24.2.1 Regional Pathology and Differential Diagnosis

Various pathologies may arise within the PA. The differential diagnosis includes infection, benign tumors, malignant tumors, congenital entities (cholesteatomas, asymmetric fatty marrow), obstructive processes (cholesterol granulomas), vascular (ICA aneurysms), and miscellaneous lesions (Paget’s disease, fibrous dysplasia).2 ^,​ 13 ^,​ 14 ^,​ 15 ^,​ 16 ^,​ 17 Embryologically the PA has a mesodermic origin and is formed by osseous and fat tissue. It is closely related to the clivus and the spheno-occipital and sphenopetrous synchondroses, which have rests of notochordal and cartilaginous matrix. The temporal bone has pneumatized cells filled with mucosa of ectodermic origin. The cranial nerves related to this region also have an ectodermic origin. Lesions of the PA may be classified on the basis of their etiology as inflammatory lesions, developmental lesions, benign and malignant tumors, vascular lesions, and osseous dysplasias. PA tumors are divided into two groups by site of origin: primary, originating within the PA, and secondary, originating from neighboring structures with secondary involvement of the PA. PA destruction is most frequently caused by a secondary process from either contiguous lesions or metastasis. The most frequent primary PA lesions are as follows:

• 
  

  Mesenchymal origin:

  
  
  
  
  • 
    

    Squamous cell carcinoma

    
    
  • 
    

    Rhabdomyosarcoma

    
    
  • 
    

    Chondrosarcoma

    
    
  • 
    

    Aneurysmal bone cyst

    
    
  • 
    

    Cholesteatoma

    
    
  • 
    

    Cholesterol granuloma

    
    
  • 
    

    Meningioma

    
    
  • 
    

    Facial nerve schwannoma

    
    
  • 
    

    Trigeminal nerve schwannoma

  
  
• 
  

  Ectodermic origin:

  
  
  
  
  • 
    

    Epidermoid cyst

    
    
  • 
    

    Chordoma

    
    
  • 
    

    Neurofibroma

    
    
  • 
    

    Mucocele

  
  
• 
  

  Mesenchymal/ectodermic origin:

  
  
  
  
  • 
    

    Multiple myeloma

    
    
  • 
    

    Lymphoma

    
    
  • 
    

    Metastasis

The most frequent secondary PA lesions are as follows:

• 
  

  Neoplastic:

  
  
  
  
  • 
    

    Adenoid cystic carcinomas

    
    
  • 
    

    Juvenile angiofibromas

    
    
  • 
    

    Vestibular nerve schwannomas

    
    
  • 
    

    Nasopharyngeal carcinoma

    
    
  • 
    

    Chondrosarcoma

    
    
  • 
    

    Meningiomas

    
    
  • 
    

    Jugular foramen paragangliomas

    
    
  • 
    

    Chordoma

    
    
  • 
    

    Metastasis

  
  
• 
  

  Nonneoplastic:

  
  
  
  
  • 
    

    Epidermoid cyst

    
    
  • 
    

    Arachnoid cyst

    
    
  • 
    

    Fibrous dysplasia

    
    
  • 
    

    Intrapetrous carotid artery aneurysm

    
    
  • 
    

    Petrous apicitis

    
    
  • 
    

    Mucocele

24.2.2 The Most Frequent Petrous Apex Lesions

Chondrosarcomas

Chondrosarcomas arise from mesenchymal cells in the embryonic cartilaginous matrix of the cranium, generally with an epicenter in the petroclival synchondrosis. They typically grow by infiltrating the bone, replacing the normal bone marrow with chondroid tissue, and spreading through Harver’s canals.18 Although similar in clinical and radiological presentation, chordomas and chondrosarcomas have very different patterns on immunohistology. Unlike chordoma (even chondroid), chondrosarcomas are nonreactive for cytokeratin and brachyury. They share reactivity to vimentin and S-100. Most chordomas are positive for epithelial membrane antigen, but a few chondrosarcomas are positive for it.19 ^,​ 20 ^,​ 21

Another marker in the last 10 years identified as an important tool in this differential is the presence of IDH1 mutation, which is associated with chondrosarcomas rather than chordoma and had a sensitivity of 71.4% and specificity of 100% in one series, although this association had no impact on prognosis.20 Prognosis of PA chondrosarcomas is related to histological grade. Grade I tumors present a 5-year survival rate of 90%, grade II 81%, and grade III 43%. Facial nerve palsy, vertigo, diplopia (cranial nerve VI dysfunction), and pulsatile tinnitus are the most frequent complaints.22 Surgical approach depends on extension of the tumor. The middle fossa approach has been used in our clinic to remove PA chondrosarcomas that extended lateral to the ICA (Fig. 24.4). Tumors with extensions medial to the ICA are approached thorough a transnasal endoscopic approach (Fig. 24.5). Radical resection is difficult in large and infiltrative tumors, and postoperative radiosurgery is performed in high-grade tumors.

