Tonsilla cerebelli – anatomy, function and its significance for neurosurgery

Summary

The goal of our work was to acquaint the reader-neurosurgeon with the detailed anatomy of the cerebellar tonsil, focusing on its individual surfaces. This is because, in most publications, the tonsil is presented only within the context of the anatomy of the entire cerebellar hemisphere, or possibly the anatomy of the cerebellomedullary fissure or the course of the arteria cerebelli posterior inferior. We conducted cadaveric dissections of the tonsil on 4 cerebellar hemispheres (divided sagittally in the plane of the vermis) and on one complete cerebellum with its peduncles and the floor of the fossa rhomboidea. We used this for demonstrating the telovelar approach. We believe that for the safe mastering of the telovelar approach in the operating room, laboratory dissection is mandatory. It allows the neurosurgeon to recognize even less known structures of the lateral recess, cerebellomedullary fissure, and understand the telovelar junction. In a comprehensive review, we also document individual surgeries related to the tonsil and telovelar approach: the surgery for Chiari malformation with syringomyelia, tumor of the IV^th ventricle, cavernoma of its lateral recess, and cystic hemangioblastoma of the medulla oblongata. Based on literary data, we document the history of the surgical approach, which is an exemplary demonstration of the collaboration between two world-renowned neurosurgeons (Rhoton and Matsushima), and was underpinned by extensive laboratory work. In the review, we address congenital variants of cerebellar tonsil herniation (Chiari malformation) as well as secondary causes and their imaging possibilities. We mention the clinical significance of the pathological descent of tonsils and their association with syringomyelia.

Keywords:

Cerebellum – foramen magnum – cerebellar tonsil – brain stem – telovelar approach – Chiari malformation – syringomyelia

This is an unauthorised machine translation into English made using the DeepL Translate Pro translator. The editors do not guarantee that the content of the article corresponds fully to the original language version.

Anatomical introduction

The tonsil represents the caudomedial part of the cerebellar hemisphere, so it is a paired structure. It is classified as a neocerebellum, although its medial vermal "partner" -⁠ the uvula -⁠ is functionally a paleocerebellum, similar to the pyramis vermis [1]. The tonsil is homologous to the mammalian ventral paraflocculus. Whereas the flocculus receives connections from the vestibular nuclei, the paraflocculus receives connections from the pontine nuclei.

The tonsil has the approximate shape of an inverted cone with an ellipsoid basal cranially, the height of the tonsil being greater than the longer diameter of the basal (Fig. 1). The lateral side (biventer) has equivalent shallow horizontal fissuring. The medial side also has horizontal and obliquely upward-facing fissuras, but on our cadaver three of them were deeper, dividing the medial aspect of the tonsil into several small "lobules" whose volume is larger than that of the foliation on the lateral side, which is clearly visible when the tonsil is viewed from the front. On the superoposterior (uvular) side, the tonsillar peduncle is more superficial, the foliae do not overlap it and run radially to its centre, with a vertical direction below and above. The anterior side, looking into the lateral recess of the IV ventricle and adjacent to the corpus restiforme, has more pronounced foliae, which are also vertical at the lower pole and overlap the tonsillar peduncle. Posteriorly, the tonsillar peduncle is smaller and truncated, directing the white matter into the uvula, anteriorly it is more elongated and directs the white matter into the lobulus biventer.

Surgical anatomy

From the point of view of surgical anatomy it is important that the tonsil is separated from the rest of the cerebellum mainly by free surfaces (Fig. 2), only superolaterally it connects to the lobulus of the biventer hemisphere and to the uvula vermis by the tonsillar peduncle (Fig. 3).

