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  • Author or Editor: Kazunari Yoshida x
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Ryosuke Tomio, Takenori Akiyama, Masahiro Toda, Takayuki Ohira and Kazunari Yoshida

OBJECTIVE

Transcranial motor evoked potential (tMEP) monitoring is popular in neurosurgery; however, the accuracy of tMEP can be impaired by craniotomy. Each craniotomy procedure and changes in the CSF levels affects the current spread. The aim of this study was to investigate the influence of several craniotomies on tMEP monitoring by using C3–4 transcranial electrical stimulation (TES).

METHODS

The authors used the finite element method to visualize the electric field in the brain, which was generated by TES, using realistic 3D head models developed from T1-weighted MR images. Surfaces of 5 layers of the head (brain, CSF, skull, subcutaneous fat, and skin layer) were separated as accurately as possible. The authors created 5 models of the head, as follows: normal head; frontotemporal craniotomy; parietal craniotomy; temporal craniotomy; and occipital craniotomy. The computer simulation was investigated by finite element methods, and clinical recordings of the stimulation threshold level of upper-extremity tMEP (UE-tMEP) during neurosurgery were also studied in 30 patients to validate the simulation study.

RESULTS

Bone removal during the craniotomy positively affected the generation of the electric field in the motor cortex if the motor cortex was just under the bone at the margin of the craniotomy window. This finding from the authors' simulation study was consistent with clinical reports of frontotemporal craniotomy cases. A major decrease in CSF levels during an operation had a significantly negative impact on the electric field when the motor cortex was exposed to air. The CSF surface level during neurosurgery depends on the body position and location of the craniotomy. The parietal craniotomy and temporal craniotomy were susceptible to the effect of the changing CSF level, based on the simulation study. A marked increase in the threshold following a decrease in CSF was actually recorded in clinical reports of the UE-tMEP threshold from a temporal craniotomy. However, most frontotemporal craniotomy cases were minimally affected by a small decrease in CSF.

CONCLUSIONS

Bone removal during a craniotomy positively affects the generation of the electric field in the motor cortex if the motor cortex is just under the bone at the margin of the craniotomy window. The CSF decrease and the shifting brain can negatively affect tMEP ignition. These changes should be minimized to maintain the original conductivity between the motor cortex and the skull, and the operation team must remember the fluctuation of the tMEP threshold.

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Mohamed Samy Elhammady and Roberto C. Heros

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Ryota Tamura, Ryosuke Tomio, Farrag Mohammad, Masahiro Toda and Kazunari Yoshida

OBJECTIVE

The anterior transpetrosal approach (ATPA) was established in 1984 and has been particularly effective for petroclival tumors. Although some complications associated with this approach, such as venous hemorrhage in the temporal lobe and nervous disturbances, have been resolved over the years, the incidence rate of CSF leaks has not greatly improved. In this study, some varieties of air cell tracts that are strongly related to CSF leaks are demonstrated. In addition, other pre- and postoperative risk factors for CSF leakage after ATPA are discussed.

METHODS

Preoperative and postoperative target imaging of the temporal bone was performed in a total of 117 patients who underwent ATPA, and various surgery-related parameters were analyzed.

RESULTS

The existence of air cells at the petrous apex, as well as fluid collection in the mastoid antrum detected by a postoperative CT scan, were possible risk factors for CSF leakage. Tracts that directly connected to the antrum from the squamous part of the temporal bone and petrous apex, rather than through numerous air cells, were significantly related to CSF leak and were defined as “direct tract.” All patients with a refractory CSF leak possessed “unusual tracts” that connected to the attic, tympanic cavity, or eustachian tube, rather than through the mastoid antrum.

CONCLUSIONS

Preoperative assessment of petrous pneumatization types is necessary to prevent CSF leaks. Direct and unusual tracts are particularly strong risk factors for CSF leaks.

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Ryosuke Tomio, Masahiro Toda, Kazunari Yoshida and Hamid Borghei-Razavi

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Shunsuke Shibao, Masahiro Toda, Maaya Orii, Hirokazu Fujiwara and Kazunari Yoshida

OBJECT

The drainage of the superficial middle cerebral vein (SMCV) has previously been classified into 4 subtypes. Extradural procedures and dural incisions during the anterior transpetrosal approach (ATPA) may interrupt the route of drainage from the SMCV. In this study, the authors examined the relationship between anatomical variations in the SMCV and the corresponding surgical modifications to the ATPA that are necessary for venous preservation.

METHODS

This study included 48 patients treated via the ATPA in whom the SMCV was examined using 3D CT venography. The drainage patterns of the SMCV were classified into 3 types: cavernous or absent (Type 1), sphenobasal (Type 2), and sphenopetrosal (Type 3). Type 2 was subdivided into medial (Type 2a) and lateral (Type 2b), and Type 3 was subdivided into vein (Type 3a), vein and sinus (Type 3b), and sinus (Type 3c). The authors performed 3 ATPA modifications to preserve the SMCV: epidural anterior petrosectomy with subdural visualization of the sphenobasal vein (SBV), modification of the dural incision, and subdural anterior petrosectomy. Standard ATPA can be performed with Type 1, Type 2a, and Type 3a drainage. With Type 2b drainage, an epidural anterior petrosectomy with subdural SBV visualization is appropriate. The dural incision should be modified in Type 3b. With Type 3c, a subdural anterior petrosectomy is required.

RESULTS

The frequency of each type was 68.7% (33/48) in Type 1, 8.3% (4/48) in Type 2a, 4.2% (2/48) in Type 2b, 14.6% (7/48) in Type 3a, 2.1% (1/48) in Type 3b, and 2.1% (1/48) in Type 3c. No venous complications were found.

CONCLUSIONS

The authors propose an SMCV modified classification based on ATPA modifications required for venous preservation.

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Ryosuke Tomio, Masahiro Toda, Agung Budi Sutiono, Takashi Horiguchi, Sadakazu Aiso and Kazunari Yoshida

OBJECT

Extended endoscopic transnasal surgeries for skull base lesions have recently been performed. Some expert surgeons have attempted to remove tumors such as chordomas, meningiomas, and pituitary adenomas in the clival region using the transnasal approach and have reported abducens nerve injury as a common complication. There have been many microsurgical anatomical studies of the abducens nerve, but none of these studies has described an anatomical landmark of the abducens nerve in the transnasal approach. In this study the authors used cadaver dissections to describe Grüber's ligament as the most reliable landmark of the abducens nerve in the transnasal transclival view.

METHODS

The petroclival segment of the abducens nerve was dissected in the interdural space—which is also called Dorello's canal, the petroclival venous gulf, or the sphenopetroclival venous confluence—using the transnasal approach in 20 specimens obtained from 10 adult cadaveric heads.

RESULTS

The petroclival segment of the abducens nerve clearly crossed and attached to Grüber's ligament in the interdural space, as noted in the transnasal view. The average length of the dural porus to the intersection on the abducens nerve was 5.2 ± 1.0 mm. The length of the posterior clinoid process (PCP) to the intersection on Grüber's ligament was 6.4 ± 2.6 mm. The average width of Grüber's ligament at the midsection was 1.6 ± 0.5 mm.

CONCLUSIONS

Grüber's ligament is considered a useful landmark, and it is visible in most adults. Thus, surgeons can find the abducens nerve safely by visualizing inferolaterally along Grüber's ligament from the PCP.