Chordomas

Chordomas correspond to 1 to 4% of all bone malignancies and 0.2 to 0.4% of all intracranial tumors, with higher incidence in men, mainly between 20 and 40 years of age. Fewer than 5% of these tumors occur in children and adolescents. The annual incidence is 0.089/100.000 in the United States.13 ^,​ 19 ^,​ 23 These slow-growing tumors arise from notochordal remnants and originate from the clivus and PA (Fig. 24.6).6 Chordomas are usually not related to any syndrome, but some reports of familial cases do exist. Duplication of a region in 6q27 plays a role in susceptibility to hereditary chordomas. This genetic abnormality is related to the brachyury gene.21 ^,​ 24 Recurrence after surgical removal is frequent, and radiotherapy/radiosurgery (proton beam) is indicated as adjuvant therapy.25 Proton therapy presents a 5-year estimate for local control of 69.6% and overall survival of 81.4%.26

Carbon ion particle therapy is increasingly being used to treat chordomas and chondrosarcomas. The Heidelberg Group has published a series of 54 cases of chondrosarcomas treated using carbon ions, for a tumor control rate of 89% and overall survival of 98% in 5 years.27 Uhl et al reported a series of chondrosarcomas treated using carbon ion therapy, for 90% local control rates in 5 years and 88% in 10 years.28 Results of carbon ion therapy for chordomas present 72 to 85% of local control and 85 to 88% of overall survival in 5 years.29

Chondromas

Chondromas arise at the base of the skull from residual rests of primordial cartilage in basilar synchondrosis entrapped during endochondral ossification.30 ^,​ 31 Usually they are located at the sphenoethmoidal, sphenopetrosal, petro-occipital, and spheno-occipital synchondroses.31 ^,​ 32 Another possible origin of chondromas is from metaplasia of meningeal fibroblasts or perivascular mesenchymal tissue, which would explain their presence in other locations, such as cerebral parenchyma, dura, and different bone sites.33 They may occur alone or as part of Ollier’s disease or Mafucci’s syndrome.34 ^,​ 35 These tumors are rare and benign and may be cured by radical resection (Fig. 24.7).

Meningiomas

Most meningiomas involving the PA originate from the petroclival region, the cerebellopontine angle, or the sphenoid ridge.36 True PA meningiomas are rare (Fig. 24.8). Involvement of the trigeminal nerve and Meckel’s cave causes often intractable trigeminal neuralgia. Meningiomas may cause hyperostosis of the PA, and in MRI studies they are hypointense on T1 and iso- to hyperintense on T2. They enhance with gadolinium, and a “dural tail” may be observed.

Schwannomas

Schwannomas affecting the PA most frequently originate from the trigeminal and facial nerves. Trigeminal nerve schwannomas (TSs) are the second most common type of intracranial schwannoma (Fig. 24.9).37 These tumors are benign in the majority of the cases and have their highest incidence between 38 and 40 years of age. They are more common in middle-aged women.37 ^,​ 38 Facial pain, hypesthesia, headaches, and diplopia are the most common symptoms. Usually trigeminal schwannomas are slow-growing tumors and when the patient presents with clinical symptoms the lesion has already reached a large size. Radical resection is curative in the majority of cases. The main challenge of surgical removal is preservation of nonaffected fibers of the trigeminal nerve. Facial nerve schwannomas are uncommon tumors that are extremely slow-growing, that are benign in the majority of cases, and that frequently present without facial dysfunction. They can be mistaken for vestibular schwannomas. The most common origin sites of these tumors are the geniculate ganglion and internal auditory canal (Fig. 24.10).

Indication for surgical removal and choice of surgical approach of facial nerve schwannomas depends on tumor extension, grade of facial nerve palsy, hearing function, and surgical experience. The best postoperative function of facial nerve after tumor resection and nerve reconstruction is a House-Brackmann grade III palsy. Patients who have preoperative facial palsy grade III or higher or those who have large tumors compressing the brainstem are the best candidates for surgery.

Cholesterol Granulomas

Cholesterol granuloma is the most common PA lesion. The granuloma contains cholesterol crystals, granulation tissue, and blood breakdown products. It may be a sequela of chronic otitis media. The cyst has a fibrous capsule without a true epithelial lining. The most common symptoms are hearing loss, tinnitus, and headache. MRI shows a hyperintense, non-enhancing lesion on T1- and T2-weighted sequences. CT scans demonstrate a well-defined PA lesion with no cortical destruction (Fig. 24.11).