The ventral side of the upper part of the tonsil lies on the inferior medullary velum (velum medullare inferius), the nodule and the tela choroidea. Above the tonsil is the distal part of the cerebellomedullary cistern, the so-called supratonsillar (telovelotonsillar) cleft -⁠ cleft. In intimate relation with the tonsil runs the tonsillomedullary and telovelotonsillar segment of the arteria cerebellaris posterior inferior (PICA). Posteriorly between the two tonsils, the posterior cerebellar incisura terminates as the fissure intertonsillaris or vallecula, and as the tonsils approach each other cranially, this becomes the intertonsillar sulcus. Even more cranially, there is a deep uvulotonsillar sulcus between the uvula and the tonsil on the inner aspect. There is also a deep fissure between the tonsil and the lobulus biventer, the tonsillobiventerica or secunda (Fig. 4A, B). It is significant for orientation during surgery that, when viewed through the suboccipital craniotomy, the folia of the tonsil are directed vertically in contrast to the oblique (lobulus biventer) and horizontally directed folia of the suboccipital surface of the cerebellar hemisphere (Figs. 3, 4A, 5). The inferior and medial margins of the tonsils are adjacent to the medulla oblongata and form the caudal border of the cerebellomedullary fissure. The vallecula, together with the obex, bring us into ventricle IV through the foramen magendia. Knowledge of the anatomy of the cerebellar tonsil will facilitate its resection in posterior fossa surgery, in its involvement by tumor, and in surgery for Chiari malformation (CM) type I (Fig. 5).

The vallecula, intertonsillar, uvulotonsillar sulcus, and cerebellomedullary fissure are also the route of the telovelar approach, or the tCMF approach. The purpose of the telovelar approach is to preserve the cerebellar nuclei, which are located very intimately -⁠ dorsolaterally to the ceiling of the IV ventricle and below the cranial portion of the vermis -⁠ and here they can be damaged by the traditional transvermal approach to the IV ventricle. This can cause so-called cerebellar mutism, especially in children [2]. For the telovelar/tCMF approach, it is important to note that the cerebellomedullary fissure is the developmental equivalent of the posterolateral fissure, separating the flocculus and nodulus from the tonsils and uvula.

To describe our laboratory dissection (Figure 6) and some clinical cases, we proceed as follows: We begin our approach by preparing the intertonsillar sulcus and then the uvulotonsillar sulcus. Next, knowledge of the course of the PICA in its tonsilomedullary, telovelotonsillar and cortical segments, its caudal (with close relation to the medulla oblongata) and then its cranial branch (loop) is necessary, keeping in mind the varieties of the course [3]. After preparation of the sulci and PICA, we can elevate and retract the tonsil craniolaterally, and then cut the tela choroidea at its abutment (tenia) along the laterocaudal margin of the fossa rhomboidea, thus gaining access to the lateral recess of the IV ventricle (Fig. 7). Eventually, if we further transduce the horizontal arm of the plexus choroideus and the velum medullare inferius, a thin layer of neural tissue connecting the flocculus to the nodule, we thereby achieve supralateral recession and, eventually, by cranial retraction of the uvula, we expose the fossa rhomboidea cranially to the orifice of the Sylvian aqueduct (Fig. 8).

This approach was proposed by the Japanese neurosurgeon Toshio Matsushima in 1992 [4], after his cadaveric study, and introduced into clinical practice in 2001 [5]. As a "telovelar" approach, it was further anatomically documented by and popularized by Mussi and Rhoton in 2000 [6,7]. The beauty of this approach combines the need to acquire detailed knowledge of anatomy in the laboratory with the subsequent elegance of microsurgical manipulation near vital structures. It is also wonderful and inspiring to observe the continued collaborative work of Matsushima and Rhoton and the continuing work in this anatomical area [8]. After all, already at the end of the 2001 article, Matsushima thanks Rhoton for allowing him to study the anatomy of ventricle IV in his laboratory in 1981.

