Cerebellar Tonsillar Ectopia and Chiari Malformations

Defining normal

CTE is defined by any descent of the cerebellar tonsils past the foramen magnum whether symptomatic or not, which is discovered on midsagittal MRI by using the basion opisthion line which is respectively the anterior to posterior point of the foramen magnum and measuring the lowest point of the cerebellar tonsils from this line. Coronal view best to diagnoseThere is no single agreed cutoff for normal. Most authors treat 5 mm or more below the B-OL (McRaes) as pathologic and diagnostic of Chiari type I; the classic definition of Chiari malformation is herniation of 3 to 5 mm below the foramen magnum, with a 6 mm threshold applied in patients under age 10. Others say  2 mm and some argue that any inferior displacement of the tonsils below the B-OL in adolescents should be regarded as CTE is abnormal. Tonsillar position is not static — the tonsils pulsate with the cardiac cycle, and dynamic herniation during the cardiac cycle has been demonstrated on dynamic MRI. And position varies with posture, A single supine measurement is therefore a snapshot of a moving structure in one of two possible gravitational states.

The Chiari spectrum

Hans Chiari in 1891 - autopsies of kids with hydrocephalus, and recognized at the outset that the size of the structural defect didnt match the severity of the hydrocephalusCM type 0: There are little to no parts of your cerebellum in the foramen magnum. But there’s crowding at that level. CM type 1: The lower parts of your cerebellum (the cerebellar tonsils) go into the foramen magnum. This is the most common type of Chiari.CM type 1.5: Your cerebellum and part of your brainstem go into the foramen magnum. Providers consider type 1.5 a progression of type 1.CM type 2: Your cerebellar vermis, brainstem and fourth ventricle (a fluid-filled cavity) are compressed. It usually happens with myelomeningocele, a severe form of spina bifida.CM type 3: Some of your cerebellum and brainstem extend through an abnormal opening in the back of your skull (not the foramen magnum). This is called an encephalocele.CM type 4: Your cerebellum is underdeveloped or has parts missing. It’s also known as cerebellar hypoplasia.Chiari V malformation – absent cerebellum, herniation of the occipital lobe through the foramen magnumcraniocervical junction abnormalities contribute to CTE by abnormal clivo-axial angle, retroflexed odontoid, occipitalization of the atlas, basilar invagination, cervicomedullary kinking, and scoliosis. much heterogeneity when it comes to the chairi eponym Traditionally the condition was considered rare, 1 in 1000 births, but improved imaging estimates in the material range from 200,000 to 2 million Americans, with a conservative figure of 300,000 corresponding to about 1 in 1000 people or 0.1% of the population.Women are affected roughly three times more often than men, genetic work supports a hereditary tendency with a transmissibility rate near 12%, and approximately 3,500 Chiari operations are performed annually in the United States. Clinicians at a dedicated conference described frequently seeing people wrongly diagnosed, undiagnosed, or underdiagnosed for years despite telltale symptoms and radiographic evidence, and called for a registry, since the true natural history remains poorly understood.

Pathogenesis I: the congenital, mesodermal account

One study showed a  prospective cohort of 364 symptomatic patients. Volumetric analysis showed a significantly reduced posterior cranial fossa volume and a 40% reduction in posterior fossa CSF volume with normal brain volume, alongside reduced supraocciput height, reduced clivus length, and increased tentorial slope. The conclusion was that Chiari I is a disorder of the paraaxial mesoderm — underdevelopment of the posterior fossa with overcrowding of a normally developed hindbrain — and therefore inherently different from types II and III. Associated findings included syringomyelia in 65%, scoliosis in 42%, and basilar invagination in 12%; 12% had positive family histories, with pedigrees consistent with autosomal dominant or recessive inheritance.Several other congenital routes converge on the same crowded posterior fossa. Craniosynostosis, particularly lambdoid synostosis, is associated with posterior fossa hypoplasia, Chiari malformation, and hydrocephalus. The craniofacial syndromes — Crouzon, Apert, Pfeiffer, Saethre-Chotzen — are frequently associated with skull base malformations and Chiari malformations that block CSF flow. Hypoplasia of the foramen magnum or the jugular foramen, anomalies of the odontoid process, abnormal clivo-axial angle, basilar invagination, and platybasia are all named as predisposing to coning of the tonsils into the foramen magnum. Where the deformity is primarily a shortage of posterior fossa volume, an inferiorly directed posterior cranial vault distraction has been described as a treatment approach — a reminder that the target of therapy follows from the mechanism.

