Chiari malformation (CM) is a disease in which the cerebellar tonsils descend through the foramen magnum into the spinal canal due to structural abnormalities of the posterior cranial fossa.
CM-1 (Chiari type I malformation) is defined as the cerebellar tonsils herniating 5 mm or more below the foramen magnum. In children, a herniation of 3 mm or more may be diagnostic5). It can be congenital or acquired and is often associated with syringomyelia.
CM-2 (Chiari type II malformation) involves herniation of the cerebellar tonsils, vermis, and medulla oblongata. It is typically congenital and associated with myelomeningocele.
The prevalence in MRI studies is reported to be up to 0.77%4). The prevalence in symptomatic adults is lower, at 0.01–0.04%2), and about 50% are asymptomatic and discovered incidentally1). The female-to-male ratio is 3:11).
Ophthalmic signs occur in up to 80% of cases and may prompt neuro-ophthalmology involvement, including symptoms of increased intracranial pressure (ICP), nystagmus, sixth cranial nerve palsy, and esotropia. Types III and IV are extremely rare and are not covered in this article.
QHow often is Chiari type I malformation found?
A
The prevalence in MRI studies is reported to be up to 0.77%4). In contrast, the prevalence in symptomatic adults is low, at 0.01–0.04%2), and about 50% are asymptomatic and discovered incidentally1). In many cases, it is found on MRI performed for evaluation of headaches or neurological symptoms.
Occipital headache: The most common symptom, observed in about 80% of cases. It is characteristically triggered by Valsalva-like maneuvers (coughing, sneezing, laughing, straining, vigorous physical activity)7).
Blurred vision: Blurring of vision. Occurs with increased intracranial pressure.
Downbeat nystagmus: Most characteristic. Associated with lesions near the foramen magnum. Often not suppressed by visual fixation.
Other nystagmus: Upbeat, gaze-evoked, seesaw, periodic alternating nystagmus (PAN), and other types can occur. Often accompanied by truncal ataxia, dysmetria, and impaired smooth pursuit.
Abducens nerve (CN VI) palsy: Most common cranial nerve palsy. May be bilateral.
Papilledema: Occurs in about 2% of symptomatic CM-I cases6). Initially, only transient visual loss, but if persistent, leads to visual field constriction and decreased visual acuity.
Findings in CM-2
Acquired comitant esotropia: In children, it is often detected as esotropia.
Alternating hypertropia: Vertical misalignment of the eyes.
Ataxia: Due to extensive compression of the cerebellum and brainstem.
Multiple cranial nerve palsies: In CM-2, the medulla oblongata also herniates, which can affect multiple cranial nerves.
QHow is papilledema due to Chiari malformation differentiated from idiopathic intracranial hypertension (IIH)?
A
In CM-I, MRI shows cerebellar tonsillar herniation, posterior fossa crowding, and CSF flow disturbance 6). In IIH, there are no anatomical abnormalities in the posterior fossa, but findings such as empty sella, optic nerve sheath distension, and transverse sinus stenosis are helpful. See “Diagnosis and Examination Methods” for details.
Several theories exist regarding the pathogenesis of CM-1.
Fetal hydrocephalus theory: Embryonic tonsillar herniation due to CSF hydrodynamic pulsations.
Posterior fossa volume insufficiency theory: Reduced CSF leads to impaired growth of the posterior fossa → mechanical crowding.
Birth trauma: Effects on posterior fossa structures due to external forces during delivery.
Molecular genetic abnormalities: Some familial cases suggest a genetic background.
Association with syringomyelia: The rate of syringomyelia associated with CM-1 is reported to be 23–85% 1), and the rate on pediatric MRI is 1.2% in girls and 0.5% in boys 1). Symptoms tend to be more severe when syringomyelia is present.
Sex differences: More common in women, with a female-to-male ratio of 3:1 1). In adults, onset often occurs in the 3rd to 4th decades 7).
Association with GH excess: It has been reported that thickening of the skull base connective tissue due to growth hormone (GH) excess may reduce posterior fossa volume and cause tonsillar herniation. Two cases of CM-1 regression after acromegaly surgery have been reported 2).
Brain MRI (sagittal): The standard for definitive diagnosis. CM-1 is diagnosed when the cerebellar tonsils are displaced downward more than 5 mm (or more than 3 mm if other features are present) beyond the foramen magnum. For CM-2, the presence of myelomeningocele and displacement of the cerebellar vermis and medulla oblongata are required for diagnosis.
CT: Used as an alternative when MRI is contraindicated. Also useful for evaluating skeletal abnormalities.
CSF flow analysis (cine MRI): Evaluates CSF flow obstruction at the foramen magnum 6).
MR venography (MRV): Evaluates venous sinus stenosis. Important for differentiation from IIH6).
Lumbar puncture (LP) opening pressure exceeding 25 cmH2O is considered abnormally high. Continuous 48-hour monitoring may capture nocturnal ICP elevation spikes (in CM-I cases, LP opening pressure >40 mmH2O and nocturnal spikes >50 mmH2O have been reported6)). If ICP elevation is suspected, CT/MRI must be performed before LP to rule out space-occupying lesions and hydrocephalus.
For asymptomatic or mildly symptomatic cases, observation is chosen. Evidence suggests that asymptomatic or mild CM-I often follows a non-progressive course 4).
Pharmacotherapy for nystagmus: GABAergic drugs such as clonazepam or baclofen are used.
Treatment of strabismus: Mild cases are corrected with prisms. For significant cases, surgery is performed. Strabismus surgery is usually performed after neurosurgical decompression.
