety of sports participation for patients with Chiari I malformation (CM-I).

A prospective survey was administered to 503 CM-I patients at 2 sites over a 46-month period. Data were gathered on imaging characteristics, treatment, sports participation, and any sport-related injuries. Additionally, 81 patients completed at least 1 subsequent survey following their initial entry into the registry and were included in a prospective group, with a mean prospective follow-up period of 11 months.

Of the 503 CM-I patients, 328 participated in sports for a cumulative duration of 4641 seasons; 205 of these patients participated in contact sports. There were no serious or catastrophic neurological injuries. One patient had temporary extremity paresthesias that resolved within hours, and this was not definitely considered to be related to the CM-I. In the prospective cohort, there were no permanent neurological injuries.

No permanent or catastrophic neurological injuries were observed in CM-I patients participating in athletic activities. The authors believe that the risk of such injuries is low and that, in most cases, sports participation by children with CM-I is safe ³³⁾.

Brock et al., analyzed prospectively 49 patients with CM operated at the Hospital das Clinicas, College of Medicine, University of São Paulo. Patients underwent decompressive surgery with or without opening of the duramater after intraoperative ultrasonography measuring flow rate. A value of 3cm/s was considered a cut-off. Quality of life before and after surgery and the improvement of neck pain and headache were evaluated.

Among 49 patients enrolled, 36 patients (73%) had CSF flow above 3 cm/s and did not undergo duraplasty. In 13 (27%) patients with initial flow <3 cm/s, a dural opening was performed together with duraplasty. All patients improved comparing pre and post operative scores and all clinical parameters evaluated did not differ between both surgical groups. Patients submitted to bone decompression alone had fewer complication rate.

Intraoperative USG with measurement of CSF allows the proper selection of patients with CM that can have a less invasive surgery with bone decompression without duraplasty ³⁴⁾.

A retrospective cohort study was performed for patients 0-18 years of age who underwent surgical correction for Chiari Type I malformation with syrinx between 1995 and 2013. Basic demographic information was collected as well as data for preoperative symptoms, prior surgical history, perioperative characteristics, and postsurgical outcomes. Descriptive statistics were performed in addition to bivariate analyses. Candidate predictor variables were identified based on an association with tonsillar cautery with p < 0.10. Forward stepwise likelihood ratio was used to select candidate predictors in a binary logistic regression model (Pin = 0.05, Pout = 0.10) most strongly associated with the outcome. RESULTS A total of 171 patients with Chiari Type I malformation with syrinx were identified, and 43 underwent tonsillar cautery. Patients who underwent tonsillar cautery had 6.11 times greater odds of improvement in their syrinx (95% CI 2.57-14.49, p < 0.001). There was no effect of tonsillar cautery on increased perioperative complications as well as the need for repeat decompressions. CONCLUSIONS Tonsillar cautery is safe and effective in the treatment of Chiari Type I malformation with syrinx and may decrease time to syrinx resolution after cervicomedullary decompression. Tonsillar cautery does not increase postoperative complications in pediatric Chiari Type I malformation patients ³⁵⁾

156 consecutive pediatric patients in whom the senior authors performed PFD without dural opening from 2003 to 2013. Patient demographics, clinical symptoms and signs, radiographic findings, intraoperative ultrasound results, and neuromonitoring findings were reviewed. Univariate and multivariate regression analyses were performed to determine risk factors for recurrence of symptoms and the need for reoperation. RESULTS Over 90% of patients had a good clinical outcome, with improvement or resolution of their symptoms at last follow-up (mean 32 months). There were no major complications. The mean length of hospital stay was 2.0 days. In a multivariate regression model, partial C-2 laminectomy was an independent risk factor associated with reoperation (p = 0.037). Motor weakness on presentation was also associated with reoperation but only with trend-level significance (p = 0.075). No patient with < 8 mm of tonsillar herniation required reoperation.

The vast majority (> 90%) of children with symptomatic CM-I will have improvement or resolution of symptoms after a PFD without dural opening. A non-dural opening approach avoids major complications. While no patient with tonsillar herniation < 8 mm required reoperation, children with tonsillar herniation at or below C-2 have a higher risk for failure when this approach is used ³⁶⁾.

Thirty-nine cases of CM-1 with and without syringomyelia (SM) were included. There were 18 patients in the nonduraplasty and 21 in the duraplasty group. Syringomyelia, tonsillar herniation (TH), preoperative symptom duration, and postoperative SM size were compared.

