Surgical Diseases of the Brain

Intracranial surgery in animals is most commonly employed for removal of intracranial masses, biopsy of intracranial lesions, placement of ventricular cerebrospinal fluid (CSF) shunting devices, decompression and debridement of intracranial tissues, and short-term treatment of increased intracranial pressure (ICP). Surgery may include removal of sizable portions of the skull (craniotomy or craniectomy) or be limited to smaller burr holes for decompression and evacuation of hematoma or stereotactic biopsy. Additional indications for intracranial surgery in humans that may be employed in animals include treatment for seizures, chronic pain, and movement disorders.

Intracranial surgery used as treatment for intracranial disease is most appropriately considered following an appropriate diagnosis (both physical and anatomic) and with an understanding of the pathophysiology associated with the disease in question. Surgical approaches to intracranial lesions are chosen based on lesion location, extent, anticipated consistency, nature (inflammatory versus neoplastic versus hemorrhagic), and the goal of the surgery (tissue removal versus biopsy versus decompression) [1]. Limited approaches are more often used for biopsy, hematoma decompression, and intraventricular shunt placement. The size and extent of bone removal are also limited by skull anatomy, surrounding soft tissues, and the associated vasculature. Surgical approaches to the brain have been reviewed elsewhere [1-4]. The more common disease processes in which intracranial surgery is currently utilized are discussed further.

Neoplasia

The most appropriate treatment for dogs with brain tumors has yet to be defined. Survival for dogs with brain tumors that are not treated, however, is reportedly dismal. In one study, median survival for dogs with varying tumor types that did not have treatment was 6 days [5]. Anecdotal reports exist, however, of dogs with brain tumors living for months to years without definitive treatment. The natural course and overall survival times of animals with brain tumors are not well established, making objective determination of treatment effects more difficult.

Surgical removal is most readily used for superficially located, encapsulated, relatively small, benign tumors of which meningiomas are the most common (Fig. 41-1). Surgery also affords the possibility of cure that is uncommon with other treatment modalities for brain tumor. Depending on the type and location of the tumor, a surgical approach is made to the skull. The overlying skull is removed by performing a craniotomy or craniectomy. The mass is identified and, ideally, its borders are determined. The tumor can then be biopsied, debulked, or ideally, removed in total via a combination of sharp, blunt, and aspiration resection. Surgical equipment and techniques have been described elsewhere [1]. Tumor removal is performed with magnification or with the aid of an operating microscope to improve visualization of the microvascular anatomy and to provide a better view of the tissue. An ultrasonic aspirator unit may be useful in some instances to more safely remove softer tumor tissue or necrotic brain at the borders of the tumor.

Many brain tumors are removed in a piecemeal fashion to avoid compression of normal brain and further brain damage. The inner portions of the tumor are removed first to collapse the volume of the mass. Next, the capsule is identified and removed, if possible. The dura in contact with the lesion is incised with as large a margin as possible. In instances of meningioma, a biopsy of normal-appearing dura at the edge of the lesion is collected for histologic evaluation of tumor margins.

Figure 41.1. (A, B) Transverse, T1-weighted MR images following intravenous contrast administration of dogs with meningioma (left) before and (right) after surgical removal (From Bagley RS: Fundamentals of Veterinary Clinical Neurology, Ames, IA: Blackwell Publishing, 2005).

In dogs, even when meningiomas are histologically benign, these tumors are often not well encapsulated and hence are difficult to differentiate from surrounding non-tumorous brain. Brain damage in areas adjacent to the tumor can make gross delineation of tumor borders difficult if not impossible. Tumor may be located in areas of the brain where access and exposure are limited, increasing the risk of surgical morbidity. Poor surgical access often leads to increased surgical mortality. If the tumor is within the parenchyma or within a ventricle, it is impossible to surgically access the lesion without damage to at least some surrounding or overlying brain. Many intracranial tumors have multiple vessels supplying them, leading to the risk of hemorrhage during resection. These factors may limit successful removal of brain tumors; in some instances, however, surgery is successful in bringing about cure or long-term remission of brain tumors [1,2,5-12].