This is not a dogmatic approach -⁠ in the midline approach, it is appropriate to adjust the complexity of tonsillar and uvular dissection to the "extensive type" (for the upper base of the IV ventricle), "lateral wall type" (for the cerebellar peduncles) and "lateral recess type" (for the lateral recess) according to the extent of the lesion [5]. In the first type, we extensively mobilize both tonsils, even by bilateral uvulotonsillar dissection, the retraction is directed more cranially, the tenia is released bilaterally, and the telovelar junctions and velum medullare inferius are equally cut. For the second type, a more pronounced lateral retraction of the tonsil and lobulus biventer is necessary. However, only unilateral opening of the tela choroidea, the telovelar junction and the velum medullare inferius (for access to the supralateral recess) is sufficient and an oblique contralateral microscopic view is appropriate. The approach to the lateral recess is similar, but compared to the approach to the lateral wall of the IV ventricle, it is not necessary to prepare the uvulotonsillar sulcus and, of course, not to spin the telovelar junction; excision of the tenia and opening of the lateral recess is sufficient, thus it is the least extensive. In addition to this classic midline approach, a lateral unilateral tCMF approach has been developed [8]. Thus, four types of lesions can be distinguished: (a) intraCMF lesions (i.e., outside the ventricle); (b) intraventricular lesions, both types suitable for the medial dissection "pathway"; (c) "pure" cerebellomedullary cistern lesions, suitable for the lateral approach; and (d) lesions extending into both the CMF and the cerebellomedullary cistern, suitable for a combination of both approaches [9]. In intraventricular lesions, the need for incision of the inferior part of the vermis for access to the rostral part of the IV ventricle or fastigium has also been described by Matsushima et al [9], a fact supported by the anatomical study of Tanriover et al [10], which was also observed in our patient with plexus papilloma (Figure 7). For all these approaches, however, the initial denominator is the anatomy of the cerebellar tonsil and its mobilization.

The vascular neurosurgeon Lawton et al. also promotes knowledge of the anatomy of the tonsil and tonsillobiventeric fissure when promoting their supratonsillar (tonsillobiventeric fissure) approach to the inferior cerebellar peduncle and posterior median [11]. He describes this more lateral approach as elegant and safe in six patients with cavernoma, but several other famous neurosurgeons (A. Rhoton, M. G. Yaşargil, A. H. Kaye, M. Samii -⁠ in order of comments on the article) disagree with him, suggesting in particular the technically easier reachability of the inferior cerebellar peduncle by the classic telovelar approach, with less bony exposure, commenting on the inaccuracy of navigation given by the liquor decompression and the risk of damage to the nucleus dentatus. The author himself admits only a greater risk of damage to the distal branches of the PICA protruding from the tonsilobiventeric fissure. Despite the objections of the reviewers, Lawton does not give up his efforts and 10 years later presents an anatomical study for the "tonsillobiventeric fissure approach" also to the lateral recession of the IV ventricle, in its inferolateral variant. He considers the vertical direction of the tonsillobiventeric fissure relative to the trunk as an advantage as opposed to the disadvantageously parallel direction of the cerebellomedullary fissure [12]. This advantage stands out more in the pronation position of the patient used by him, not so obvious for us used to operate in the "semisitting" position. Lawton applauds the absence of performing a C1 laminectomy, but in a previous article on the supratonsillar approach to the inferior cerebellar peduncle [11], he acknowledges it as "optional" due to the higher trajectory of the approach, and in his opinion, no significant head anteflexion is necessary in the case of the lateral recess approach. He considers the fact that preparation of the telovelotonsillar segment of the PICA is not necessary as an additional advantage, thus avoiding manipulation of the medullary perforators of the tonsillomedullary and proximal telovelotonsillar segments; however, the distal branches of the PICA (distal telovelotonsillar and cortical) remain at risk, as do the retrotonsillar veins within the fissura.

Another author dealing with the topic of the midline subtonzillar approach is M. Tatagiba [13]. He promotes the dissection and elevation of the tonsil in midline approaches for hypoglossal canal meningioma [14], jugular tubercle meningioma [15] or even glossopharyngeal neuralgia [16]. We have appreciated the possibility of tonsillar manipulation in the lateral subtonsillar approach for cystic hemangioblastoma of the medulla oblongata [17].

