Anatomical study and clinical practices of
transcerebellomedullary fissure approach

Yang Yang, Jiangang Wang, Yi Shao, Xingang Li

Dept. Neurosurgery, Qilu Hospital ,Shandong University, Jinan , Shandong , P.R. China

 

Abstract

Objective. To evaluate the advantages of transcerebellomedullary fissure approach over conventional surgical approaches in exposing the fourth ventricle on the base of our anatomical study and recent clinical practices.

Materials and Methods. In the anatomical study, 10 formalin-fixed cadaveric heads were selected , and the bilateral internal carotid and vertebral arteries were cannulated and perfused with red colored latex . The stepwise dissections were then performed and important structures were identified. Based on our findings in cadaver dissections, 8 patients with their lesions in the fourth ventricle, including a intrinsic tumor in brainstem were selected and transcerebellomedullary fissure approach was performed. With the patient in the prone or lateral recumbent position, a suboccipital midline craniotomy is performed. What is different from the conventional strategies is that in stead of splitting the vermis , the cerebellomedullary fissure is opened by dissecting and separating the tonsils from the medulla oblongata and uvula, cutting the tenia(e), the tela choroidea and/or the inferior velum to extensively exlpose the whole fourth ventricle from obex superiorly to the aqueduct and laterally to the lateral recesss.

Results. In cadaver dissection, the cerebellomedullary fissure was opened, the tonsil, the foramen of Magendie, the tenia(e), the tela choroidea, the inferior velum, the telovelar junction, choroidal branches of the PICA, the choroidal plexus, the nodule, the lateral recess and structures in the floor of the fourth ventricle were exposed step by step and the feasibilities of the brand-new techniques reevaluated. In the clinical practices, the surgical exposure were proved wider with transcerebellomedullary fissure approach than that in our previous experiences with conventional techniques. Total resection was achieved in 7 patients and subtotal resection achieved in 1; The clinical condition improved in 7 patients following operation. Acute obstructive hydrocephalus occurred in 1 of them 12 hours after surgery and his condition improved immediately following a ventricular tapping and had a satisfactory recovery and 1 patients died from respiratory dysfunction.

Conclusions. Transcerebellomedullary fissure approach is a practicable and promising surgical technique which have advantages over the conventional techniques in dealing with lesions in fourth ventricle.

Introduction

In conventional surgical approaches to expose the fourth ventricle, the midline suboccipital approach is used and the operative access to the fourth ventricle was obtained by splitting the cerebellar vermis or even by removing part of a cerebellar hemisphere [1-7]. An incision of the vermis and a lateral retraction of the dentate nuclei and dentatonucleocortical projections have been implicated in the so-called "cerebellar mutism" syndrome [4-10], which usually occurs within 12 to 48 hours following surgery and lasts for 1.5 to 12 weeks [8]. The visualization of the lateral portion of the fourth ventricle, however, is usually inadequate even at the cost of splitting the vermis [11, 12].

The natural clefts in the cerebellomedullary fissure were not considered as a route previously just probably because of the its complexity and poor understanding of the anatomical structures by neurosurgeons and it was not until recently when several reports of the use of this fissure for approaching the fourth ventricle have appeared [4-6] and the advocators of the new approach were probably inspired by the idea of opening the sylvian fissure during the pterional approach [4-6]. Our present study is composed of two parts: An anatomical dissection including 10 formalin-fixed cadaveric heads and clinical practices of 8 patients with lesions in the fourth ventricle.

Materials and Methods

Cadaver dissection

10 formalin-fixed cadaveric heads were selected, and the bilateral internal carotid and vertebral arteries were cannulated irrigated with water and perfused with red latex. The stepwise dissections were performed by retracting the cerebellar tonsils, opening the tela choroidea and inferior medullalry velum to exposed the whole ventricle.

Clinical practices

Based on the anatomical findings, we adopted transcerebellomedullary fissure approach to 8 patients with their lesions in fourth ventricle from a period of January, 2002 to December, 2003. The general information of the patients is illustrated in the table 1.

Table 1

The preoperative information of the patients

Case No.

