The first original image of cranial MRI in 2014 at Affiliated Hospital of Harbin Medical University was available, but the corresponding MRI report showed only present the unruptured AVM, without hydrocephalus and other abnormalities (Supporting materials 1).

In 2018, the patient visited a prestigious neurosurgery clinic in Shanghai, China, due to occasional dizziness and headaches. A follow-up cranial MRI showed no significant changes in the AVM, but revealed the emergence of mild hydrocephalus (Figure 1A). Considering the mild symptoms and the high-risk associated with surgical intervention for the AVM, the neurosurgeon recommended continued observation through follow-up visits (Table 1).

In 2020, the patient visited a neurosurgery outpatient clinic at a hospital in Xi’an due to mild dizziness and headaches. A follow-up cranial MRI revealed increased hydrocephalus compared to previous scans, along with cerebellar tonsillar herniation (known as “secondary Chiari malformation”) (Figures 1B,C) (5, 6). Considering the risks associated with surgery for this AVM, the neurosurgeon suggested using Antinociception for oral pain relief treatment (Table 1).

In May 2022, the patient sought care at a neurosurgery outpatient clinic at a hospital in Xi’an due to intolerable dizziness and headaches. A follow-up cranial MRI revealed increased hydrocephalus, along with secondary Chiari malformation and a partially empty sella (Figure 1D). Considering the operative risk, the neurosurgeon recommended combining acetazolamide to reduce CSF secretion, continuously with Antinociception for oral pain relief treatment (Table 1).

In June 2022, the patient presented with severe dizziness, headaches, vomiting, and altered consciousness and was admitted to emergency department of our institution. A cranial MRI revealed pronounced AVMP throughout (Figures 1E,F). The patient was promptly transferred to the neurosurgery department for further treatment and comprehensive examinations.

A whole brain angiography was performed using a Dutch Philips digital UNIQ FD20 angiography machine (DSA). The left middle cerebral artery (▽) and branches of the anterior cerebral artery (◇) from the left doublestem artery (▽) supplied the deep cerebral arteriovenous malformation in the left temporal lobe, draining to the straight sinus (○) via the deep vein (△). Two hemodynamic aneurysms were observed at the junction of the end of the left internal carotid artery (#) and the beginning of the M1 segment of the left middle cerebral artery (□) (Figure 2).

A 2D-Q-FLOW sequence was performed with retrospective cardiac gating, and the patient’s CVJ cerebrospinal fluid flow rate was measured in a supine median sagittal scan. The patient’s mean CSF flow rate increased, whereas the mean flow amount decreased (Figure 3A).

The OCT exhibited patient’s bilateral disks were congested and swollen, with flame-like hemorrhages, infarcted cotton wool-like changes in the nerve fiber layer, and heart-shaped exudates and hemorrhages in the macula. Bilateral severe progressive papilledema has been identified (Figure 3B).

Pituitary hormone 6 test: the PRL = 138.14 ng/mL. All other blood tests were unremarkable.

The patient returned to our hospital 1 year later for a review: no complaints of any headache, dizziness, blurred vision, unsteady walking, vomiting, etc. MRI of the head showed: a completely alleviated obstructive hydrocephalus (Figure 1G), CMI, syringomyelia, an improvement empty sella (Figure 1H) and an unprogressive scoliosis (Figure 1I). However, the patient still exhibited significant congestion in the posterior cranial fossa, with the cerebellar tonsils notably displaced downward, although not to the extent that meets the diagnostic criteria for Chiari malformation (Figure 1J).

A 2D-Q-FLOW sequence was performed under retrospective cardiac gating, and the patient’s cranio-cervical junction (CVJ) cerebrospinal fluid flow rate was measured using a supine median sagittal scan. The results indicated that the patient’s mean CSF flow rate was slower, and the mean flow amount was higher than before (Figure 3C).

The follow-up examination revealed that the patient’s bilateral papilledema has completely resolved (Figure 3D).

Initially, dilation of draining veins can lead to the compression of narrow passageways within the ventricular system, such as the foramen of Monro or the aqueduct of Sylvius (1, 10–14), typically resulting in obstructive hydrocephalus. This form accounts for 94.4% of such presentations, characterized by moderate to severe hydrocephalus and representing approximately 72% of all identified mechanisms (3, 11). Secondly, chronic venous hypertension from impaired drainage can hinder the reabsorption of cerebrospinal fluid (CSF), usually leading to communicating hydrocephalus (15). This is observed in almost all cases, where the hydrocephalus is primarily mild to moderate, constituting about 28% of the mechanisms (7, 16).