Cholesteatomas

PA cholesteatomas are uncommon and may be congenital or acquired (Fig. 24.12). They constitute 4 to 9% of all PA lesions.39 Congenital lesions are rare, arise from aberrant ectoderm that is trapped during embryogenesis, and develop behind the tympanic membrane.22 Headaches, hearing loss, and facial nerve palsy are the most frequent symptoms. Large lesions may produce symptoms of other cranial nerves. Persistent otorrhea after previous mastoid surgery is an indication of an acquired cholesteatoma. A high recurrence rate is associated with difficulty clearing all the disease at primary surgery, and long-term follow-up is required.

24.3.1 Diagnostic Imaging

The differential diagnosis for PA lesions is extensive. Evaluation of the temporal bone with standard radiography includes Towne’s and Stenver’s views and polytomography. However, accurate diagnosis is possible only using imaging studies, such as CT scanning and MRI; usually both studies are required for elucidation of a more definitive diagnosis.41 CT scanning with contrast enhancement, thin slices, and 3D reconstruction is useful to evaluate the extension of the lesion and erosion of the cranial base bone and in planning the surgical approach. Bone erosion with smooth or scalloped margins suggests a slow-growing benign lesion (cholesterol granuloma, meningocele, mucocele, schwannoma). Infections and aggressive tumors may present on CT as osteolytic lesions with ill-defined margins and a moth-eaten pattern. CT scanning is helpful in the differential diagnosis of lesions in this region (Table 24.1).42

Tumors such as fibrous dysplasia, multiple myeloma, and calcified chondrosarcomas can be well demonstrated using CT examination.43 ^,​ 44 about the nature and extension of the lesion and involvement of other structures, such as vessels, nerves, and the brainstem (Table 24.2).10 ^,​ 42

Inflammatory diseases (e.g., petrositis, osteomyelitis) and epidermoid cyst have characteristic findings on MRI. Petrous carotid and cavernous carotid aneurysms as well as venous sinus variations are well demonstrated using MR angiography and venography. Special sequences are helpful to visualize cranial nerves and relations of the tumor with the brainstem. Digital angiography is performed when an aneurysm is suspected, when a balloon occlusion test is needed, and for preoperative embolization (e.g., paragangliomas and other highly vascularized tumors).

24.3.2 Preoperative Preparation

Careful preoperative evaluation is critical in minimizing intraoperative and postoperative complications. Preoperative preparation involves evaluation of the patient’s clinical condition. Because PA tumors occur commonly in elderly patients, adequate evaluation of clinical condition and associated comorbidities is mandatory (Table 24.3).

Management of PA pathologies poses a challenge. A correct preoperative diagnosis is needed to define treatment modality and surgical approach. The central location in the skull base, with adjacent critical neurovascular structures, makes diagnostic biopsy difficult and hazardous. Preoperative imaging studies may define the exact size and location of the lesion. If the tumor has extension into the sphenoid sinus, then an endoscopic biopsy may be performed. Careful evaluation of the involved cranial nerves is needed to define the best form of treatment. Facial nerve function and the quality of preoperative hearing are important factors in choosing the surgical approach. Removal of the labyrinth should be avoided in patients who have good hearing. Patients who have associated infection, such as otitis, require antibiotic treatment. Radiosurgery may be an option in some cases.

24.3.3 Surgical Approaches

Different surgical approaches to this region have been described, each of which has its advantages and disadvantages. Choice of surgical approach depends on the clinical status of the patient, etiology of the lesion, extent of the tumor, involvement of ICA and cavernous sinus, presence of facial nerve paralysis, quality of preoperative hearing, vestibular function, presence of infection, and experience level of the surgeon.5 ^,​ 45 Skull morphology may also be a factor, for the distance between the external cortical table of the skull and the PA varies with skull types.5 In selected cases the endoscopic endonasal approach offers advantages over transcranial approaches by avoiding risks to the facial nerve and hearing.

24.4.1 Indications

The main indications for this approach are PA cholesteatomas, facial nerve schwannomas, and malignant lesions involving the temporal bone and the PA.46 This approach should be avoided in patients who have preserved preoperative hearing. Many authors use the translabyrinthine approach to resect vestibular schwannomas and labyrinthectomy in cases of intractable vertigo. We prefer the retrosigmoid approach for treatment of vestibular schwannomas.

24.4.2 Surgical Technique

In supine position with head turned toward the opposite side, a retroauricular skin incision is made 2 cm posterior to the postauricular sulcus. The mastoid cortex is exposed after periosteal incisions and an extended mastoidectomy with removal of bone over the sigmoid sinus and the middle cranial fossa is performed (Fig. 24.13).

