Telovelar approach to the fourth ventricle
A., artery; Cer.Med., cerebellomedullary; Chor., choroidal; Fiss., fissure; For., foramen; Inf., inferior; Med., medullary; P.I.C.A., posteroinferior cerebellar artery; Telovel., telovelar; V., vein; Ve., vermian; Vel., velum
A formalin-fixed cadaveric specimen is dissected to simulate a median suboccipital craniotomy with C1 posterior arch osteotomy
The cerebellomedullary fissure extends upward between the tonsils posteriorly and the medulla anteriorly. The vallecula opens between the tonsils into the fourth ventricle. .
Both tonsils have been retracted laterally to expose the inferior medullary velum and tela choroidea that form the lower part of the ventricular roof. The nodule of the vermis, on which the inferior medullary arises, is hidden deep to the uvula
The telovelar approach is initiated by slight lateral retraction of the cerebellar tonsils, exposing and dividing both the tela choroidea and the inferior medullary velum
Enlarged view of the left half of the cerebellomedullary fissure. The choroidal arteries course along the tela choroidea from which the choroid plexus projects into the roof of the fourth ventricle. The vein of the cerebellomedullary fissure, which crosses the inferior medullary velum, is the largest vein in the cerebellomedullary fissure.
The interrupted line shows the site of the incision in the tela to provide the exposure seen in the next step. The telovelar junction is the line of attachment of the tela to the velum.
The tela choroidea has been opened extending from the foramen of Magendie to the junction with the inferior medullary velum. The uvula has been displaced to the right side to provide this view extending from the aqueduct to the obex.
The tip of a nerve hook placed inside the fourth ventricle is seen through the paper-thin inferior medullary velum.
The left half of the inferior medullary velum has been divided to expose the superolateral recess and the ventricular surface formed by the superior and inferior peduncles.
The uvula has been retracted to the right to expose all of the floor and much of the roof of the ventricle.
A wide view of the rhomboid fossa is provided without cutting the vermis, from the cerebral aqueduct to the obex.
The right half of the cerebellum was removed by dividing the vermis sagittally and the cerebellar peduncles transversely. The tonsil has been removed and the inferior medullary velum and the cranial loop of the PICA have been displaced downward to expose the opening into the lateral recess. The dentate nucleus forms a prominence, the dentate tubercle, in the superolateral recess of the roof of the fourth ventricle near the site of attachment of the inferior medullary velum.
- Lesions of the fourth ventricle are a challenge because of the
- Severe deficits that may follow injury to the structures in the ventricular walls and floor.
- Other techniques
- Splitting the cerebellar vermis
- Removing part of a cerebellar hemisphere
- Telovelar approach
- Opening the tela alone will provide adequate ventricular exposure in most cases without splitting the vermis
- The inferior medullary velum, another paper-thin layer, can also be opened if opening the tela does not provide adequate exposure.
- Opening the tela alone provides access to the full length of the floor and all of the ventricular cavity except, possibly, the fastigium, superolateral recess, and the superior half of the roof.
- Opening the inferior medullary velum accesses the latter areas and the superior half of the roof.
- Extending the telar opening laterally toward the foramen of Luschka opens the lateral recess and exposes the peduncular surfaces bordering the recess.
- Tumors in the fourth ventricle may stretch and thin these two semi-translucent membranes to a degree that one may not be aware that they are being opened in exposing a fourth ventricular tumour.
- There are no reports of deficits following isolate opening of the tela and velum.
- Structures exposed in the ventricle walls and at risk for producing the deficits
- Dentate nuclei,
- Cerebellar peduncles,
- Floor of the fourth ventricle,
- PICA.
- Occlusion of the branches of the PICA distal to the medullary branches at the level of roof of the fourth ventricle avoids the syndrome of medullary infarction but produces a syndrome resembling labyrinthitis, which includes rotatory dizziness, nausea, vomiting, inability to stand or walk unaided, and nystagmus without appendicular dysmetria
- The main trunk of the AICA is infrequently exposed in opening the cerebellomedullary fissure, but it may also send choroidal branches to the tela and choroid plexus in the lateral recess.
- During an operation on the caudal part 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 part of the roof where they are wrapped around the superolateral recesses near the lateral edges of the inferior medullary velum
- All of the cerebellar peduncles converge on the lateral wall and roof where they may be damaged.