Clinical significance of the cerebellar tonsils

The cerebellar tonsils are functionally classified as part of the neocerebellum, the developmentally youngest part of the cerebellum. The functional connections of the neocerebellum are predominantly ponto-cortico-ponto-cerebellar and are responsible for the smooth execution of learned free movements [1,18]. The tonsils (and paraflocullus) appear to play a key role in the generation of downbeating nystagmus [19]. According to some studies, the functional involvement of the cerebellar tonsils in the pathophysiology of migraine has been suggested [20]. According to a study by Dartory, changes in glucose metabolism (as determined by fludeoxyglucose [FDG] PET) in the tonsils may mark conversion to mild cognitive impairment in otherwise healthy adults [21]. However, the importance of the cerebellar tonsils is mentioned in the literature for their potential herniation into the foramen magnum with its specific symptoms. The clinical presentation of Chiari malformation type I can be divided into three subtypes based on the anatomy of the dislocated structures of the hindbrain: 1) cerebellar compression; 2) compression of the brainstem and cranial spinal cord; 3) alteration of cerebrospinal fluid flow through the foramen magnum. Ataxia, dysmetria, dyscoordination were referred from cerebellar symptoms. Spinal symptoms include mainly weakness and impaired limb sensation, hyperreflexia and muscle atrophy. Among the trunk symptoms, nystagmus (horizontal and downbeating nystagmus), diplopia, dysarthria, dysphonia, vocal cord paralysis, palatal paralysis, and tongue atrophy have been reported [22]. However, headache is the most frequently reported symptom in CM1 [23]. The 2018 International Headache Classification lists the diagnostic criteria for headache in CM. The criteria include MR-specific findings and occipital or suboccipital pain associated with coughing or Valsalva maneuver that resolves within 5 min and fully resolves after successful (surgical) resolution of CM, yet is not better explained by another etiology. There is a common association with the aforementioned trunk, spinal or cerebellar symptoms [24]. Less common are neurocognitive symptoms -⁠ memory impairment, aphasia, depression, anxiety, and fear is a frequently described emotion [23]. According to Kokurkin et al. a significant relationship between the severity of cognitive deficit and the degree of cerebellar tonsillar ectopia has been demonstrated [25]. A neglected symptom in CM is sleep disturbance; according to some sources, sleep apnoea (hypopnoea) may be present in up to 50% of CM cases [26]. A total of 14-30% of CM1 cases are asymptomatic [27,28]. Since both asymptomatic CM1 and CM1 with mild clinicopathology are relatively benign and non-progressive diseases, it is reasonable to rather monitor the patient even if he/she has significant tonsillar herniation or syringomyelia [29]. With significant intracranial extension, acute herniation of the cerebellar tonsils through the foramen magnum is a life-threatening condition. Compression of the inferior trunk, spinal cord and the feeding cerebral arteries -PICA, vertebral arteries and their branches, and the outlying segment of the anterior spinal artery may occur. Compression of these vessels leads to ischemia of the brainstem, tonsils, and caudal cerebellum with signs of lesions of these important neural structures [30]. Herniations of the cerebellar tonsils and CM are associated with syringomyelia. Syringomyelia syndrome is due to lesion of the anterior spinal cord commissure and has a specific clinical picture. It involves a segmental lesion of alginic and thermal sensation (tactile is preserved) and segmental flaccid paresis of the upper limbs with atrophy and fasciculations [31]. In the case of syringobulbia, there are also symptoms of brainstem lesions, most commonly manifesting as bulbar syndrome, or Horner's syndrome, nystagmus or respiratory disturbances. It can be confusing for the diagnosis that the lesion can also manifest as unilateral hemiparesis [32,33].

Cerebellar ectopia and Chiari malformation

Cerebellar ectropion (and herniation of the cerebellar tonsils) can be congenital (CM) or acquired during the pressure effect of surrounding structures (mass effect) or changes in cerebrospinal fluid pressure. CMs represent a spectrum of the most common congenital malformations of the cervicocranial nucleus with a decrease in the cerebellar tonsils and sometimes the brainstem through the foramen magnum. They differ in their symptoms, age of onset, and etiology associated with syringomyelia [26,34,35]. Hans Chiari, an Austrian pathologist and anatomist who was active in Bohemia for a long time (he was even a member of the Czech Provincial Assembly), described three variants of cerebellar malformations in 1891, which are associated with herniation of the structures of the posterior cranial fossa into the cervicocranial junction; he subsequently described a fourth variant in 1896. Chiari I (CM1) is characterized by descent of the cerebellar tonsils below the foramen magnum and is the most common type of CM [35,36]. Chiari II (CM2) already represents a larger herniation of the rhombencephalon through the cervicocranial junction and is associated with syringomyelia, lumbosacral dysraphia, and neural tube defects [34]. However, the incidence of CM2-associated myelomeningocele decreases after the introduction of prenatal folate supplementation [37]. Chiari III was described based on a single case report of a 5-month-old infant with spina bifida, enlarged skull, convergent strabismus, absence of tentorium, cervical hydromyelocele communicating with ventricle IV, and complete herniation of the cerebellum into the spinal canal [34]. Chiari IV (CM4) represents cerebellar hypoplasia; the cerebellum is not herniated into the craniocervical junction. In contrast to CM4, the Dandy Walker variant is characterized by agenesis or hypotrophy of the cerebellar vermis with cystic dilatation of the IV ventricle and dislocation of the tentorium cranially [38].

