Anterior Midline Approaches to the Skull Base

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Chapter 52 Anterior Midline Approaches to the Skull Base

Comprehensive oncologic management of neoplasms involving the cranial base is an expanding field. Surgery has emerged as the primary modality of treatment for most tumors in this region, either as a single modality (for benign tumors) or in combination with irradiation and chemotherapy (for most malignant tumors). As with most tumors, the control of the primary site is one of the most important determinants of the ultimate outcome of treated patients, and thus a three-dimensional (3D) tumor resection with histologically clear margins is the primary goal of cranial base oncologic surgery. Extensive resections must be balanced with an acceptable functional and esthetic morbidity. Gross central nervous system involvement or internal carotid encasement in a patient with poor collateral cerebral circulation is considered a relative contraindication to oncologic surgery.

Neoplastic growth affecting the anterolateral skull base often originates from central and paracentral craniofacial anatomic structures (dura, orbit, ethmoid, sphenoid and maxillary sinus, pterygoids, infratemporal fossa, clivus, etc.). Histologically this group of tumors includes a great diversity of cell origin.

The surgical approach to such tumors should accommodate the following features: (1) tumor-specific predilectional neoplastic growth; (2) when feasible, protection of key anatomic structures such as the internal carotid artery (ICA), the optic nerves, and the content of the cavernous sinus and the superior orbital fissure; (3) the best cosmetic result; and (4) the stability of the craniovertebral junction.

The principal goal of an anterolateral approach to the skull base is to achieve an unobstructed view of the midline and paramedian skull base region. Strictly midline lesions of the anterior cranial fossa are treated with craniofacial resection using a low (basal) subfrontal approach combined with a midfacial translocation approach. For small clival (central) skull base lesions, a transoral approach may be satisfactory. For larger lesions, we have combined that with midfacial translocation. Midline lesions extending laterally can be resected through various units of the facial translocation system of approaches.

Anatomic Considerations

The intimate relationship of the skull base to the cranial as well as facial structures requires tissue displacement of one or both of these compartments to reach the desired section of the skull base. It is important to consider the effects of surgery on the normal but surgically manipulated tissues so as to select the most optimal approach. Any operative tissue displacement produces alterations in the anatomy and physiology of affected structures. Such changes have variable consequences when they occur in the neurocranium or the facial viscero-cranium. For example, facial swelling is usually self-limiting, with minimal long-term consequences for the patient. Similar edema, however, may be very deleterious when it involves the neurocranium.

The anterolateral skull base constitutes the floor of the anterior and middle cranial fossa. The proximate paranasal sinuses (ethmoid, sphenoid) with the nasal cavity and the orbits are intimate components of this skull base section.

The proximity of the face to the anterior cranial base gives this region a unique significance. The craniofacial skeleton gives protection to the organs of olfaction and vision and provides support for the configuration of the soft-tissue facial anatomy. This arrangement, however, hinders a direct surgical approach to the cranial base for tumors and requires planning of incisions and osteotomies that respect not only function but esthetics as well.

Anterolateral skull base approaches permit visualization of the surgical anatomy of the skull base that may extend from the ipsilateral temporomandibular joint and geniculate ganglion through trigeminal nerve branches of V3 and V2 as well as the ICA to the cavernous sinus, inferior and superior orbital fissures, and both anterior clinoids with corresponding optic nerves.

Diagnostic Evaluation

Several essential issues guide our evaluation of cranial base neoplasms: (1) tumor biology and its extent, (2) tumor composition, and (3) relationship of the tumor to the ICA and its importance to cerebral circulation (Fig. 52-1).

Tumor biology is best determined by preoperative histologic evaluation obtained after biopsy. New endoscopic instrumentation permits access to many skull base sites for direct visualization and tissue biopsy, or an open biopsy can be performed. The tumor extent determines the potential for surgical resection of the neoplasm. This is currently best determined by multiplanar CT as well as MRI. Both tests are also very useful in assessing the character of the lesion in terms of its vascular, bony, or soft-tissue content. The location of the ICA, and its contribution to the tumor vascularity as well as the relationship of this vessel to the tumor perimeter, are assessed by MRI, MR angiography, or invasive angiography. The tolerance of the patient to temporary occlusion of the ICA can be evaluated with a series of tests known as temporary balloon occlusion test and xenon blood flow studies.1 These tests permit us to estimate the risk of neurologic deficit with a permanent occlusion in the ICA.

For orbital or periorbital tumors, a detailed neuro-ophthalmologic evaluation is valuable. Not only must the precise level of visual acuity, extent of visual fields, and ocular mobility be ascertained, but the completeness of function or the degree of dysfunction of the superior orbital fissure structures, optic nerve, and lacrimal apparatus should be known as well.