Pathogenesis II: the acquired and mechanical accounts

Running in parallel is a set of mechanisms in which the tonsils descend into a normally formed fossa, or in which the junction moves relative to the brainstem.Trauma. Failure to decompress -  Twenty-four percent of the Milhorat cohort cited trauma as the precipitating event, and separate reports describe symptomatic conversion of previously quiescent Chiari I after minor head or neck trauma, discovery of symptomatic Chiari I following motor vehicle crashes, presentation after a flexion injury to the neck, and — at the extreme — death following minor head trauma in two adults with the Chiari I deformity. A crucial diagnostic problem follows from this: acquired tonsillar herniation is radiographically indistinguishable from pre-existing ectopia, and the ectopia is very often discovered only because imaging was performed after the trauma.Traction from below: weakening or disruption of craniocervical junction stabilizers permits head-forward posture and loss or reversal of the cervical lordosis, which effectively lengthens the the canal relative to the cord and is tethered segmentally by the dentate ligaments, the result is caudal traction transmitted upward to brainstem and cerebellum. Related work has described compromised cranio-spinal suspension in Chiari I as a secondary pathophysiology, and abnormal spinal cord motion at the craniocervical junction in hypermobile Ehlers-Danlos patients.Low CSF volume and pressure cones. Dural tears, CSF leaks, and over-drainage produce pressure cones that cause tonsillar ectopia by an entirely different route — the tonsils are pulled down by a pressure gradient rather than pushed down by crowding. Two iatrogenic causes are named explicitly: shunting, with acquired Chiari I described as a late complication of CSF shunting, and lumboperitoneal shunting in particular, after which tonsillar herniation has been called "the rule rather than the exception" in the pediatric population. Lumbar puncture is likewise listed among the common procedures known to cause coning. Connective tissue disorders such as Ehlers-Danlos appear predisposed to leaks and are therefore doubly vulnerable — both to spontaneous leak and to post-procedural leak.Cranial settling — the mirror image. Connective tissue disorders including rheumatoid arthritis and Ehlers-Danlos cause ligament tears, degeneration, and laxity, and thereby cranial settling: the skull rocks back on the craniocervical junction and sinks onto the upper cervical spine during upright posture. This is worth stating precisely because it inverts the usual picture. In Chiari, the brainstem is displaced downward into the foramen magnum; in cranial settling, the junction shifts upward relative to the brainstem. The outcome resembles basilar invagination and can likewise cause hydrocephalus. Clinically this matters because the two produce overlapping symptoms by opposite kinematics, and a syndrome of occipitoatlantoaxial hypermobility, cranial settling, and Chiari I malformation has been described in patients with hereditary disorders of connective tissue — meaning both can coexist in the same patient.