This is the main treatment for symptomatic cases, including non-ophthalmologic symptoms. The standard procedure is suboccipital craniectomy plus C1 laminectomy 7).
Extradural decompression vs. intradural manipulation: In CM-I without syringomyelia, extradural decompression is reported to result in good outcomes and shorter hospital stays 6).
Expansile duraplasty: Performed in severe cases 7).
Tonsillar herniation below the C2 lamina doubles the risk of reoperation 7). Headache persists or recurs postoperatively at a 70% higher rate than other symptoms 7).
Posterior fossa decompression can improve papilledema6). Intracranial pressure-lowering drugs such as acetazolamide (Diamox®) are adjunctive and have limited efficacy when used alone. Early treatment allows papilledema to resolve without visual impairment, but delayed treatment can lead to irreversible damage.
QIf I undergo surgery, is there a possibility that symptoms will recur?
A
Postoperative symptom persistence or recurrence is observed in approximately 22% of cases 7). Headaches are 70% more likely than other symptoms to persist or recur after surgery 7). Tonsillar herniation below the C2 lamina is associated with a doubled risk of reoperation.
Insufficient volume of the posterior cranial fossa causes the cerebellar tonsils to descend into the foramen magnum. This leads to direct compression of the brainstem and spinal cord, as well as impaired CSF flow at the foramen magnum. The latter contributes to increased ICP and the formation of syringomyelia.
Impaired CSF flow leads to the formation of fluid-filled cavities (syringomyelia) within the spinal cord parenchyma. The exact pathophysiology remains largely unclear. It commonly occurs in the cervicothoracic region (C2 to T9) but can extend throughout the entire spinal cord 1). When the syrinx damages the dorsal horn, central pain occurs, which persists in 40% of cases even after foramen magnum decompression 3).
Crowding of the posterior fossa causes intermittent CSF pressure imbalance, leading to transient ICP elevation. This is the cause of papilledema6). Although demographics, symptoms, and treatment response are similar to IIH, CM-I differs in that MRI reveals anatomical abnormalities of the posterior fossa (small skull, posterior fossa crowding) 6).
Chronic blood flow impairment to the herniated tonsils may progress to ischemia and cystic degeneration. The incidence of this change is extremely rare at 0.01%, and the rate of syringomyelia complication is reported to be 75% 1).
QWhy does syringomyelia form?
A
It is thought that CSF flow disturbance at the foramen magnum leads to fluid inflow and accumulation within the spinal cord parenchyma. The complication rate is reported to be 23–85% 1), and the presence of syringomyelia is associated with symptom severity. The exact mechanism of formation remains speculative in part.
7. Latest Research and Future Prospects (Reports at the Research Stage)
There are a certain number of cases where pain persists even after foramen magnum decompression for syringomyelia associated with CM-1.
Yamana et al. (2024) performed spinal cord stimulation (SCS) in a case with persistent refractory central pain after foramen magnum decompression 3). Combined FAST therapy + Contour therapy improved VAS from 9.5 to 5 and McGill emotional pain score from 7 to 3. A review of 6 cases in the literature reported pain improvement in all cases with no adverse events.
Complete spontaneous resolution of CM-1 in adults is extremely rare.
Cuthbert et al. (2021) reported a case of a 25-year-old man with complete resolution of CM-1 and syringomyelia over 4 years 4). Only 16 cases of spontaneous resolution have been reported in the literature. Proposed mechanisms include cerebellar tonsil atrophy, spontaneous tonsillar ascent (average 2.8 mm with age), and restoration of CSF dynamics due to Valsalva maneuvers.
This report supports conservative management for asymptomatic or mild CM-1.
Zielinski et al. (2024) reported a case of CM-1 complicated by nonfunctioning pituitary adenoma and adrenal tumor, and discussed the possibility of CM-1 development through mechanisms other than GH excess 2). Two cases of acromegaly with regression of CM-1 after transsphenoidal adenomectomy have been reported previously, and the mechanism of CM-1 regression by GH-related treatment is being studied.
Henderson R, Lakshmanan R, McLaughlin A, et al. A complicated Chiari type 1 malformation and holocord syrinx as a likely cause for heel pain. Childs Nerv Syst. 2024;40:997-1003.
Zielinski R, Khan A, Mirza FS. Chiari I malformation with concomitant nonfunctioning pituitary and adrenal tumors. JCEM Case Rep. 2024;2:luae113.
Yamana S, Oiwa A, Nogami R, et al. Successful spinal cord stimulation using fast-acting sub-perception therapy for postoperative neuropathic pain of syringomyelia with Chiari malformation type 1: a case report and literature review. BMC Neurol. 2024;24:284.
Cuthbert H, Pepper J, Price R. Spontaneous resolution of a Chiari malformation with syringomyelia. BMJ Case Rep. 2021;14:e241789.
Snee IA, Mazzola CA, Sikorskyj T. Chiari I malformation with Klippel-Trenaunay syndrome: case report and review of the literature. Childs Nerv Syst. 2021;37:2369-2373.
Jiang L, Thant KZ, Bao H, Ivanova T. Course of papilloedema following surgical decompression in Chiari I malformation with raised intracranial pressure. BMJ Case Rep. 2021;14:e218228.
Turk ML, Schmidt K, McGrath ML. Diagnosis, management, and return to sport of a 16-year-old patient with a Chiari I malformation: a case report and literature review. J Athl Train. 2022;57(2):177-183.
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