Surgical treatments for brain-stem tumors are often hindered by inadequate exposure of the lesion and infiltration or compression of vital brain structures. Cranial nerves are often entwined within these masses, making complete resection difficult. Tumors of the cranial nerves may also require surgical removal similar to other tumors of peripheral nerves [7]. Median survival times reported in dogs with all types of brain tumors after surgery vary but tend to cluster around 140 to 150 days [1,2,5-12]. For meningiomas, median survival times may be slightly longer (240 to 300+ days). Mortality is significant within the first 30 days of surgery for animals with infratentorial compared with supratentorial tumors.

Surgical excision is more readily accomplished in cats, as meningiomas in this species tend to be well encapsulated and easily delineated from normal brain. Studies have determined a median survival interval of cats after meningioma resection to be 22 and 27 months [13,14].

Pituitary tumors may be treated surgically primarily through a transsphenoidal approach [15-18]. It can be difficult, however, to access larger tumors (> 1.0 cm) surgically, leading to incomplete removal and increased surgical morbidity.

Treatment of metastatic tumor to the brain is rarely performed in animals. With humans, however, surgical removal of metastases often improves survival. Tumors invading the skull, such as multilobular chondrosarcoma or osteosarcoma, may also be surgically removable depending primarily on size and anatomic location at the time of diagnosis [19,20].

Many factors may affect survival following brain surgery, including neurologic and systemic abnormalities. In general, most dogs are probably diagnosed relatively late in the course of their disease and only when clinical signs become obvious. By this time, the tumors are relatively large, have caused significant associated brain injury, and are often infiltrative or tightly adherent to the nervous tissue. An overall deterioration of systemic health may have occurred prior to definitive treatment because of lack of adequate nutrition, chronic stress, pain, or deleterious effects of medications administered to control clinical signs, namely corticosteroids. Non-nervous system organ effects such as brain-heart syndrome may adversely compromise systemic health. In addition, as intracranial surgery in small animals has a relatively short history, the learning curve is still steep in regard to intracranial surgical techniques. All of these factors affect historical morbidity and mortality statistics of animals with brain tumors.

Unfortunately, primarily because of relatively small numbers of dogs with brain tumors that have been definitively treated, all dogs with tumors tend to be grouped for analysis. This means that all dogs with brain tumors, regardless of tumor location or tumor growth characteristics, are regarded as being equal, when the behavior and aggressiveness of the various tumor types and locations are ignored. Additionally, treatments given are variable even for animals with the same clinical characteristics. These factors introduce bias into the determination of overall survival. Finally, some lesions are diagnosed as brain tumors based primarily on imaging characteristics without a subsequent biopsy diagnosis. All of these potential influences make it difficult to communicate accurately with owners the prognosis of the many different types and locations of brain tumor. In an attempt to begin to determine whether certain characteristics influence overall survival, we have evaluated dogs with biopsy-diagnosed tumors regarding the influence of tumor type and location on survival after initial surgical treatment. Preliminary conclusions indicate that dogs with supratentorial tumors are more likely to survive longer than 30 days following surgery compared with dogs with infratentorial tumors. The mortality associated with supratentorial surgery is also lower than for infratentorial surgery in dogs with brain tumor. Overall, dogs have a 30-day survival rate of approximately 60% following intracranial surgery for brain tumor. Conversely, there is a 30-day mortality rate of approximately 40% for dogs with brain tumors. Survival for longer than six months depends on tumor type and location.

Surgical treatment of brain tumor should ideally not only cure the dog of the tumor but also improve or resolve the clinical signs associated with the tumor. Surgical resection of tumors, for example, may increase the likelihood of seizure control or help in the control of effects of increased ICP.