Effects of cerebellar neural injury
- The operative approaches to the cerebellum and fourth ventricle may require
- Splitting of the vermis,
- Resection of part of the hemisphere,
- Removal of the tonsil,
- Opening of the inferior medullary velum,
- Separation of tumor from the floor and roof,
- Dissection in the region of the cerebellar peduncles and deep cerebellar nuclei, and retraction or removal of the flocculus.
- Horsley pointed out that large amounts of cerebellar tissue could be sacrificed with little or no demonstrable loss of function
- Splitting the vermis on the suboccipital surface
- Dandy stated that the vermis could be opened at its center to gain access to fourth ventricular tumors without causing a disturbance of function, provided that the operator carefully avoided the dentate nuclei
- Small lesions in the vermis caused no symptoms or deficit,
- Larger lesions of the uvula, nodule, and flocculus,
- Involving cerebellar fibers related to the vestibular system, cause
- Equilibratory disturbances,
- With truncal ataxia,
- Staggering gait,
- Oscillation of the head and trunk on assumption of the erect position without ataxia on voluntary movement of the extremities.
- Injury to the vestibular projections from the brainstem to the flocculonodular lobe also causes nystagmus that is present in all directions of gaze.
- Cerebellar mutism
- Is a transient complication that may appear after removal of cerebellar tumors
- Usually in children
- Characterized by
- Lack of speech output in the awake patient,
- Intact speech comprehension,
- Sometimes associated with oropharyngeal apraxia
- Exact anatomic cause is still unknown
- Occurred after removal of midline tumors involving the vermis
- The inferior part of the vermis, including the pyramid, uvula, and nodule has been implicated.
- Hemispheric resection
- Required to reach lesions of the
- Lateral part of the roof
- Lateral recess of the fourth ventricle.
- Frazier resected the lateral part of the hemisphere without permanent sequelae (6).
- Unilateral resection of the part of the hemisphere lateral to the dentate nuclei results in
- Ataxia of voluntary movement,
- Hypotonia,
- Adiadochokinesia in the ipsilateral limbs with errors in rate, range, direction, and force of movement,
- Often transient
- If the ablation involves the dentate nucleus
- Above symptoms are more severe and enduring +
- Intention tremor with voluntary movement of the extremities.
- Caudal part of the roof surgery
- Dendate nuclei are located just rostral to the superior pole of the tonsils and are wrapped around the superolateral recess of the ventricle near the inferior medullary velum.
- Dysarthria
- Results when resection extends into the paravermian part of the cerebellar hemisphere
- Occurs more frequently from left hemisphere injury than from vermal or right hemisphere injury
- Nystagmus
- With hemispheric lesions is associated with an ocular rest point 10 to 30 degrees toward the unaffected side, with greater oscillation upon looking to the side of the lesion.
- The addition of a vermian lesion or a lesion extending to the contralateral hemisphere produces more marked symptoms than a unilateral hemispheric lesion and is associated with disturbances of standing, walking, and speech.
- Lesions of the anterior part of the tentorial surface result in
- Increased tone in the muscles used for maintaining the erect posture.
- If the lateral half of this area is damaged, the hypertonia is predominantly in the ipsilateral extremities.
- All of the cerebellar peduncles converge on the lateral wall and roof and may be damaged here.
- Inferior and superior cerebellar peduncles
- Are more likely to be injured during procedures within the ventricle
- Because they abut directly on the ventricular surface;
- Lesions of the superior cerebellar peduncle cause
- Severe ipsilateral intention tremor, dysmetria, and decomposition of movement.
- The syndrome is mild and subsides rapidly if there is only a partial section of the peduncle.
- Lesions of the inferior cerebellar peduncle causes
- Disturbances of equilibrium (similar to those produced by ablation of the flocculonodular lobe)
- Truncal ataxia
- Staggering gait
- Middle cerebellar peduncle
- Would be more susceptible to injury in procedures near the external wall such as those in the cerebellopontine angle
- Because it forms a major part of the cisternal surface of the ventricular wall.
- Lesions here (similar to that produced by damage to the lateral part of the hemisphere)
- Ataxia
- Dysmetria during voluntary movement of the ipsilateral extremities
- Hypotonia
Floor of the fourth ventricle injury
- Intraoperative blood pressure decrease,
- Apnea,
- Respiratory rate increase
- Postoperative diplopia,
- Disturbances of speech and swallowing,
- Poor cough reflex associated with
- Incidental disturbances of gastrointestinal bleeding aspiration pneumonia
- Electrolyte disturbances
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