Safe entry zones to brainstem

1. Approaches to the Brainstem

4. Ventral aspect

5. Lateral aspect

6. How to approach the brain stem ?
• Possible transcranial routes
• Approaching lesions in the brainstem
• Possible safe entry zones
Intent of management?
• Biopsy
• Decompression
• Total excision

7. Approach selection
Preoperative Planning
• The ultimate success depends largely on preoperative planning and
preparation.
• Selection of the proper approach - a key component of preparation.
• Ideally, the approach uses -
the shortest distance, although this is not possible in every case.
minimally disturbing adjacent neural pathways.

8. Intraoperative monitoring
• Cranial nerves
EMG monitoring -III IV V VI VII VIII IX
X XI XII
BAEP
• MEP/SSEP

13. • IOM - the mapping of cranial nerve nuclei.
• Based on intraoperative electrophysiological findings and the
compound muscle action potentials from related muscles.
• Facilitates the identification of “safe” entry zones to the brainstem.
• Useful in midline tumors or floor of the fourth ventricle.
• Not useful for ventrally or laterally located lesions.

14. Anesthesia in the Setting of Brainstem IOM
• Ultra-short-acting paralytics limited to induction
• Constant infusion of iv agents for steady state of anesthesia
• Avoidance of agents that degrade IOM –
Hypothermia, hypotension
inhaled halogenated anesthetic agents
intermittent injection of intravenous anesthetics

15. Method -
• Starting with a current of 0.2-0.3 mA, probe is applied to an area of
interest for no longer than 5 seconds and is moved at 1-mm intervals
to create a functional map of the floor of the fourth ventricle.
• Distortion of anatomy by tumor mapped.
• Update map during resection.
• MEPs of the corticospinal tract can be used for monitoring
descending motor tracts during resections of brainstem tumors.
• SSEP useful for understanding surgery’s impact on sensory function.

17. Image Guidance for Brainstem Surgery
• The use of image guidance often directs intra- operative decision
making as to the location of the lesion and the boundary of pathology
with normal tissues.
• In cases where the lesion does not abut a pial surface, neuronavi-
gation can assist with the selection of entry points.

18. Surgical Tools for Brainstem Surgery
• Because the visual axis and light source of the
microscope are 3° to 6° apart, depending on the
focal length used, the small deep exposure is
frequently poorly illuminated.
• Lighted suction and bipolar devices eliminate this
problem.
• Use of dynamic retraction.
Bayonetted lighted bipolar forceps

19. The Two-Point Method
• To guide the surgical approach selection for deep-
seated lesions.
• The surgeon places a point at the center of
the lesion (Point A).
• A second point (Point B) is selected where the
lesion most closely approximates a pial or an
ependymal surface.
• A straight line is drawn from Point A to Point B and
then extended to the skull.
• This line defines the optimal trajectory for
approaching the lesion.

20. Application of two-point method with selection of an alternative

21. Approaches to Brainstem
Schematic drawing illustrating the
most common surgical approaches
used for different areas of the
brainstem.

23. A solid/cystic anterior
mesencephalic lesion with
expansion towards the
interpeduncular cistern,
resection done by fronto-
orbito zygomatic approach

24. Tumors located at the central
portion of the midbrain and
growing towards the pineal
region.
Infratentorial supracerebellar
approach.

25. Tumor in the quadrigeminal plate
growing towards the fourth ventricle.
Transtentorial occipital approach

26. A large tumor growing towards third
and fourth ventricle. Approached by
combined infratentorial
supracerebellar followed by
subocciptal telovelar approach.

27. Endoscopic access to tumors
located in the anterior and
superior portion of the midbrain.

28. A large lesion in the anterior and
superior portion of the pons,
approached via orbito-fronto
zygomatic via the supratrigeminal
entry zone.

29. Superior and posterior pontine tumor
approached via suboccipital telovelar
approach with the point of entry into the
pons through the suprafacial triangle.

30. A large tumor anterior to the
medulla.
The far lateral approach and trans-
olivary point of entry used for
resection .