Syringomyelia and hydromyelia are increasingly diagnosed as a cause of spinal cord dysfunction in dogs and, rarely, in cats. Because several possible pathogenic mechanisms can result in or perpetuate syringomyelia and hydromyelia, an appropriate pathophysiologic diagnosis of the actual cause of these diseases is imperative for appropriate treatment planning. Commonly, syringomyelia and hydromyelia are associated with abnormal CSF dynamics at the level of the fourth ventricle/foramen magnum area. This may be the result of malformations of the foramen magnum or cranial cervical region, and may be associated with Chiari-like malformation of the infratentorial area.

If clinical signs are mild, non-surgical treatments with confinement, restricted activity, corticosteroids, or pain medication may provide symptomatic relief. Medical management of this condition may improve clinical signs in some dogs, and it is often attempted prior to, or concurrent with, surgical treatments. Corticosteroids (prednisone) are most often used at anti-inflammatory dosages (0.5 - 1 mg/kg every 12 to 24 hours). If medical treatment is the sole treatment to be used (i.e., because an owner does not wish to pursue surgical treatment), corticosteroid dosages are adjusted (increased) if clinical signs are not improved, and the animal is monitored closely for side effects. If improvement or resolution of clinical signs occurs within the first one to two weeks following corticosteroid therapy, this medication can be used in decreasing doses to achieve the lowest dose necessary to achieve remission of clinical signs.

If the clinical signs are the result of a Chiari-like malformation or other foramen magnum or infratentorial abnormality, surgical removal of any stenosis and decompression of the brain stem, cerebellum, and cranial spinal cord may alleviate clinical signs. Decompressive surgery of the foramen magnum usually includes a suboccipital craniectomy and durotomy [21,22]. After portions of the occipital bone and associated fibrous tissues are removed, a dura graft, such as a temporalis fascia graft, is placed over the foramen magnum area in an attempt to prevent further scaring of tissues in this area. In other situations where syringomyelia is associated with hydrocephalus, ventriculoperitoneal shunting may improve clinical signs (see further under hydrocephalus).

Primary cystic structures may also be present in the intracranial region, usually in or around the caudal and dorsal third ventricle. These cystic structures are often referred to as arachnoid or subarachnoid cysts. In some instances, surgical fenestration through a craniotomy or craniectomy can produce improvement in clinical signs. Similarly, epidermoid or dermoid cysts may be surgically resectable. Although there is no primary treatment for occipital dysplasia, the clinical consequences of Chiari-like malformation such as syringomyelia may be improved with a foramen magnum decompression via a suboccipital craniectomy with subsequent surgical reconstruction of this region.

Hydrocephalus

Several medical and surgical treatment options may be beneficial for animals with hydrocephalus. The choice of treatments is generally dictated by the degree of physical impairment, age of the animal, and cause of the hydrocephalus, if known. Medical treatment may include general supportive care and medications to limit CSF production and reduce intracranial pressure. Surgical treatment is designed to provide drainage of CSF from the brain to another site within the body for reabsorption.

Surgical treatment for hydrocephalus is generally required for those animals that do not improve within two weeks of the institution of medical therapy or for those animals whose clinical signs deteriorate during medical therapy. Surgical procedures are designed to provide controlled CSF flow from the ventricles of the brain to either the peritoneal cavity or the right atrium. Shunt systems designed for use in humans seem to be adaptable for animals. Ventriculoperitoneal (VP) shunts are technically easier to install and are most commonly used in human neurosurgery. Emergency placement of shorter-term CSF-removal devices may be used in the management of secondary hydrocephalus. Vascular access ports or similar subcutaneously implanted devices have been used for this purpose.

Ventriculoperitoneal shunt systems are used in the long-term management of hydrocephalus in humans and dogs. These shunt systems have three required components: a ventricular catheter, a siphon control mechanism, and a distal catheter. The three components can be obtained separately and fitted together at surgery, or purchased as a unit with the three parts permanently connected. The ventricular catheter is a fenestrated section of silicone tubing that is usually placed in a lateral ventricle via a small burr hole placed in the skull. This ventricular catheter is connected to a siphon control valve. The valve is designed to reduce the siphon effect of gravity-induced hydrostatic pressure in the distal catheter, thereby maintaining physiologic ventricular pressures. Hydrostatic pressure, which is greatest when the animal stands up, would cause overdrainage of CSF if not attenuated by the valve. There are low-, medium-, and high-pressure valves to maintain ventricular pressures within preset ranges. As normal ICP in dogs is thought to be between 8 and 12 mm Hg, a shunt that works at or above these pressures is usually used. The distal catheter carries CSF from the valve to the peritoneal cavity. Surgical placement of these shunts has been described elsewhere.