In our patient’s case, both mechanisms of hydrocephalus were likely at play. The initial CT scan indicated mild hydrocephalus, presenting with symptoms of vertigo and headache, potentially due to increased venous pressure from the AVM and compromised CSF absorption. As the condition progressed, the patient exhibited symptoms indicative of severe hydrocephalus, such as vomiting and blurred vision, which may correlate with the development of non-communicating hydrocephalus. This could be attributed to the thickening of the draining vein, leading to complete obstruction of the Sylvius aqueduct or, less commonly, the foramen magnum.

Dhandapani (17) posits that chronic increased intracranial pressure is a likely cause of empty sella. The proposed mechanism involves the enhanced compliance of the ventricular system due to hydrocephalus and intracranial hypertension, which allows CSF to encroach upon the pituitary fossa, resulting in vacuolation of the sella. Consequently, this may lead to hyperprolactinemia-associated symptoms such as lactation, menstrual irregularities, and amenorrhea. In the case of our patient, amenorrhea was the sole presentation in 2022, coinciding with radiological evidence of an empty sella (Figure 1H).

Pre-operative prolactin levels were elevated at 138.14 ng/mL, while post-operative levels decreased to 11.35 nmol/L. Despite minimal radiological improvement, the alleviated pressure on the pituitary gland led to a gradual restoration of its endocrine function. Reviewing prior reports, we noted instances where patients exhibited a significant empty sella, which authors either overlooked or dismissed as coincidental (2, 18). This suggests that pituitary hormone disorders in these patients might have been underestimated or entirely missed.

CMI is primarily recognized as a congenital disorder characterized by a reduced posterior cranial fossa (PCF) size, which is attributed to mesodermal dysplasia (19–21). Yet, recent literature suggests that acquired factors may also play a role in the pathogenesis of this condition (18). Fric et al. (22) proposed that intracranial hypertension could lead to a widespread secondary form of CMI, known as Chiari syndrome. Surgical decompression and duroplasty are the recommended treatments for congenital CMI, while acquired CMI should be managed by addressing the underlying etiology. Identifying the precise cause of acquired CMI is crucial for effective management, and treatment should be etiology-specific.

Iampreechakul (2) described a case involving a 35-year-old woman afflicted with a sizable right parieto-occipital AVM. The author initially detected tonsillar herniation concurrent with syringomyelia during a routine cervical MRI, which was presumed to be a “congenital Chiari malformation (5),” leading to an Atlanto-occipital decompression. Post-surgery, however, the patient’s condition deteriorated. Subsequent brain MRI uncovered a large unruptured AVM, establishing the AVM as the initial cause; this prompted a vascular intervention, which ameliorated the patient’s symptoms.

In our case, no congenital cranial abnormalities were found on cranial MRI. The initial brain imaging from 2014 and a subsequent MRI in 2018 (Figure 1) displayed only mild hydrocephalus without any indication of tonsillar herniation or syringomyelia. However, as the hydrocephalus progressed, tonsillar herniation, syringomyelia, and even scoliosis emerged, indicative of an acquired CMI.

Moreover, previous studies have suggested that AVM can induce tonsillar herniation and syringomyelia through venous hypertension of PCF. O’Shaughnessy et al. (23) documented the inaugural case of a unruptured AVM leading directly to tonsillar herniation without accompanying hydrocephalus in 2006, with venous congestion and PCF structure swelling due to venous hypertension posited as contributing factors (23). This results in increased PCF pressure, causing tonsillar herniation, the so-called CMI, extended herniation, syringomyelia, and scoliosis, which are evidently not congenital deformities (24).

Although there have been five reported cases of AVM coexisting with CMI, the mechanism by which AVM lead to CMI remains highly contentious. Some scholars believe it is due to hydrocephalus causing midbrain sagging (4), while others attribute it to venous hypertension leading to swelling in the PCF (25). Remaining reports either arbitrarily deem it coincidental (3, 26) or overlook this diagnosis altogether (9). However, our analysis of current case suggests that both midbrain sagging and increased pressure in the PCF might contribute to the downward displacement of the cerebellar tonsils. This is supported by prior MRI findings in the patient showing simultaneous cerebellar tonsillar herniation and midbrain sagging (Figures 1A, C–E). After comprehensively addressing the hydrocephalus with a ventriculoperitoneal (V-P) shunt surgery, a significant rebound of the cerebellar tonsillar herniation was observed (Figures 1H,J), yet it remained below the craniocervical junction (CCJ; Figures 1H,J), with persistent abnormal swelling in the PCF and notable compression of the brainstem (Figures 1H,J). Therefore, we propose that for AVMP-associated CMI, hydrocephalus plays a dominant role, while the direct impact of AVM on CMI is secondary.