Craniotomia em Cães

Principles in Intracranial Surgery

Access to the brain structures requires familiarity with intracranial anatomy and experience in neuroimaging diagnosis. Intracranial surgery is indicated for tumor excision, decompression and removal of injured brain tissue, removal of bone fragments that penetrate the brain after fracture, brain biopsies, control of the intracranial pressure (ICP), drainage of abscesses and granulomas, treatment of congenital anomalies (e.g., arachnoid cysts and Chiari-like syndrome) and treatment of hydrocephalus by ventriculoperitoneal shunt (VPS).

Basic Anatomy

The frontal and parietal bones join together among of transverse and sagittal sutures. The meeting of these two sutures forms an anatomical point called "bregma." Caudally, the parietal and occipital bones are joined by the sagittal and lambdoid sutures. The meeting of these two sutures is called "lambda" and in this region is the junction of the dorsal sagittal and transverse venous sinuses that drain the brain tissue. While accessing the intracranial structures, the surgeon should observe the course of these venous sinuses to avoid vascular injury. The laceration of the dorsal sagittal and transverse venous sinuses can cause uncontrollable life-threatening bleeding to the patient and the ligature of these sinuses (especially the two-thirds caudally) can cause severe brain edema.

The frontal bone covers in its rostral portion the frontal sinus and, below this, the cribriform plate of the ethmoidal bone is observed overlying the olfactory bulb.

Emissary veins run through the supraoccipital region forming an arc that follows the transverse sinus.

The middle meningeal artery is positioned caudolaterally and runs through the dura mater in a transversal direction from the temporal region until the dorsal parietal portion.

Preoperative Management

Patients with mass effect lesions, such as tumors and arachnoid cysts, and those with hydrocephalus may benefit from low doses of prednisone (0.5 mg/kg every 12 hours) given 5–7 days before the surgery. This dose of glucocorticoids reduces the peritumoral edema and cerebrospinal fluid (CSF) production. On the other hand, patients with brain trauma or ischemic processes should not receive glucocorticoids preoperatively because its use can increase acidosis and accumulation of intraneuronal liquids.

Patients with brain trauma may present edema and elevated ICP. Due to the action of osmotic pressure in the cerebral vasculature, cerebral interstitial and intracellular space, it is recommended to use 0.9% saline solution as fluid of choice for management of patients suspected of increased ICP.

The use of mannitol at a dose of 0.25 to 1.0 g/kg (over10–20 minutes) every 8 hours for 24 hours drastically reduces the production of CSF and promotes reduction in vasogenic cerebral edema. Reports that the use of mannitol may exacerbate bleeding in patients with intracranial trauma are unfounded.

Patients submitted to intracranial surgery should receive oxygen in the pre-anesthetic time in order to avoid hypercapnia. The compression of the jugular vein should be avoided because it promotes increase in ICP. Throughout the pre-, trans- and post-surgery period it is recommended that the patient be kept in a bed that allows a 30° skull elevation relative to the body to facilitate the jugular drainage.

Standard Approaches

The transfrontal craniotomy is used to expose the frontal lobe, the olfactory bulb and the ethmoidal area. The animal is positioned in sternal recumbence with jaws parallel to the operating table (Figure 1A). The rostrotentorial craniotomy is used to expose lesions found in the cortical region of the temporal and parietal lobes, as well as the cranial portion of the occipital lobe. The head is positioned in lateral recumbence with the injured antimere upward (Figure 1B).

The suboccipital craniotomy is used to expose the caudal portion of the cerebellum, the medulla oblongata (bulb) and the initial portion of the spinal cord (foramen magnum region). The animal must be positioned in sternal recumbence and the ventral cervical region bent through 90° while avoiding compression of the jugular vein to allow the venous return (Figure 1C).

Figure 1. Standard craniotomies techniques in dogs. (A) Transfrontal craniotomy used to expose the frontal lobe and olfactory bulb. (B) Rostrotentorial craniotomy used to expose the temporal and parietal lobes and cranial portion of the occipital lobe. (C) Suboccipital craniotomy used to expose the caudal portion of the cerebellum, medulla oblongata and initial portion of the spinal cord.

(VIN editor: Figures were not provided at the time of publication)

Materials and Basic Instruments

The craniotomy can be achieved by electrical or pneumatic drills or craniotomes that promote faster cutting with greater security. Hemostasis by diathermy should always be performed with bipolar electronic forceps. The use of bipolar diathermy for the hemostasis of middle meningeal artery greatly reduces the bleeding observed right after rostrotentorial craniotomy. In the author's experience, the use of bipolar diathermy controlled between 13–15 Watts has allowed good control of bleeding in peritumoral brain parenchyma without trans- and postoperative complications.

The rayon fiber (Cotonoide®) helps not only in the maintenance of the liquid on the exposed surface but also the control of bleeding. Oxidized cellulose (Surgicel®) is widely used in the control of brain parenchymal hemorrhage. The use of additional sterile adhesive fields (Ioban®) prevents contact with the skin and reduces the risk of trans-surgery contamination.

If repair of the skull defect is considered necessary, titanium mesh and polymethylmethacrylate (PMMA) can be used. Postoperative CSF leakage is not a problem commonly seen in dogs and it can be avoided by using a layer of Gelfoam® over the exposed brain before the time of closure.

Still not widely available in the veterinary market, due to the high cost, equipment such as Cavitron Ultrasonic Surgical Aspirator (CUSA®) that promotes ultrasonic aspiration of injured tissues with a higher degree of protection to the surrounding tissues and the Neuronavigato Stereotactic System (Brainsight®) which gives greater precision to the surgical access with minimal invasion, demonstrate the prospect of veterinary neurosurgical routine in the coming years.

Healing of the Brain

Normally, the brain tissue heals via glial proliferation. A connective tissue is formed over the incised dura ("neodura") within two months after surgery. The surrounding musculature will adhere to the outer surface of this neodura, but CSF separates the brain from the inner surface of this membrane. Attention must be taken if the bone flap is replaced or a synthetic covering is applied to the craniotomy site because neodura may be adherent to the bone or synthetic flap if the patient undergoes reoperation for tumor recurrence.

References

1.  Carreira LM, et al. The dorsal sagittal venous sinus anatomical variations in brachycephalic, dolichocephalic, and mesocephalic dogs and their significance for brain surgery. Anat Rec. 2011;294:1920–1929.

2.  Lapierre F, et al. Hemostatic agents in neurosurgery. In: Francesco Signorell, ed. Explicative Cases of Controversial Issues in Neurosurgery. ISBN: 978-953-51-0623-4, InTech, 2012. Available from: www.intechopen.com/books/explicative-cases-of-controversial-issues-in-neurosurgery/topical-hemostatic-agents-and-neurosurgery.

3.  Bagley RS. Surgical approaches to the central nervous system. Brain. In: Slater D. Textbook of Small Animal Surgery. 3rd ed. Philadelphia, PA: Saunders; 2003.

4.  Wininger F. Neuronavigation in small animals: development, techniques and applications. Vet Clin North Am Small Anim Pract. 2014;44:1235–1248.

5.  Fossum TW. Small Animal Surgery. 4th ed. St. Louis, MO: Elsevier; 2013.