Regardless of the type of shunt used, strict aseptic technique and thorough hemostasis must be used to avoid shunt failure. The two most common complications of shunt placement in humans are infection and undershunting. Overshunting and seizures are less commonly encountered, but possible. Placement of VP shunts is contraindicated if systemic infections are present. All infections should be resolved prior to surgery.

Even if patients do not have a systemic infection at the time of surgery, they may develop an infection at a later time. Fever and deterioration of neurologic functions are the most common signs of shunt infection. If an infection is suspected, CSF can be collected from the subcutaneous shunt reservoir for culture and sensitivity examination. Prophylactic antibiotics are suggested for patients with VP shunts at times of potential for development of bacteremia or septicemia (e.g., dental procedures). Shunt nephritis has been reported in humans owing to chronic, low-level shunt infection causing immune complex deposition in renal glomeruli.

Undershunting (not removing enough CSF from the ventricle through the shunt) results from blockage, disconnection, or kinking of the catheter system. Possible causes of blockage include obstruction by choroid plexus and buildup of proteinaceous accretions, blood, or cellular debris (inflammatory or neoplastic). The blockage may occur in the ventricular catheter, siphon control valve, or distal catheter. Several methods, including shunt tapping and radiographic examinations, have been developed to evaluate shunt patency and locate the site of blockage.

Complications associated with overshunting have not been well described in animals, but they most likely do occur. In humans, overshunting results in formation of subdural hematomas and/or very small ventricular size (slit ventricle syndrome). Overshunting of CSF can cause collapse of the brain with tearing of vessels, which produces subdural hematoma formation. Animals with large dilated lateral ventricles and only a thin rim of cerebral cortex remaining have the highest risk for brain collapse and hemorrhage. The risk of brain collapse can be minimized by not allowing large amounts of CSF to escape during shunt placement. Additionally, a siphon control valve with the proper pressure range must be selected. Intracranial bleeding and brain collapse are most easily confirmed with advanced imaging studies such as MR or CT imaging following shunt placement.

Successful, long-term resolution of clinical signs of hydrocephalus has been achieved in dogs with these types of surgical shunting procedures. Experimental evidence suggests that reconstitution of the cerebral hemispheres after shunting occurs only in the white matter. Reconstituted white matter is characterized by myelin destruction, remyelination, and reactive astrocytosis. Because neuronal loss and cortical laminar destruction are irreversible, hydrocephalus should be treated aggressively as early as possible.

Inflammatory Diseases

Treatment of encephalitis and meningitis is ideally directed at a specific causative organism. In some instances, intracranial inflammation or infection will coalesce to form a discrete mass or abscess. As an abscess and its associated inflammation can rapidly increase intracranial pressure, surgical drainage and decompression of a focal abscess should occur as soon as possible after detection. Similarly, penetrating intracranial wounds may require debridement to avoid infectious complications.

Intracranial Hemorrhage and Vascular-Based Disease

Treatment for CNS bleeding includes treatment of any underlying bleeding disorder and stopping or resolving the local effects of any accumulated hemorrhage. Various treatments exist for individual coagulopathies and for thrombocytopenia. Current recommendations for treatment of these bleeding disorders should be reviewed.