Age (yrs)

Sex

Headache

Papilledema

Nystagmus

Romberg sign

Overshoot of "finger to nose"

Inaccurate "heel- knee- shin" test

Staggering gait

Diagnosis

1

15

male

+

+

+

+

-

-

+

Anaplastic astrocytoma

2

7

female

+

+

-

+

+ (bilateral)

+ (bilateral)

+

Ependymoma

3

24

male

+

+

-

-

-

-

-

Anaplastic ependymoma

4

40

male

+

-

-

-

-

-

-

Anaplastic choroid plexus papilloma

5

47

male

+

+

-

-

-

-

-

Choroid plexus papilloma

6

14

male

+

+

-

+

+ (left)

+ (left)

+

Medulloblastoma

7

51

female

+

+

-

-

-

+ (bilateral)

-

Choroids plexus paplilloma

8

16

female

+

+

-

-

-

-

-

Astrocytoma
(Grade II)

Surgical techniques

The surgical techniques used here is described in detail previously [4-7, 12, 13]. Lateral recumbent position or prone position if necessary, is applied. A suboccipital midline craniotomy is performed as usual. What is different is that, in stead of splitting the vermis, the tonsillar surfaces are dissected, separated from the medulla oblongata and uvula, and retracted superiorly and laterally. The tenia(e) and posterior margin(s) of the lateral recess are cut along the lateral margin of the ventricle, the cutting line is turned laterally to the posterior margin of the lateral recess at its ventricular entrance and the cerebellar hemisphere(s) is then retracted more superolaterally . An alternative variation, namely the so called "telovelar approach", is that the cutting begins from the foramen Magendie towards the tela choroidea and the inferior velum can also be opened if opening the tela alone does not provide adequate exposure and, as a result, the whole floor of the ventricle, from the obex to the aqueduct and to the lateral recesses is then extensively exposed. Various intraoperative strategies can then be used.

Results

Cadaver dissection

The extention of the cerebellomedullary cistern (cisterna magna) (Fig.1) is the cerebellomedullary fissure between the cerebellum and the medulla to the inferior portion of the roof of fourth ventricle. The cerebellomedullary fissure consists of the medullotonsillar space between the tonsil and medulla (Fig.2) and the uvulotonsillar space between the uvula and the tonsil (Fig.3), [4].

The vermis surface, located behind the fourth ventricle and above the foramen of Magendie, has a diamond shape. The upper portion is called the pyramid; The lower half projects downward between the tonsils is called the uvula. The majority of the uvula is hidden by the cerebellar tonsils. The latter are superolaterally attached to the cerebellar white matter by the tonsillar peduncle whereas the inferior and medial surfaces of the tonsils are free (Fig.4).

The tela choroidea and inferior medullary velum are located at the floor of the cerebellomedullary fissure and form the majority inferior half of the roof of the fourth ventricle, except in the rostral midline where the ventricular surface is formed by the nodule, there are some branches posterior inferior cerebellar artery in the tela choroidea. Covered by the uvula immediately from behind, the nodule cannot be seen from outside.

The teniae, the attachment of the tela choroidea to the medulla inferolaterally, continues along the ventricular entrance into the lateral recess. The choroid plexus is attached to the ventricular ( interior ) surface of the tela choroidea (Fig.5).

The attachment of the inferior medullary velum to the tela choroidea, the telovelar junction, extends from the nodule into each lateral recess (Fig.6), (Fig.7). The inferior medullary velum is a membranous layer connecting the nodule and the flocculus and blends into the dorsal margin of each lateral recess and forms the peduncle to which the flocculi attach to the margins of the foramina of Luschka (Fig.8).

The roof of the lateral recess is formed by the extension of the tela choroidea and the inferior medullary velum. The wall of the fourth ventricle become visible after removing the tela choroidea. The floor is the rhomboid fossa. Its apex is located at the level of the cerebral aqueduct, and its lateral angles open into the cerebellopontine angles through the lateral recess. The medullary striae connects the apertures of the lateral recesses (Foramen of luschka) is located at the level of the junction of the pons and the medulla. Its caudal tip, is the obex (Fig.9).