Endocrinologic evaluation is necessary preoperatively and in the follow-up period for tumors of the sellar or parasellar region.

Selected Tumors


Carcinoma that involves the anterior cranial base originates primarily in the paranasal sinuses, the nose and nasopharynx, or occasionally as a metastatic disease. Carcinoma of the nose and sinuses makes up less than 1% of all malignancies. It carries an overall 30% 5-year survival rate. In general, the prognosis of a patient with a carcinoma is very much related to the histologic type. Anaplastic carcinoma must be differentiated from lymphoma and melanoma with leukocyte common antigen and S-100 protein. Anaplastic carcinoma appears to be a separate entity from poorly differentiated squamous cell carcinoma, which still exhibits some squamous differentiation. It is found more often in women, with occurrence on the left side predominant. Among these patients, 33% develop cervical metastases, but only 70% of these have obvious evidence of bone destruction on radiographs. The survival of patients with anaplastic carcinoma varies with the site of origin. If it occurs in the nose, the 5-year survival rate is 40%. If it originates in the sinuses, the 5-year survival rate decreases to 15% (see Thorup et al.2).

The signs and symptoms common to most malignancies in the sinus–nose region include nasal obstruction, discharge, epistaxis, facial pain, as well as swelling, proptosis, or cervical node metastases.

The nasal passages and the sinuses are intimately related, permitting tumor to spread easily from one cavity to the other. Therefore, ethmoid sinuses are often involved secondarily by tumor spread from the nasal cavity or the maxillary sinus. This is reflected in the fact that isolated ethmoid carcinomas compose no more than 5% to 20% of all carcinomas involving the ethmoid sinus. The initial symptoms are usually insidious and trivial, accounting for a significant delay of diagnosis from the onset of symptoms. Sixty to 75% of patients with malignant tumors of the ethmoid sinuses do not survive for 5 years. The ethmoid sinus is closely related to the orbit. Both the orbit and the ethmoid sinus are simultaneously involved in 60% of malignant sinus neoplasms, and 45% of the patients are likely to require orbital exenteration. Most sinus tumors arise from the mucous membrane lining that is in continuity with the mucosa of the remaining sinuses, nasopharynx, and lacrimal draining system. The respiratory mucosa of the ethmoid sinus gives rise to two types of neoplasm. The first is squamous cell carcinoma, arising from the metaplastic epithelium. Of all malignant neoplasms of the sinuses, 75% to 95% will be squamous cell carcinomas, and the ethmoid sinus is the second most common site for this neoplasm. The second is a glandular tumor, arising from mucous glands. The submucosal glands give rise to adenocarcinomas or adenoid cystic carcinomas. Adenocarcinoma occurs most frequently in the ethmoid sinus, and its behavior is similar to that of squamous cell carcinoma. There is some suggestion that this tumor is found more frequently among workers in woodworking industries than in the population in general.3 The lymphatic drainage from the ethmoid sinus is into the superior cervical chain and the retropharyngeal nodes. The incidence of metastases at the time of diagnosis is low, but 25% to 35% of patients will eventually develop metastatic disease. Distant metastases may occur in up to 18% of the cases.4


This is a rare tumor originating from the olfactory epithelium and represents 3% of all intranasal neoplasms. It was originally described by Berger and Luc.5 This tumor has been identified under different terms, including olfactory neuroblastoma, esthesioneurocytoma, and olfactory esthesioneuroblastoma. It arises from cells of neural crest origin and resembles childhood neuroblastoma. The tumor does contain neurosecretory granules and is linked to other neural crest tumors, such as carcinoid, chemodectoma, and pheochromocytoma. It occurs most frequently in the third decade of life and is more common in males. Unilateral nasal obstruction and epistaxis are the most common symptoms. The tumor may fill the nose and paranasal sinuses and involve the cribriform plate.

A staging system has been proposed by Kadish and colleagues that recognizes three stages6:

However, correlation of tumor extent with prognosis has not been as accurate as the relationship of clear surgical margins.

Esthesioneuroblastoma is known to have a slow but insidious malignant course, and death comes from local recurrence, intracranial invasion, and/or metastatic disease. Differential diagnosis must exclude lymphoma, melanoma, and metastatic neuroblastoma. The characteristic histologic picture includes a fibrillary intercytoplasmic background that on electron microscopy is identified as representing neuronal cell processes. The 5-year survival rate is approximately 50%, with a median survival of 58 months. When the cranial base is invaded, the survival rate drops to about 40%. Long-term recurrence has also been observed 10 to 20 years after the original diagnosis. This tumor is characterized by local persistence and recurrence. There is a 20% to 40% potential that this tumor will metastasize into cervical lymph nodes, lungs, and bones. The current modality of treatment includes a radical resection of the area involved that includes the cribriform plate with or without the attached dura followed by a full course of irradiation and possibly chemotherapy as well.

Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma is a rare tumor among non-Chinese patients, with an incidence of 1 in 100,000 among the North American population as compared with 2 in 100,000 among Chinese, especially those living in the Canton province of the People’s Republic of China. Several etiologic factors have been implicated in the development of nasopharyngeal carcinoma, for example, the Epstein-Barr virus and numerous external inhalation as well as dietary carcinogens. The male-to-female ratio heavily favors male patients (3:1), with an average age of onset of 45 years.

Clinically, the tumor appears to arise primarily at the superior or lateral aspect of the nasopharynx. The symptomatology often includes epistaxis, nasal and Eustachian tube obstruction, and eventual cranial nerve neuropathies (the fifth cranial nerve is most commonly involved). Histologically, these tumors are predominantly poorly differentiated carcinomas with a high propensity for metastatic regional spread, so that at the time of diagnosis, 50% of patients are expected to have regional disease. In the diagnostic evaluation, direct nasopharyngoscopy and biopsy, as well as a CT scan and MRI, provide for full assessment of the primary site. Irradiation is still considered a primary therapeutic modality for the nonkeratinizing squamous cell carcinoma of the primary site and the regional lymph node draining area.7 The cure rate, however, varies tremendously depending on the histologic type of the tumor, stage of the disease, and subsequent therapy. The most frequent recurrence of nasopharyngeal carcinoma is in the neck.8 Reirradiation of recurrent nasopharyngeal carcinoma gives a 5-year cure rate of only 14%, with a high chance of radiation-induced complications. It is important prognostically to separate patients with metastatic nasopharyngeal carcinoma in the neck on the basis of their response to the primary irradiation. If metastatic neck nodes disappeared completely following irradiation, the recurrence rate was only 13%. If nodes persisted throughout the course of irradiation, the recurrence rate was 91%.

With the advent of new approaches to the nasopharynx, surgery is becoming a therapeutic option for the treatment of resectable recurrent nasopharyngeal cancer with expected survival of over 50% (5 years). For tumors with very poor response to the primary radiotherapy (e.g., keratinizing squamous cell carcinoma, adenoid cystic carcinoma), surgery should be considered as the initial treatment.

Fibrous Dysplasia

Fibrous dysplasia is a progressive benign fibro-osseous lesion. Its natural growth is one of gradual expansion beyond its bony margins with concomitant displacement of surrounding soft tissue. It was first described by Lichtenstein in 1938.9 It may be placed into three categories on the basis of its clinical presentation. The monostotic form represents a localized disease to one osseous structure and is the most frequent form (up to 70%). A polyostotic form involves several bones but usually on the same side of the body. Here the frequency ranges from 30% to 50%. The third form is disseminated, in which numerous bones are involved, along with the possibility of extraskeletal developments such as skin pigmentation and precocious puberty. The incidence ranges from 3% to 30%. These individual clinical forms retain their categorization during the course of the disease and do not seem to change from, for example, the monostotic to the polyostotic form. Fibrous dysplasia is more common in females. In the head and neck region (0.5% of all head and neck tumors), it is the maxilla, frontal bone, mandible, and parietal and temporal bones that are most frequently involved. It is of interest that the progression of the disease is often limited after completion of skeletal maturation.10

The clinical symptomatology usually includes swelling at the tumor site with displacement of surrounding soft tissues. For example, diplopia, when present, is usually caused by mechanical displacement of the globe. If the cribriform plate is directly involved, alteration in olfaction can be perceived. Histologic verification can be considered in addition to the clinical and radiographic examination. In the differential histologic diagnosis, fibrous dysplasia may mimic meningioma, and sarcoma is also a possibility. Radiographically, a sclerotic form manifests itself with dense bone. The cystic and pagetoid forms are distinguished radiographically from each other by the greater amount of fibrous component in the former.

Fibrous dysplasia can be treated by surgical resection when functional or esthetic deformity warrants it.11 Full preoperative evaluation should include CT scan, with and without contrast, in the axial and coronal planes with bone algorithms.

Osseous reconstruction of the surgical defect is necessary only when the tumor involves key aspects of the craniofacial skeleton. Autogenous bone graft or alloplastic materials can be used. In the orbital region, most of the fibro-osseous lesions involve the orbital roof. Prolonged ocular displacement by the tumor often produces a secondary concavity in the orbital floor. This must be taken into account, since after orbital tumor removal superiorly, the globe may not return to the expected normal level. Secondary bone grafting of the deformed orbital floor may have to be considered.