Treatment for a localized hematoma within the CNS is focused on evacuation of the hematoma, usually via a craniotomy or craniectomy. If the underlying bleeding disorder is still active, a difficult choice exists between the need for surgical drainage of the hematoma and the risk of causing increasing amounts of hemorrhage. Short-term hemostatic support with blood products or platelets may provide a window of patient stability for a surgical procedure to be performed. Ultimately, if the accumulation of bleeding in the central nervous system is becoming life-threatening, surgical drainage is a necessity, which is undertaken in full recognition of the increased risk of hemorrhagic complications. While it is impossible to reverse all of the effects of the hemorrhage, decreasing the size of an expanding hematoma with the resultant decrease in nervous tissue pressure may improve clinical signs. Two caveats should be considered. One is that, by decreasing intraparenchymal pressure, blood flow to the area may be restored and a rebound hemorrhage may occur. Second, along the same lines, if the original cause of the bleeding was a ruptured vessel (aneurysm or arteriovenous malformation), hematoma removal may result in rebleeding. In both of these situations, however, hemostasis can be employed intraoperatively to decrease this possibility.

In chronic hematomas, especially if the initial bleeding tendency has subsided, surgical drainage is reasonably straightforward if the extent of the hematoma can be localized. Usually, a black, sometimes greenish, soft lesion consistent with an established hematoma is found. This can be removed with suction aspiration. The edges of the hematoma tend to be more organized and require blunt dissection for removal.

Intracranial Traumatic Injury

Surgical treatments for intracranial trauma are indicated primarily for decompression and debridement of abnormal tissue or blood. A major tenet of the Monro-Kellie doctrine is that intracranial contents are confined within the cranium. It would follow, therefore, that surgical removal of this structure would potentially relieve ICP elevations. Unilateral or bilateral craniectomy has been used as a treatment for ICP elevations that cannot be decreased by the more conventional methods stated earlier. Although skull removal alone may be beneficial, subsequent dural incision appears significantly more effective in lowering ICP [23]. This has been shown clinically in humans and experimentally in dogs as well as cats. Ultimate functional recovery remains dependent on the underlying primary brain damage inflicted by the disease process. Because ICP may increase during wound closure and anesthetic recovery, the magnitude of ICP decrease subsequent to surgery remains uncertain.

Craniectomy and durotomy have been shown to lower ICP by 15% and 65%, respectively, in dogs and cats, and humans. Intracranial pressures in normal dogs approached atmospheric pressure when a lateral rostrotentorial craniectomy and durotomy were performed. Adequate decompression of the brain in animals with structural disease would be suggested if intraoperative ICP approached similar levels.

Surgical Treatment for Diseases of Cranial Nerves

Cranial nerves can occasionally be affected with disease independent of the intracranial nervous structures [24]. Tumors, for example, often arise within or around the trigeminal nerve. Trigeminal nerve abnormalities can occur with infiltrating neoplasia (lymphosarcomas) that involve a branch or the entire nerve. Myelomonocytic leukemias and other hematocellular neoplasia may affect this nerve. Tumors of the coverings of the nerve (nerve sheath tumors) may arise on or around the trigeminal nerve. These tumors can infiltrate or compress the nerve, resulting in dysfunction. Unilateral muscle atrophy of the temporalis and masseter muscles is suggestive of an isolated disease of this cranial nerve. Surgical removal of these tumors has been performed, but can result in cosmetic consequences owing to complete trigeminal denervation of head muscles. Radiation therapy has been used in a limited number of cases with apparently encouraging results.

The vestibular nerve may be affected either peripherally (usually by disease of the middle or inner ear) or centrally at the brainstem level. Tumors of the ear more often occur in older animals. Squamous cell carcinoma and adenocarcinoma are most common. Treatment may include surgical resection or radiation therapy. Inflammatory polyps occur in cats. Surgical removal of these polyps is often helpful. Otitis media/interna may require systemic antibiotics (clavulanic acid), or surgical debridement (bulla osteotomy) may be necessary.

Future Directions-Surgical Treatments of Seizures

Surgical treatments for seizure control are used for selected human patients with seizures that are not controlled adequately with medical therapies. Similar surgical treatments are rare in animals, but we hope will increase with a better understanding of seizure generation and location [25,26]. This will occur through advances in intracranial imaging and electrophysiologic monitoring for animals.