Syringomyelia may be present even in minimal tonsillar herniation, and therefore some authors have proposed an additional grade -⁠ Chiari 0 (CM0). CM0 corresponds to cerebellar tonsillar herniation of less than 3 mm with the presence of syringomyelia (an interval of 3-5 mm is considered borderline) [34]. Symptoms specific to CM1 are needed to diagnose CM0 without syringomyelia, as many MR images corresponding to CM0 are asymptomatic. The benefit of surgical management of CM0 should be further investigated in prospective controlled studies [39]. Over the years, various authors have defined other types of CM. Chiari 0.5 is defined by Morgenstern by the term "tonsillar wrapping" when the radiological findings do not meet the criteria of CM1, but the cerebellar tonsils already ventrolaterally "wrap" the medulla oblongata [40]. In contrast to CM1, Chiari 1.5 also represents a brainstem herniation [26]. Chiari 3,5 was proposed by Fisahn on the basis of one historical case report of extensive congenital malformation of occipitocervical encephalocele, absence of the neck with pathological location of the intestine in the posterior mediastinum and communication of the encephalocele with the esophagus and stomach [41]. In 2012, Tubbs proposed to add Chiari V to the classification based on one case of hydrocephalus with cerebellar agenesis, occipital herniation through the foramen magnum, and sacral myelomeningocele [42]. Another classification can be found in the literature that divides CM into type A (with syringomyelia) and type B (without syringomyelia) [43].

Acquired cerebellar ectopia (Chiari-like)

In clinical practice, there may be cases that mimic CM but arise from a different pathophysiological basis. Recognizing and understanding these conditions may help to select the optimal treatment strategy and avoid unnecessary surgical procedures. Post-traumatic arachnoiditis of the craniocervical junction may result from birth trauma and perinatal hemorrhage. The craniocervical junction in CM often shows thickening of the dura (referred to in the literature as a "dural band" ["dural band"]); however, this thickening can also arise post-traumatically, reducing the space in the cisterna magna, compressing the tonsils and mimicking CM [44]. Herniation of the cerebellar tonsils is also associated with spontaneous intracranial hypotension with clinical signs of postural headache, nausea, vomiting and cognitive deficit. CT myelogram, MR myelogram or digital subtraction myelogram can be used for diagnosis. In addition, there is enhancement of the dura on brain MRI, the brain appears flaccid, the venous sinuses are enlarged, and the pituitary gland is usually enlarged. Intracranial hypotension can be managed with a blood plug or surgically [44-46]. Idiopathic intracranial hypertension can lead to a CM picture. On MRI, there are usually signs of intracranial hypertension, including findings of empty sella, tortuosity of the optic nerves and increased accumulation of cerebrospinal fluid along them. High intrathecal pressure is measured during the liquor examination. MRI images of CM are also commonly associated with cysts (e.g., arachnoid or choroid plexus cysts) that lead to a decrease in the cerebellar tonsils by their pressure/volume effect in the posterior fossa [44]. Similarly, spatially significant posterior fossa changes, intracranial hematomas, hydrocephalus, or cerebral edema can cause similar effects [34]. Symptomatic herniation of the cerebellar tonsils after lumbar drainage, lumboperitoneal shunt, and supratentorial shunt surgery has been described [26,47,48]. Abnormal mesoderm development can lead to cervicocranial junction pathology, in which the cerebellar tonsils drop; examples include basilar impingement/invagination, clivus hypoplasia, retrocurvation of the dens of the condyle, occcipitalization of the atlas, and Klippel-Feil syndrome [26]. Basilar impression/invagination is defined as a pathology of the cervicocranial junction, whether congenital or degenerative, in which the peripheral bony structures (or dens axis) are invaginated into the posterior fossa and compress the posterior fossa structures during relative immobility of the tentorium [49,50]. Some authors specify invagination as a congenital anomaly, whereas impingement as an acquired lesion [51]. CM can arise secondary to a small posterior fossa and is often seen in conditions such as achondroplasia, fibrous dysplasia, or craniosynostosis [26,34].

