Skull Base Tumors

The cranial base often is conceptually divided into three anatomic regions that are named for their intracranial relations to the overlying cranial fossae: anterior, middle and posterior. Cerebellopontine angle (CPA) lesions are the predominant skull base neoplasms that affect the posterior fossa. In published series of CPA neoplasms, acoustic neuromas are the most common tumors, accounting for more than 90%. The remaining primary tumors were meningiomas (3%), primary cholesteatomas (2.5%), and facial nerve schwannomas (1%), with less common tumors composing the remaining lesions.

Meningiomas represent up to 18% of all intracranial tumors. The CPA is the most common site for posterior fossa meningiomas. The cells lining the arachnoid villi are the cells of origin. These cells are distributed throughout the intracranial space predominantly in relation to veins and dural sinuses. Meningiomas are benign but locally aggressive tumors. Audiovestibular symptoms usually are the first indication of a posterior fossa meningioma. The signs and symptoms are similar to those of acoustic neuromas. Small tumors produce hearing loss, tinnitus and imbalance. Large tumors also produce signs and symptoms of involvement of other cranial nerves and hydrocephalus.

Glomus tumor of the temporal bone is the most frequently encountered neoplasm after the acoustic neuroma. Glomus tumors are divided into two broad categories on the basis

of their site of origin: glomus tympanicum and glomus jugulare. The glomus tympanicum most frequently manifests with pulsatile tinnitus of insidious onset and a conductive hearing loss, although it may also be an incidental finding on routine physical examination. The glomus jugulare tumor originates within the dome of the jugular bulb and commonly appears late, after considerable growth and bone destruction. It may cause dysfunction of cranial nerves passing through the jugular bulb (cranial nerves IX to XII), facial nerve paresis caused by tumor extension into the mastoid, or sensorineural hearing loss caused by bony erosion of the labyrinth.

Cranial chordomas originate from the clivus and progressively destroy the skull base. They may extend ventrally into the nasopharynx, nose or sinuses and cause obstruction. Headache, loss of vision, and cranial nerve deficits with involvement of the abducens, trigeminal, facial, and acoustic nerves are common presenting complaints. Extension of clivus chordomas to the petrous apex, sphenoid, or CPA is not unusual.

Chondrosarcomas of the skull base also may arise in the CPA. They are clinically indistinguishable from chordomas except that they are centered more laterally. Computed tomography shows characteristic bone destruction and invasiveness.

Lipomas are harmartomas that are thinly encapsulated and poorly delineated. They appear as soft, multilobular masses of typical adult adipose tissue. Lipomas within the

internal carotid artery can produce symptoms typical of an acoustic neuroma.

Primary cholesteatomas (epidermoids) are usually slow-growing lesions, which originate from epithelial rests within the temporal bone or CPA, and symptoms often do not become apparent until the second to fourth decades of life. As the lesions expand, compression and irritation of the surrounding structures produce the signs and symptoms. Although primary cholesteatomas produce pressure on cranial nerves VII and VIII with continued growth, these lesions can become quite large without producing any symptoms. Facial twitching is a distinguishing clinical manifestation of primary cholesteatomas. Progressive facial paralysis is more common with these lesions than with schwannomas.

The most common primary lesion involving the temporal bone is squamous cell carcinoma originating in the skin of the external auditory canal. The lesion may remain confined to this anatomic area; however, it may extend medially through the tympanic membrane and into the various recesses of the middle ear and temporal bone and petrous apex. The origin of this tumor remains a mystery, for cutaneous squamous cell carcinoma most frequently results from the effect of solar ultraviolet radiation. In addition to squamous cell carcinoma, there are lesions involving the auricle, such as basal cell carcinoma, which may extend to the bony portion of the external auditory canal and into the temporal bone. Rhabdomyosarcoma, aggressive middle ear adenoma, and

adenocarcinoma of the endolymphatic sac are rare and require individualized treatment.

An uncommon cause of fluctuating hearing loss is an endolymphatic sac tumor. These tumors originate from epithelium of the vestibular aqueduct and endolymphatic sac. A high index of suspicion is needed to diagnose these tumors, which manifest with symptoms similar to Meniere’s disease.

The modern era of neurotologic skull base surgery began in 1961 when William House popularized the operating microscope and multidisciplinary surgery for removal of acoustic neuromas. With its low mortality rate and enhanced facial nerve preservation rate, House established the translabyrinthine procedure and made it a practical approach, and a technique to which all other transtemporal microsurgical approaches to the CPA are compared. Subsequently, multiple approaches to the CPA have been developed, each with its own advantages an indications.

At House Clinic, cutting edge technology and skull base procedures are tailored to the individual patient’s pathology and physiologic status. Skull base surgery is a team endeavor, and the full array of experts familiar with the specific needs of these patients is necessary for a successful outcome.