The floor is divided longitudinally into symmetrical halves by the median sulcus. The sulcus limitans, another longitudinal sulcus, divides each half of the floor into the median eminence bordering the midline and a lateral region called the vestibular area (Fig.10).

The median eminence, contains from above to below, the facial colliculus, the hypoglossal and vagus triangle and the area postrema; The vestibular area, lateral to the median eminence and sulcus limitans, forms a rounded elevation that extends into the lateral recess, is crossed by the striae medullares, and overlies the vestibular nuclei (Fig.11). The auditory tubercle, a prominence in the lateral portion of the vestibular area, overlies the dorsal cochlear nucleus and the cochlear part of the vestibulocochlear nerve.

Clinical practices

Total resection was achieved in 7 patients and subtotal resection achieved in 1; The clinical condition improved in 7 patients following operation. Acute obstructive hydrocephalus occurred in 1 of them 12 hours after surgery and his condition improved immediately following a ventricular taping and had a satisfactory recovery. 1 patient died from respiratory dysfunction 19 hours after the surgery. The detailed information is recorded in table 2.

Table 2

The postoperative information of the patients

Case No.

The extent of resection

The postoperative condition

1

Subtotal

Improved

2

Total

Improved

3

Total

Died

4

Total

Postoperative hydrocephalus occurred and remitted after ventricular taping and had a satisfactory recovery.

5

Total

Improved

6

Total

Improved

7

Total

Improved

8

Total

Improved

Representative cases

Case 1. A 15-year-old male who had experienced a intermittent headache for 1 year accompanied by nystagmus and dysfunction of the left abducens, was admitted to the hospital. Papilledema, diplopia, nystagmus and positive Romberg sign were disclosed in physical exam. A solid lesion in the dorsal medulla with a large cyst in the left cerebellar hemisphere as well as hydrocephalus was demonstrated by MRI (Fig.12). With the patient in his prone position , the suboccipital midline craniotomy (Fig.13) and the transcerebellomedullary fissure approach was performed and lesion was found situated at the low left quadrant of the rhomboid fossa (equivalent to the left hypoglossal triangle), and was continuous with a intracerebellar cyst via the inferior peduncle. A subtotal resection (Fig.14), (Fig.15) was achieved and the patient recovered without complication. The lesion was determined as anaplastic astrocytoma by histological exam.

Case 2. A seven-year-old female with a headache and vomiting for 6 months accompanied by staggering gait for about 20 days. Physical exam found papilledema, inaccurate "heel- knee- shin" test, overshoot of "finger to nose" test and positive Romberg sign. Preoperative MRI displayed a inhomogenous mass lesion in the IV ventricle and obstructive hydrocephalus (Fig.16). With the lateral recumbent position, the suboccipital midline craniotomy was performed and the lesion was totally removed via transcerebellomedullary fissure approach. The tumor is verified as ependymoma by pathological exam. The patient recovered uneventfully. MRI after 1 year found the the patient is still remain tumor-free.

Case 3. 14-year-old male patient ,with a intermittent headache and vomiting for 10 days. Papilledema, positive Romberg sign, left-sided overshoot of "finger to nose" and inaccurate "heel- knee- shin" test was found in the physical exam. A inhomogenous space-occuping lesion was demonstrated in the fourth ventricle hydrocephalus by MRI scanning (Fig.17) and CT (рис.18). The prone position was applied and a total resesction was achieved with transcerebellomedullary fissure approach. No residual lesion was indicated in postoperative CT scan (Fig.19). and the lesion was verified as a medulloblastoma histologically.

Discussions

The pioneer [4, 14] auther had previously stated that the vermis could be opened without causing a disturbance in function, as long as the the dentate nuclei is carefully spared. Recent research, however, has verified that incision of the vermis has been implicated in the development of so-called "posterior vermal split syndrome." [5-10, 12, 13].Vermian lesions may cause equilibratory disturbances with truncal ataxia, staggering gait, oscillation of the head and trunk, and nystagmus on assuming the erect position, without ataxia on voluntary movement of the extremities [4]. Cerebellar mutism is a transient complication and is usually observed in children and is characterized by a lack of speech output in the awake patient with intact speech comprehension, which is sometimes associated with oral pharyngeal apraxia. [4, 8, 10, 13]. If the dentate nucleus, whose surface on the ventricular wall is marked by the dentate tubercle, is involved, equilibratory disturbances are more severe and enduring than those observed with vermian lesions alone; in addition, they are accompanied by intention tremor during voluntary movement of the extremities [4].