Imaging in Chiari malformation

Radiologically, CM1 is inferred when the cerebellar tonsils are herniated 5 or more below the foramen magnum [34]. Baseline MR imaging is used to diagnose tonsillar drop below the foramen magnum [22]. It should be noted that the assessment of cerebellar ectopia in CM1 on MR can vary considerably between assessors, and according to Lawrence, MR assessment by multiple specialists is recommended prior to definitive diagnosis of CM1, especially prior to surgical intervention. The position of the tonsils relative to the foramen magnum is assessed in the mid-sagittal plane in relation to McRae's line (McRae's line connects the basion and opisthion, the most anterior and posterior points of the foramen magnum in the mid-sagittal plane) [52,53]. However, the cerebellar tonsils are paramedial structures, and this must be taken into account when assessing herniation in the mid-sagittal plane. Some authors recommend measuring the position of the tonsils relative to the foramen magnum in the coronal plane as well [54]. Moreover, basic MR imaging tells us nothing about the dynamics of the cerebrospinal fluid. MR examination with phase contrast (cine flow MR) will provide information about the cerebrospinal fluid circulation in the foramen magnum. Normally, in systole, the fluid travels caudally through the foramen through the posterior subarachnoid space (behind the brainstem), and in diastole, cranially through the anterior subarachnoid space (in front of the brainstem). MR examination with phase contrast (cine flow) can reveal the pathology of the circulating fluid in CM. Some studies have highlighted the use of these MR sequences as a diagnostic tool for CM [22]. Normally, the cerebrospinal fluid flow velocity in the subarachnoid space of the craniocervical junction is fairly constant with lower peak flow velocities (peak systolic velocity; PSV) (systolic PSV 1.2-3.3 cm/s, diastolic PSV 1.6-4.5 cm/s). In symptomatic patients with CM1, flows on MR phase contrast imaging are not uniform (there is acceleration in places and deceleration in places in the subarachnoid space) and there is a significantly higher acceleration of flow (systolic PSV 1.8-4.8 cm/s, diastolic PSV 2.5-5.3 cm/s) [55]. Ultrasound can also be used to measure the dynamics of the liquor in the foramen magnum, which can be advantageously performed perioperatively during surgical decompression [56].

Syringomyelia and syrinx

Chiari and other scientists began to investigate the link between cerebellar ectopia and liquor pathology (hydrocephalus and syringomyelia). Many theories have emerged for the formation of the syrinx in CM1. Modern imaging methods in the study by Oldfield and Heiss support the theory that herniation of the cerebellar tonsils into the foramen magnum disrupts the circulation of the liquor during the cardiac cycle; during systole, the cerebellum descends into the foramen, restricting the flow of liquor through the foramen magnum and increasing the pressure exerted on the spinal cord in the subarachnoid spaces, thereby forming a cavity, the syrinx [34,57]. The syrinx is the pathological substrate for a specific type of myelopathy, syringomyelia. In contrast, the term hydromyelia is used for the pathological enlargement of the central spinal canal [58]. In the case of syringomyelia encroaching on the brainstem, we speak of syringobulbia [32]. Surgical intervention enlarging the liquor space in the foramen magnum will release the circulation of the liquor, the cerebellar tonsils regain normal position and shape (if not resected), and the syrinx regresses [34]. It should be noted that there is no evidence for surgical management of asymptomatic syringomyelia or hydromyelia [58]. In general, syringomyelia is associated with impaired circulation of fluid in the foramen magnum or at the spinal level, e.g., in CM1, arachnoiditis, or basilar invagination [59]. Clinically, it is significant that syringomyelia can also occur in spontaneous intracranial hypotension (thus not primarily resolved by decompression surgery) [60].

