Meningiomas are extra-axial mesenchymal tumours that arise from neoplastic arachnoid cap cells, which are themselves a unique population of mesothelial-like cells. These display morphology similar to the embryologic cells found at the interface of the developing arachnoid and dura within the cranial and spinal cavities. The primitive meningeal cells originate from cells of the neural crest and from mesodermal cells that migrate into the area of the developing neural tube. In adults, the arachnoid cap cells are found at the apices of arachnoid granulations and are morphologically similar, in many respects, to the cells of the arachnoid barrier (Nafe 1979, Yasargil 1994). The majority of meningiomas develop from these cells and grow as unilateral hemispherical lesions with a flat dural base. They initially grow outward into the arachnoidal spaces and onto the brain.
Meningiomas are the most common primary intracranial tumour in cats. (Summers et al. 1995; Troxel 2003; Vite 2005). They are slowly growing tumours, and about 50% of affected cats do not manifest any clinical signs. The diagnosis is therefore often an incidental finding during pathological examination (Summers et al. 1995; Pocknell et al. 2003). Several characteristics, including a median age of 13 years and a male to female ratio of 2-3:1, are characteristic of feline meningiomas (Troxel et al. 2003). The domestic shorthair is the most reported breed, most likely because of the relative frequency of this breed in the general cat population (Nafe 1979, Gordon et al. 1994, Troxel et al. 2003). Clinical signs are dictated by the localization of the lesion. Behavioural changes (pacing, aggressivity,...) are more common, seizures less common.
In addition to single meningioma, there is also a high incidence of multiple meningiomas in cats (Summers et al. 1995, Vite 2005). The base of the skull has been described as a predilection site for meningiomas (olfactory, sphenoid, suprasellar, foramen magnum, pontocerebellar angle, tentorium cerebelli). Tentorial meningiomas are not uncommon in cats (Jaggy 2004) and arise in the transverse fissure from the inner ring of the tentorium, and less frequently, from the outer ring along the transverse sinus.
Most typical clinical signs are behavioural changes (pacing, aggressivity, dullness,...) and seizures but are at least linked with the localization of the tumour. Clinical signs are commonly responsive on steroids. The clinical effects of corticosteroids appear to be the result of directly decreasing the permeability of tumour capillaries. Steroid administration was found to decrease the blood supply to a tumour by 29% within six hours of administration and further decreases tumour blood volume by 21% within 24 hours. These changes can result in reduced intracranial pressure, decreased brain oedema and an attenuation of clinical signs (Jarden et al. 1989, Adamo et al. 2004).
Criteria for the MRI diagnosis of meningioma have been described (Pocknell et al. 2003, Troxel et al. 2003). These are extra-axial space-occupying lesions, iso- to hyperintense signal in T2, iso- to slightly hypointense signal in T1, contrast uptake and occasionally a dural tail sign. MRI is the imaging modality of choice for detection of brain tumors and provides superior resolution of intracranial lesions compared to CT. The sensitivity in detecting brain tumors in cats is approximately 98% (Troxel et al. 2004). Meningioma is the most common tumor overall (58%) and the most common extra-axial tumor (83%) in cats. Therefore, the odds are high that an extraaxial mass in a cat is a meningioma (Troxel et al. 2003).
Surgical approaches to intracranial lesions are based on the location, extent and nature of the lesion and the purpose of surgery (removal, biopsy, decompression). Good exposure of the lesion is a key aspect contributing to successful intracranial surgical outcome (Bagley 2003). Limited approaches make exposure of the lesion difficult and increase the risk of iatrogenic brain damage from excessive manipulation of the brain (Andrews and Bringas, 1993).
Cats are intravenously premedicated with methylprednisolone sodium succinate (Medrate soluble®, Pfizer, Karlsruhe, Germany, 30mg/kg) and mannitol (Braun-melsungen®, Melsungen, Germany, 0.5g/kg over 20 minutes) to minimize brain oedema and inflammation during surgery. Mild mechanical hyperventilation (end-tidal CO2 3-3.5%) is applied and anaesthesia is maintained with 1.5-2% isoflurane (Abbott®, Wiesbaden, Germany) with oxygen. Lactated Ringer's solution (3ml/kg/h IV) is administered throughout surgery. Prophylactic antimicrobial therapy with cefazolin (Kefzol®, Medica, Aesch, Switzerland, 25mg/kg IV) is administered immediately before induction and again at the end of surgery.
For surgery of convexity meningiomas, cats are positioned in sternal recumbency with the head slightly elevated, taking care to maintain airway patency. The head is shaved from the eyebrows to the level of C2, scrubbed with chlorhexidine and disinfected with alcohol and povidone-iodine. Rostrotentorial craniotomy/craniectomy exposes the cerebrum and is performed centered over the tumour. The opening should always be broader than the tumour superficial diameter.