The epidemiology of the traumatic association

The largest case-control series bearing on the traumatic hypothesis reviewed 1200 cervical MRI scans in neck pain patients, 600 with recent motor vehicle crash trauma and 600 without, half scanned recumbent and half upright. Tonsils 1 mm or below 5.7% and 5.3% of the recumbent and upright non-trauma groups, versus 9.8% and 23.3% of the recumbent and upright trauma groups roughly a fourfold difference when imaged upright. Upright imaging increased sensitivity to ectopia about 2.5-fold over recumbent imaging, and the effect was significantly larger among women, whose average tonsil position was lower across groups. Notably, herniation of 5 mm or more was rare in every group, with only six cases in total across all 1195 scans read. The authors observed, as an incidental point, that the proportion of upright scans showing ectopia in the trauma group approximates the proportion of whiplash-injured patients who go on to report chronic symptoms.Two design limitations deserve carrying forward. The non-trauma controls were defined by absence of recent trauma, and since roughly half of people with chronic neck pain attribute onset to a whiplash event, some controls almost certainly had remote injuries — an error that would shrink rather than inflate the observed difference. And symptom-level data were not collected, so the study establishes an imaging association rather than a clinical one.Ectopia is also over-represented in other populations. Among adolescents with idiopathic scoliosis, 37% had tonsillar ectopia versus 13% of controls (OR 3.8), though there was no association with Cobb angle or curve progression. In fibromyalgia, 20% of 270 patients had Chiari I and more than 70% had some degree of ectopia — raising the hypothesis that fibromyalgia may be linked to whiplash injury by way of an acquired ectopia, possibly secondary to a dural leak.

Pathology: what the descent actually does

Downstream, the mechanism is CSF obstruction. The most consistent imaging findings in the Milhorat cohort were obliteration of the retrocerebellar CSF spaces (all 364 patients) and a meniscus sign at the lower pole of the tonsils; CINE-MRI demonstrated reduced CSF velocity and flow behind the cerebellum and in the cisterna magna.Flow at the foramen magnum in symptomatic patients - non-laminar, like jets, regions with a preponderance of flow in one direction, and synchronous bidirectional flow. This is not a technical footnote: measurements averaged spatially across the subarachnoid space or temporally across the cycle substantially underestimate the abnormality, which likely explains much of the disagreement in the published flow literature. Independently, occipital headaches have been found to be strongly associated with hindbrain CSF flow abnormalities regardless of the degree of tonsillar ectopia — the flow disturbance tracks the symptom better than the millimetres do.The most quantitatively persuasive evidence that obstruction at the junction drives intracranial dynamics comes from measuring a site the surgeon never touches. In ten pediatric Chiari I patients studied before and about 14 months after suboccipital decompression, amplitude of mean CSF velocity within the cerebral aqueduct was significantly elevated preoperatively compared with ten healthy volunteers and fell significantly after surgery, with no residual difference from volunteers. Amplitude of peak velocity followed the same trend (8.8 → 6.4 cm/s). Net flow at the aqueduct was zero in every scan — which is precisely why amplitude, capturing both cranial and caudal phases, detects what net flow cannot. Since the aqueduct is anatomically untouched by craniovertebral surgery, the most plausible reading is that decompression at the junction alters intracranial compliance, and that the aqueductal changes are a remote readout of that. Clinically, headache resolved completely in seven of nine patients, one of four syrinxes resolved and three shrank, and all other neurological symptoms resolved or improved.The average total magnitude of motion was 0.43 mm in subjects with normal tonsils (controls) and 0.57 mm in patients with Chiari malformations, 33% greater than that in controls. Tonsil motion was 0.61 mm in the patients with syringomyelia and 0.50 mm in those without it (22% difference).In CSF flow simulations in idealized models of a patient with Chiari and a healthy adult, increasing the pulse rate decreases the duration of the diastolic phase of CSF flow, increases the peak diastolic velocity, increases the magnitude of synchronous bidirectional flow, and doubles the pressure load on the spinal cord.Flow velocities in the foramen magnum reach 12 cm/s or more in patients with Chiari I compared with 5 cm/s in healthy adultsPressure gradients effect flow velocities near the dura and pia differently than flow in the middle of the channel. • As flow oscillates between craniad and caudad directions, flow direction reverses throughout the subarachnoid space at slightly different times with the result that, for short periods of time, CSF flows in both craniad and caudad directions (synchronous bidirectional flow). The clinical objective of most PCMR imaging is to help distinguish patients who are symptomatic from the Chiari malformation from those whose symptoms are not related to their ectopic tonsil position. Readers interpreting PCMR images have an accuracy of 60–70% in differentiating symptomatic from asymptomatic cases. Symptomatic cases of Chiari I typically have evidence of synchronous bidirectional flowCraniovertebral Decompression. Craniovertebral decompression and removal of a portion of the inferior posterior calvaria and the posterior part of C1 modify CSF flow usually reverses the growth of syrinx and relieves neurologic signs and symptoms in the patient with Chiari I. This surgical procedure reduces CSF velocities in the foramen magnum but not to healthy levels. Simulations show that a craniovertebral surgical defect normalizes velocities in the midcervical spine and reduces them in the upper cervical spine. Time-SLIP - Historic CSF flow theories of progressive hydrocephalus propose that blockage of the arachnoid granulations increases the resistance to the absorption of CSF into the bloodstream, leading to an accumulation of CSF in the ventricles. In the pulsatile models of hydrocephalus, a “waterhammer” effect is hypothesized, in which large undampened pulsations produce increased pressure gradients and asymmetric pulsation distributions lead to ventricular dilation.Enlarged pulse-wave amplitudes of CSF movement in patients with NPH have been observed and measured with MR imaging for a number of years. This effort has attempted to improve our understanding of hydrocephalus. However, it has failed to correctly identify those patients who would benefit from CSF diversion. PC cannot reliably visualize CSF flow within the ventricular or subarachnoid spaces or within cysts; this visualization can be repeatedly accomplished with timeslip. 