31. Thirteen zones were selected:
1) anterior mesencephalic zone
2) lateral mesencephalic sulcus
3) intercollicular region
4) peritrigeminal zone
5) supratrigeminal zone
6) lateral pontine zone
7) supracollicular zone
8) infracollicular zone
9) median sulcus of the fourth ventricle
10) anterolateral and
11) posterior median sulci of the medulla
12) olivary zone
13) lateral medullary zone
PONS
MIDBRAIN
MEDULLA

32. MIDBRAIN
Anterior Mesencephalic Zone
• Lesions involving the anterior midbrain can
be accessed through a limited area on the
cerebral peduncle bounded medially by the
oculomotor tract and nerve and laterally by
the corticospinal tract.
• The entry point inside the interpeduncular
cistern is limited superiorly by the posterior
cerebral artery (PCA) and inferiorly by the
main trunk of the superior cerebellar artery
(SCA).

34. Lateral Mesencephalic Sulcus
• The lateral mesencephalic sulcus extends
downward in a concave fashion from the
medial geniculate body to the
pontomesencephalic sulcus.
• The average total length of the sulcus was 9.6
mm (range 7.4–13.3 mm) with an average
working-channel length of 8.0 mm.

35. Intercollicular Region
• The most appropriate area for a small
neurotomy has been described as the
intercollicular region, because of its
sparseness of fibers.

36. PONS
Peritrigeminal Zone
• The anterolateral surface of the pons has
traditionally been considered a safe zone
for entering the brainstem.
• On the axial plane, a mean distance of
4.64 mm (range 3.8–5.6 mm) between CN
V and the corticospinal tract, and a mean
depth of dissection of 11.2 mm (range
9.5–13.1 mm) to the trigeminal nuclei.

37. Supratrigeminal Zone
• Taking advantage of the posterolateral
location of the middle cerebellar peduncle
and the thick pontine transverse fibers, it is
possible to carefully dissect along these
fibers, medially or anteromedially, posterior
to the trajectory of the corticospinal tract.

39. Lateral Pontine Zone
• A safe corridor on the junction between
the middle cerebellar peduncle and the
pons and between the trigeminal and the
facial-vestibulocochlear complex root
entry zones.

40. Median Sulcus of the Fourth Ventricle
• An approach through the midline, between
the projection of the CN VI nuclei and the
projection of the CN III nuclei on the
midbrain surface.
• Even the slightest lateral retraction may
provoke extraocular movement disorders
caused by damage to the medial longitudinal
fascicle.

41. Supracollicular and Infracollicular Zones
• Suprafacial triangle - caudally by the facial nerve, laterally by the
cerebellar peduncles, and medially by the medial longitudinal fascicle.
• Infrafacial triangle - striae medullaris caudally, the facial nerve laterally,
and the medial longitudinal fascicle medially.

42. MEDULLA
Anterolateral Sulcus
• Just lateral to the pyramid, the rootlets of
the hypoglossal nerve leave the brainstem
on the anterolateral sulcus.
• The short space between these rootlets and
those of the C-1 nerve coincides with the
decussation of the corticospinal tract.
• A paramedian oblique dissection may avoid
the corticospinal tract and address lesions of
the anterior lower medullary region.

43. Posterior Median Sulcus
• A neurotomy on the median sulcus provides
a corridor near the center of the medulla.
Lateral Medullary Zone “Inferior Cerebellar
Peduncle Approach”
• Safe entry zone for resection of dorsolateral
medullary lesions.
• Lesions were approached through the
foramen of Luschka with an incision in the
inferior cerebellar peduncle.

44. Olivary Zone
• The olives are marked oval eminences on the
anterolateral surface of the medulla, limited
medially by the anterolateral sulcus and the
pyramids and posteriorly by the
posterolateral sulcus.
• A safe depth of dissection via the olive,
ranging from 4.7 to 6.9 mm, with a vertical
length of 13.5 mm.

45. General Technique for Resection of well-encapsulated Lesions
• Vertically open a pial or an ependymal brainstem surface, parallel to
the fibers at the level of the safe-entry zone, using bayonet
microforceps instead of a blade.
• The opening of the forceps is used to stretch and displace fibers to
allow the surgeon to reach the lesion.
• After exposure of the lesion, use micropituitary forceps, dissectors,
suction, and cautery to remove the lesion in a piecemeal fashion.

46. • In cases of cavernous malformations, the gliotic tissue surrounding
the cavernous malformation and the developmental venous anomaly
should be preserved.
• After complete removal of the lesion, hemostasis is achieved using
electrocautery on a low setting.
• Alternatively, hemostatics may be placed into the resection cavity to
assist with hemostasis but should be removed upon completion of
the procedure