One of two techniques can be used to remove the tumours: en-bloc resection for small superficial meningiomas, and a single-window technique for large meningiomas with a broad-based deep contact area with brain parenchyma. In the latter cases, the visible surface of the tumour is fenestrated and the centre of the tumour is enucleated using tumour forceps through this window. The first phase of the single-window technique permits reduction of the tumour mass and requires very little space. In fact, it creates the space necessary to dissect and free the capsule or pseudocapsule from the surrounding tissue and to remove the remainder of the tumour piece by piece from the wound.
For tentorial meningiomas, cats are positioned in sternal recumbency with the head flexed at 60°, slightly rotated and elevated, taking care to maintain airway patency. A horseshoe shaped incision and a unilateral temporal craniectomy, extending over the tentorium, is performed. After release of 2 to 3 ml cerebrospinal fluid (CSF) from the cisterna magna, the cerebellum becomes slack and shifts downward due to gravity. Minimal retraction of the cerebellum permits sufficient visualisation of the tentorium. Moreover, a small part of the meningioma is found protruding medially from the tentorial edge in most cases. In cases with ventricular enlargement, a puncture of the intraventricular CSF (3 to 5 ml) can also be performed. The osseous part of the tentorium can carefully be removed under magnification using a fine bone rongeur. Small bleeding vessels are cauterized with bipolar microforceps. Larger venous structures are obliterated with methylcellulose. After removal of the tentorium, the tumour can be internally decompressed through a single-window technique. The tumour margins could then be carefully dissected away from the midbrain favoured by a good cleavage plane.
For basal meningiomas, cats can be positioned in lateral recumbency with the head slightly tilted (20°) and elevated, taking care to maintain airway patency. A straight vertical skin incision is performed dorsal to the zygomatic bone. Small subcutaneous bleeding vessels are cauterized with bipolar microforceps. The platysma is incised vertically. The superficial temporal nerve and the rostral auricular nerve are retracted caudally and the zygomatic branch of the facial nerve cranially. After incision of its zygomatical attachment, the caudal margin of the temporal muscle is elevated from the temporal bone in a blunt fashion. A craniotomy/craniectomy is performed dorsally to the caudal attachment of the zygomatic arch. The medial meningeal artery is identified and ligated. Removal of the tumour occurs with a single-window technique.
In case of multiple meningiomas, a broad straight or T-shaped incision is performed, followed by multiple craniotomies, centered over the meningiomas. Another possibility would be to make tumours approaches through multiple skin incisions.
After removal of all blood clots and copious lavage of the craniotomy site with saline solution, the operation site is closed by suture of the temporal muscle to its attachment, and closure of subcutaneous tissue (Vicryl 3-0, Ethicon, Norderstedt, Germany) and the skin (Prolene 4-0, Ethicon, Norderstedt, Germany).
Cats are kept in an oxygen box for 12 hours after the operation. Postoperative analgesia is provided with fentanyl (Janssen®, Neuss, Germany, 0.02 mg/kg CRI) for the first twelve hours, followed by buprenorphine (Temgesic®, Essex, München, Germany, 0.01 mg/kg SC q8h) for the next 24-48 hours. Lactated Ringer's solution (2 ml/kg/h CRI) is administered for 24-36 hours.
Cats will receive cefalexine (Cefaseptin mite®, Chassot, Ravensburg, Germany, 25 mg/kg PO q12h) for one week following surgery and phenobarbital (Aphenylbarbit®, Streuli, Switzerland, 2 mg/kg PO q12h) for four weeks postoperatively. In addition, we recommend hydroxyurea (Litalir®, Bristol-Myers Squibb, München, Germany, 20 mg/kg PO daily) for all cats that underwent surgical treatment of meningioma as adjunctive therapy. The rationale for this was based on in vitro studies that showed that multiplication of feline meningioma cells was slowed or completely arrested by the application of hydroxyurea, and on subsequent clinical studies, corroborating this in vitro effect (Forterre et al. 2000).
We recommend a second MRI immediately after surgery; sequences include FSE T2 in transverse and FE 3D MPR T1 (plain and contrast enhanced) in dorsal orientation.
Postoperative MRI reveals if complete gross tumour removal was achieved by surgery. At the tumour site, pneumocephalus can be observed in all cases.
Histomorphology and immunohistochemistry will confirm the diagnosis of a meningioma.
Patients without complications are discharged from the clinic two to three days after surgery. Most of the preoperative presenting symptoms will resolve by the follow-up examinations after four weeks in most cases. Surgically-induced or exacerbated neurological deficits if reversible will take longer to resolve (2-3 months).
The surgical prognosis of single intracranial meningiomas in cats is good and survival is significantly prolonged with surgical intervention (Lawson et al. 1984, Gallagher et al. 1993). Median postoperative survival time is 685 days (23 months) compared to 18 days for cats that are treated medically (Gordon et al. 1994). Similar survival would be obtained with radiotherapy. Median survival time for dogs after irradiation of meningiomas ranged from 4.9 to 16 months. Axlund et al. (2002) compared the outcome in canine patients with intracranial meningiomas treated with surgical resection alone or followed by radiation therapy and found median survival times of 7 and 16 months, respectively. The same results could be expected in cats, but to our knowledge no concluding comparative studies are available at this time (Rohrer Bley et al. 2005).
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