Surgical techniques for Chiari malformation type I

The surgical approach to CM1 has no clear consensus, in 2020 Arnautovic et al. [61] published a review of 145 surgical series from 1965-2013 from the USA and Europe, the average number of patients in each series was 31, i.e. a total of 4 495 patients. The majority (99%) referred to posterior fossa/foramen magnum decompression as the operative procedure, with the dura being opened in 92 %, followed by some degree of arachnoid dissection in 65%, which resulted in tonsil resection in 27%. An American prospective multicentre study published in the same year [62] included 68 paediatric patients operated on by 14 surgeons; the primary outcome studied was the effect on syringomyelia, with a threshold for satisfactory outcome being its > 50% reduction; 42 patients underwent posterior fossa decompression with duroplasty and tonsil volume reduction (PFDD-T) and 26 underwent the same procedure without tonsil manipulation (PFDD). The desired outcome occurred in 52% of the PFDD group and 63.6% of the PFDD-T group, but without a statistically significant benefit of tonsil volume reduction. A much larger but retrospective cohort of 437 children was processed in 2023 by Braga et al [63], here the tonsil volume reduction was clearly better, with syringomyelia improving after 79.8% of PFDD-T versus 58.7% of PFDD type operations. Recommendations made in a systematic review of CNS (Congress of Neurological Surgeons) published in 2023 in the journal Neurosurgery [64], state when asked about the benefit of tonsil reduction in CM1, that "surgeons can perform resection or reduction of the cerebellar tonsils to improve syrinx and/or symptoms". In our opinion, after ruling out instability of the craniocervical transition, a neurosurgeon for whom intradural surgery is a routine procedure and who feels that there is no risk during tonsil resection in a given patient should perform tonsil resection in the treatment of CM1, to the extent that the flow of fluid in the area of the craniocervical transition is anatomically restituted, the obex is completely free, and there is no risk of future descent of the rest of the tonsils. The "technical note" of three European departments grading the manipulation of tonsils into [65] the "Three R's" may be useful for this issue -⁠ these are (1) intracranial repositioning by coagulation; then (2) subpial aspiration and reduction of the tonsillar volume; and finally (3) resection, which is performed especially in sclerotic tonsils. The different procedures are complementary and the extent of the procedure is performed according to the "feel" of the adequacy of decompression. The individual decision in a given patient is suggested by the recent experience of the Vienna institution with 81 patients [66]. The procedure with only incision or removal of the outer layer of the thickened dura (i.e. without opening the dura) is the least risky, but carries the highest probability of failure; it was performed in 11 patients in the cohort (14%). The highest number of surgical complications was due to arachnoid dissection and manipulation of the tonsils performed in 21 patients during foramen magnum decompression associated with duroplasty (PFDD performed in 45/81 patients overall, i.e. 56%). It can be argued that in the vast majority of cases it was "only" a liquor fistula, with only one patient experiencing intradural bleeding requiring subsequent tonsil resection. Resection or subpial tonsil volume reduction was primarily performed in 25 patients (31%) and was associated with the best clinical outcome in the Chicago Chiari Outcome Scale (COOS), however, again in one case a serious complication due to thrombosis of one PICA occurred requiring urgent revision.

"The choice of the ideal neurosurgical procedure for CM1 remains one of the biggest debates in the field of paediatric neurosurgery" [63] and our review paper does not have the ambition and cannot solve this problem, however, we believe that even for this diagnosis a detailed knowledge of the microanatomy of the cerebellar tonsil will allow the neurosurgeon to make a rational decision during the procedure and to perform the operation as uncomplicated as possible.

Conclusion

The cerebellar tonsils are part of the neocerebellum and have a specific position in the craniocervical junction. Herniation of the cerebellar tonsils through the foramen magnum has a variable clinical picture, ranging from the absence of symptoms to a life-threatening condition with a trunk lesion. The spectrum of congenital CMs is quite broad, with varying clinical significance -⁠ the most common type is type I. Obstruction of the liquor pathways in the foramen magnum can result in syringomyelia, a condition often associated with CM. MRI is the method of choice for imaging cerebellar tonsillar herniation and pathological cerebrospinal fluid circulation. Knowledge of the anatomy and surgical approach options allows us to choose the appropriate surgical technique for posterior fossa and cervicocranial junction surgery.

Grant support

The work was partially supported by grant IGA-KZ-2021 -⁠ 1-16.

Conflict of interest

The authors declare that they have no conflict of interest in relation to the subject of the study.