Syrinx formation. 

Three theories are represented. Syrinx comes from water-hammer effect due to insufficient drainage through the foramina of Magendie and Luschka. descent of the cerebellum into the foramen magnum creates a piston-like effect forcing fluid into the syrinx. dissociation of craniospinal hydrodynamics,lower pressure in the subarachnoid space relative to the central canal: reduced flow through the foramen magnum lowers the volume and pressure of the CSF surrounding the cord that would normally oppose enlargement of the cavity, and also reduces venous blood flow in the epidural vertebral veins. The synthesis in the Milhorat material is closest to Oldfield: obstruction at the foramen magnum exaggerates the systolic pulse wave in the spinal subarachnoid space and drives CSF through anatomically continuous perivascular and interstitial spaces into the central canal. Most syrinxes do not communicate with the fourth ventricle and are separated from it by an occluded or stenotic canal segment. Patients with Chiari I and syrinx have steeper tapering of the upper cervical spinal canal than matched controls. A syrinx in the lower third of the cord is called terminal and is often associated with a tethered cord; one extending the full length of the cord is a holocord, and both are frequently associated with Chiari malformations.The muscular and neural layer. A newer strand implicates the suboccipital musculature. Needle EMG in 40 Chiari I patients (mean tonsillar herniation 8.76 mm; 62.5% with headache, 67.5% with syringomyelia adjacent to C1) versus 30 controls showed substantial bilateral denervation of rectus capitis posterior minor, most conspicuous during Valsalva, and worse in those with headache, greater tonsillar descent, and syringomyelia involving C1. The proposed physiology is that RCPmi normally contracts during a Valsalva, pulling the craniocervical dura dorsally through the myodural bridge, expanding the subarachnoid space at the junction and assisting the CSF jet through it; herniated tonsils compress the posterior branch of the C1 nerve root, denervating the muscle and removing this compensation. Four mechanisms of head and neck pain are proposed in that model: direct compression of the C1 nerve root by tonsillar herniation or pulsation, stimulation of craniocervical dural receptors by the tonsils, junction instability, and inflammation of the C1 branch.