The transcerebellomedullary fissure approach makes it unnecessary to incise the inferior vermis [4-7, 12]. To open the transcerebellomedullary fissure sufficiently and remain impunity, a thorough understanding of the detailed anatomy of the fissure is mandatory. The tonsil(s) and uvula and/or the tonsil(s) and medulla oblongata must be separated from each other and retracted superolaterally. In the next step, the tenia, tela choroidea, and/or lateral recess, which constitute the roof of the ventricle, have to be incised to enter the interior of the ventricle [5]. Although some auther insisted that to expose the fourth ventricle walls widely, the roof of the ventricle also has to be widely opened, and not only the tenia but also the posterior margin of the lateral recess should thus be incised on both sides [5], our experiment of cadaver dissection and clinical experience found that opening merely the tela choroidae did provided sufficient surgical exposure under most circumstances [4].

Excessive exposure increases the chance of injury to cerebellar peduncles and therefore cause undesirable complications. The superior cerebellar peduncle is likely to be injured during operations on lesions involving the superior portion of the roof above the level of the dentate tubercles and postoperative ipsilateral intention tremor, dysmetria, and decomposition of movement may occur; The inferior peduncles are more susceptible to damage and lead to disturbances of equilibrium with truncal ataxia and staggering gait when exposing lesions within the lateral recess [4]; Middle cerebellar peduncle is susceptible to injury only when during procedures that take place near the external wall of the superior half of the roof .The common symptoms under such circumstances include ataxia and dysmetria during voluntary movement of the ipsilateral extremities, with hypotonia similar to that produced by damage to the lateral portion of the hemisphere [4].

Opening the tela may require that some choroidal branches of the posterior inferior cerebellar artery (PICA) will be obliterated; however, these choroidal branches rarely have neural branches once they enter the tela [4]. It is not very difficult to control intraoperative bleeding and provides a clear observation of the boundary between the lesion and the floor of the fourth ventricle, which makes the surgery safe and effective because the PICA can be exposed and properly protected at the beginning of the approach and its operative view from below is parallel to the ventricle floor [5]. We have no experience of troublesome hemorrhage.

During an operation performed on the caudal portion of the roof, one should remember that the dentate nuclei are located just rostral to the superior pole of the tonsils underlying the dentate tubercles, in the posterolateral portion of the roof, where they are wrapped around the superolateral recesses near the lateral edges of the inferior medullary velum [4]. In all the present cases, the great cares were taken during the manipulation of these structures in an effort to spare the dentate tubercles to minimize postoperative ataxia.

Exposure and accurate identification of foramen Magendie which is an important landmark of the surgery is almost indispensable because it's just from here that the incision of the tenia, the opening of tela choroidea and sometimes the inferior velum start. What is of importance is that too much manipulation at the site of obex may result in severe and even disastrous outcomes. Some animal experience demonstrated that a small cutting into the obex can induce irreversible respiratory dysfunction, which indicates that some important passage from the respiratory center may be exist around the area [15]. The only one patient died in our present series is probably because some injury to this area.

Vagal triangles is another venerable structures should be preserved as much as possible and injury of bilateral vagus nerve lethal palsy of vocal cord. There is no such complication observed in the present 8 cases, but it did occurred in our earlier experiences.

Our clinical practices with techniques of transcerebellomedullary fissure approach are optimistic and the brand new strategies is proved to be advantageous compared with conventional vermian-splitting techniques since it offers neurosurgeons a wider exposure, a minimally traumatic manipulation, a more radical resection of lesions and what is most important, a smoother postoperative course and a potentially better prognosis.

Conclusions

Transcerebellomedullary fissure approach is a practicable and promising surgical technique which have advantages over the conventional techniques in dealing with lesions in fourth ventricle.

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