No significant difference was found between improvement in the duraplasty group (81%) and the non-duraplasty group (61.1%). In cases whose symptom duration was 0-36 months, improvement in the duraplasty group (93%) was significantly better than in the nonduraplasty group (50%) (p < 0.01). The rate of syrinx regression was 92.3% in the duraplasty group and 12.5% in the non-duraplasty group (p < 0.05). In cases with SM, the improvement was 21.4% in the non-duraplasty group compared to 78.6% in the duraplasty group (p=0.056). In cases with TH greater than 10 mm, the improvement was 66.7% in the non-duraplasty group, whereas all six cases (100%) in the duraplasty group had improved.

In SM associated cases, cases with TH greater than 10 mm, and whose symptom duration is less than 36 months, duraplasty is a more reliable choice despite a slightly higher rate of complications ³⁷⁾.

In 21 patients, 12 cases had osteo-compression on the cerebellar hemisphere, 18 cases had thickened adhered fabric ring that stretched from arachnoid membrane to cerebellar hemisphere, and 15 cases with syringomyelia. The patients were followed up for 6 months to 3 years after the surgery. All patients showed a remarkable recovery of syringomyelia. There were no morbidity or death related to the surgery. Most of ACM-1 patients, the osteo- and membrane compression on cerebellar hemisphere and tonsil were observed during the operation. Therefore, decompression of foramen magnum and posterior craniocervical combined with the removal of cerebellomedullary fissure arachnoid membrane and placement of an artificial dural graft should be considered as a comprehensive option of minimally invasive surgery and rational and radical treatment of ACM-1. Our experience showed that, by using our procedure, shunting becomes no longer necessary in the treatment of ACM-1-associated syringomyelia ³⁸⁾.

Of the 25 patients who presented with spinal symptoms, 23 (92%) proved to have a syrinx at MR imaging. When the syrinx extended into the medulla (n = 3), however, brain stem symptoms predominated. Patients with objective brain stem or cerebellar signs had the largest mean tonsillar herniations. Patients with tonsillar herniations greater than 12 mm were invariably symptomatic, but approximately 30% of patients with tonsils herniating 5-10 mm below the foramen magnum were asymptomatic at MR imaging. “Incidental” Chiari I malformations are thus much more common than previously recognized, and careful clinical assessment remains the cornerstone for proper diagnosis and management ³⁹⁾.

In a series of 71 patients, pain was the commonest symptom (69% of patients); other symptoms included weakness (56%), numbness (52%), and unsteadiness (40%). The presenting physical signs consisted of a foramen magnum compression syndrome (22%), central cord syndrome (65%), or a cerebellar syndrome (11%). Myelography was performed in 69 patients, and was the most useful investigation. Only 23% of plain radiographs were abnormal. In addition to tonsillar descent, the operative findings included arachnoid adhesions (41%) and syringomyelia (32%). All patients underwent suboccipital craniectomy and C1-3 laminectomy. Respiratory depression was the most frequent postoperative complication (14%), and one patient died from sleep apnea. Early postoperative improvement of both symptoms (82%) and signs (70%) was followed by later relapse in 21% of patients, showing an initial benefit following surgery. None of the patients with a cerebellar syndrome deteriorated, whereas 56% of patients with evidence of foramen magnum compression and 66% of those with a central cord syndrome maintained their initial improvement. The authors conclude that posterior fossa decompression appears to benefit some patients, although a significant proportion might be expected to relapse within 2 to 3 years after operation, depending upon the presenting syndrome ⁴⁰⁾.

A 13-year-old obese boy with a 3-week history of headaches, neck pain, torticollis and progressive visual deterioration was admitted. Bilateral chronic papilledema and decrease in visual acuity were found in the presence of a previously diagnosed CMI.

Intracranial pressure monitoring demonstrating increased pressure levels was followed by a suboccipital decompression, C1 laminectomy and duroplasty. Post-operatively, the boy improved markedly, the 6 months follow-up opthalmological examination demonstrated resolution of papilloedema, but consecutive bi-lateral optic nerve atrophy.

IH with progressive visual deterioration represents one of the varying clinical presentations of CMI and may be classified as a secondary form of idiopathic intracranial hypertension (IH). Neuro-ophthalmological examination in all patients with CMI is recommended to identify the real incidence of this presentation. Altered CSF dynamics, venous hypertension and obesity as co-factors may be causative pathophysiologic factors ⁴¹⁾.