Patient presentation

Symptoms with chiari most common in third decade and w/o chiari but with basilar invagination seconds decade. The clinical syndrome consists of five elements: headaches, head pressure, a Ménière's disease-like syndrome, lower cranial nerve signs, and spinal cord disturbances even in the absence of syringomyelia.Headache was the commonest symptom (81%), described as heavy, crushing, or pressure-like at the back of the head, radiating to the vertex, behind the eyes, and down to neck and shoulders — pounding when severe but otherwise non-throbbing. The distinguishing feature is accentuation by physical exertion, Valsalva, cough or sneeze, head dependency, and sudden postural change, with premenstrual accentuation in women of menstrual age. Occipitocervical pain is reported by up to 78% of patients in more recent series, with occipital Valsalva headaches comprising 48%.Ocular disturbances occurred in 78%: retro-orbital pressure or pain, floaters, flashing lights, blurred vision, photophobia, diplopia, difficulty tracking objects, and field cuts — with few objective findings on neuro-ophthalmological examination, and a leading cause of erroneous MS diagnoses in young women.Vestibular occurred in 74%: dizziness, disequilibrium, aural pressure, tinnitus, hearing loss or hyperacusis, vertigo, and oscillopsia, with nystagmus the main objective sign. Of 24 patients with disabling dizziness or vertigo who had complete otological testing, 16 showed low-frequency sensorineural hearing loss with two vestibulopathy patterns — a peripheral type with impaired caloric responses and no central abnormalities (12 patients), and a central type with normal calorics but impaired optokinetic nystagmus, impaired smooth pursuit, saccadic dysmetria, and downbeat, positional, or periodic alternating nystagmus (2 patients). These findings fulfil the criteria for endolymphatic hydrops and suggest that CSF displacement contributes to disturbances of CSF-perilymph dynamics.Lower cranial nerve, brainstem, and cerebellar disturbances were documented in 191 patients (52%), most commonly dysphagia, sleep apnea, dysarthria, tremors, palpitations, and poor coordination, with objective cranial nerve deficits and cerebellar signs in only a minority. Sleep apnea was confirmed by 24-hour monitoring in 30 patients. Of 42 patients with palpitations or bradycardia on examination, continuous cardiac monitoring demonstrated paroxysmal atrial tachycardia in 23 and episodes of tachycardia and bradycardia in 5 — an unusually high rate of cardiac irregularity for a cohort of predominantly young adult women. Also described are vocal cord paralysis, frequent gagging and choking, coughing while swallowing, frequent respiratory tract infections, and sinus bradycardia.Spinal cord disturbances were present in 83 of 126 patients (66%) who did not have syringomyelia — attributable partly to mechanical compression of the cervicomedullary junction, and partly to the possibility that the exaggerated systolic pulse wave in the spinal canal can itself mimic syringomyelia. Where a syrinx is present the picture is the classic one: suspended dissociated sensory loss with arms affected and legs spared, central cord weakness greater in arms than legs, and spasticity.Where basilar invagination coexists, the presentations diverge. Patients with Chiari showed slower progression over a longer time, with weakness (94%), paresthesia (79%), posterior column and spinothalamic disturbance (56%), and ataxia (47%). Those without Chiari presented with weakness (100%), neck pain (59%), isolated posterior column dysfunction (39%), bowel and bladder disturbance (28%), and paresthesia (25%).Perhaps the most clinically consequential figure: by the time of definitive diagnosis, 59% of patients had been told by at least one physician that their disorder was psychogenic, and misdiagnosis as migraine, fibromyalgia, or multiple sclerosis was common.