Removal of bone over the sigmoid sinus must be done carefully. The middle fossa dura is dissected, the antrum is opened, and the lateral semicircular canal is identified. The short process of the incus and the semicircular canal are landmarks for the horizontal segment of the facial nerve. After identification of facial nerve a complete labyrinthectomy is performed. If infection is present (PA cholesteatoma with secondary infection), marsupialization of the cavity and a wide meatoplasty using a skin flap are carried out to allow postoperative care of the cavity. Management of the facial nerve depends on the preoperative facial nerve function, presence of infection, and whether the proximal and distal stumps of the nerve can be identified. If the facial nerve is involved by the tumor but not infiltrated and its function is normal, the nerve is dissected from the lesion, preserving the perineural tissues (neurolysis). If the nerve is infiltrated by the tumor and there is infection, reconstruction of cranial nerve VII should be postponed. If there is no infection, the affected portion of the nerve is resected and an end-to-end reconstruction or interposition nerve graft (sural nerve or great auricular nerve) is performed. When identification of the proximal stump of the facial nerve is possible only at the brainstem, a sural graft is sutured with 10–0 nylon or glued with fibrin glue at the brainstem and at the stylomastoid foramen (Fig. 24.14).

When direct reconstruction of the facial nerve by direct suture or grafting is not possible, a facial–hypoglossal anastomosis is carried out. In such cases we prefer to perform an end-to-side anastomosis to avoid atrophy of the tongue. In our experience the postoperative results with this technique are similar to an end-to-end VII–XII anastomosis. After tumor removal and reconstruction of the facial nerve, the dural opening and the mastoid cavity are obliterated with fat tissue and fibrin glue to avoid CSF leak. The wound is closed using vascularized muscle–periosteal flaps.

24.5.1 Indications

The most frequent pathologies treated by this approach are petroclival meningioma with its main portion in the middle fossa, facial and trigeminal schwannomas, small vestibular schwannomas, cholesteatomas, cholesterol granulomas, chondrosarcomas, teratomas, CSF fistulas, and facial nerve lesions at or medial to the geniculate ganglion. This approach exposes the PA intradurally and extradurally and allows dissection of the second and third divisions of the trigeminal nerve, the gasserian ganglion, the petrosal portion of the ICA, and the meatal and petrosal portions of the facial nerve.47 Both damage of the temporal lobe with excessive retraction and injury to the temporal lobe draining veins can be avoided by using adequate anesthetic and microsurgical techniques.

24.5.2 Surgical Technique

Under general anesthesia, and with head rotated to the opposite side, a semicircular or straight skin incision is cut, beginning at the tragus and extending anteriorly to the frontal region. A temporalis muscle fascia flap is dissected, exposing the temporalis muscle, the zygomatic arch, and the upper portion of the external auditory canal. The temporal muscle is cut and the bone of the temporal bone exposed. If exposure of the infratemporal fossa is needed, the temporal muscle is rotated down, the zygomatic arch is temporarily removed, and in some selected cases opening of the glenoid fossa or resection of the mandibular condyle is performed. A craniotomy flap is cut, exposing the basal portion of the temporal fossa from the middle portion of the zygomatic arch to the transverse sinus (Fig. 24.15).

Temporal lobe retraction is reduced by this basal approach, assisted by the infusion of mannitol and lumbar CSF drainage. The extradural approach to the PA is carried out by elevating the dura mater of the middle fossa so as to expose the middle meningeal artery, the greater superficial petrosal nerve, and the second and third divisions of the trigeminal nerve. The middle meningeal artery is coagulated and cut. The greater superficial petrosal nerve is carefully dissected from the dura up to its exit at the facial nerve hiatus. The arcuate eminence (superior semicircular canal) is identified. This extradural approach allows exposure of the PA, internal auditory canal, labyrinthine and tympanic portions of the facial nerve, and petrosal and horizontal portions of the ICA by drilling the bone medial to the Eustachian tube. To intradurally expose the PA region, the dura is incised parallel to the sylvian fissure. The temporal lobe is carefully retracted, avoiding damage to the draining veins—especially the vein of Labbé. The free border of the tentorium is dissected; the fourth cranial nerve is identified at the margin of the tentorium and the oculomotor nerve, medial to the troclear nerve. Splitting the tentorial border allows exposure of the posterior fossa and cranial nerves V, VI, VII, and VIII (Fig. 24.16).

The posterior communicating artery and its branches, as well as the posterior cerebral and superior cerebellar arteries, are identified. All these anatomical structures may be displaced or embedded in the lesion and must be preserved. The posterior clinoid process and bone between the trigeminal nerve and the internal auditory meatus may be removed using a high-speed drill to enlarge the exposure of the middle line. In cases of petroclival meningiomas, perforating branches from the basilar artery may be embedded in the tumor so that complete removal may be impossible. After tumor removal, watertight dura closure is performed. All opened mastoid cells are closed using temporal muscle graft or wax. The temporalis fascial flap is rotated to cover the dural opening and the craniotomy flap is replaced.