Overlap syndromes and differential diagnosis

POTS, EDS, and orthostatic intolerance. Because Ehlers-Danlos is comorbid with POTS and associated with Chiari, the triad of EDS, POTS, and Chiari I is not uncommon in practice. The evidence, however, should be handled carefully. One study of 23 women with POTS found that cerebellar tonsillar herniation is not a common cause of orthostatic intolerance — though the authors conceded that hindbrain compression may still be present and that a single measurement of tonsillar depression might underestimate how many patients have it. The alternative proposed is craniocervical instability, which manifests with CSF flow obstruction and compression of the medulla or pons where the central autonomic networks reside. To date there are no formal studies of brainstem compression as an etiology of autonomic dysfunction, only anecdotal reports of dysautonomia resolving after surgery. There is also a genuine overlap between POTS and spontaneous intracranial hypotension — orthostatic headache is the hallmark of SIH and a symptom in nearly 30% of POTS patients, all patients in one SIH series met POTS criteria, and one patient's POTS resolved after an epidural blood patch. Given that both leak and ectopia can produce orthostatic symptoms, and that a leak can cause ectopia, this is a differential worth keeping open rather than collapsing.Vascular mimics. Compression of the medulla or upper cord by an elongated or ectatic vertebral artery produces an overlapping symptom set, including downbeat nystagmus attributed to caudal brainstem compression — and Chiari malformation with medullary compression by the vertebral arteries has itself been reported. Since downbeat nystagmus is one of the more specific signs in the Chiari examination, the vascular differential matters.Post-traumatic intracranial hypotension has been described as an overlooked cause of post-traumatic head and neck pain, with epidural blood patch reported as therapy for chronic whiplash-associated disorder — again pointing at leak rather than crowding as the operative mechanism in a subset of post-traumatic patients.

The measurement is not the diagnosis. Thirty-two of 364 patients (9%) had less than 5 mm of descent with typical symptoms, and 53% of those had syringomyelia; all showed hindbrain overcrowding and CINE-MRI flow abnormalities comparable to those with ≥5 mm. Conversely, 5 mm or more is encountered incidentally in asymptomatic people. Symptom severity was not confirmed to relate directly to the extent of herniation. Tonsillar position is best read as a general index of hindbrain overcrowding and only one of several factors shaping the clinical picture.Position and dynamics matter. Upright MRI more than doubles detection. Tonsils move with the cardiac cycle. Upright dynamic MRI is also the modality argued to best reveal positional craniocervical instability, which frequently accompanies ectopia and may be the more relevant finding in some patients.When to look. Consider imaging for tonsillar ectopia when there is a history of whiplash trauma with persisting suboccipital headache, particularly combined with cough-exacerbated headache or bilateral upper-extremity sensory or motor deficits. Where the headache is relieved by lying down, radionuclide cisternography for a dural leak is appropriate. Where flow rather than position is the question, cine phase-contrast imaging with attention to amplitude rather than net or averaged flow is the better instrument.Red flags. Patients should be instructed to report immediately any sign of severe brainstem compression — breathing difficulty, swallowing or choking problems, wild heart rate fluctuation, or drop attacks — and these reports should be acted on. Brainstem compression from Chiari can develop at a variable pace, and it has been known to kill.Downstream decisions. In scoliosis, detecting a syrinx before corrective surgery matters because correction in the presence of syringomyelia carries a high risk of postoperative paraplegia; craniocervical decompression before age 10 has been shown to improve or arrest curve progression, and spontaneous resolution of syrinx and Chiari has also been reported with non-surgical treatment of the scoliosis. Not every patient needs surgery: patients with "benign" Chiari I require decompression at a low rate, conservative management has been studied in both pediatric and adult cohorts using phase-contrast MRI to guide selection, enlarging syringomyelia in clinically stable patients has been managed non-operatively after decompression, and exercise regimes have been trialled. Where decompression is performed, the newer literature argues for protecting the suboccipital muscles and myodural bridges, minimally invasive small-portal approaches, avoiding excessive dural repair that could fold or collapse the dura, and limiting bone removal that could destabilize the atlanto-occipital joint — roughly 7% of patients develop CSF flow disturbance or hydrocephalus after posterior fossa decompression, including after bony decompression alone. Decompression also does not reliably relieve headache in every patient, which is a further argument for characterizing the mechanism before operating on the measurement.