This article describes various epidemiologic trends for vestibular schwannomas over the last 35 years, including a brief note on terminology. Additionally, it provides information on the natural history of tumor growth and hearing level following the diagnosis of a vestibular schwannoma. A treatment strategy based on the natural history of tumor growth and hearing also is discussed.
This article describes various epidemiologic trends for vestibular schwannomas over the last 35 years and includes a brief note on terminology.
Knowledge of the spontaneous course or natural history of a disease is imperative to evaluate the outcome of various treatments properly. Accordingly, this article provides information on the natural history of tumor growth and hearing level following the diagnosis of a vestibular schwannoma (VS).
Finally, a treatment strategy based on the natural history of tumor growth and hearing is discussed.
Terminology of acoustic neuroma and vestibular schwannoma
The term neuroma was originally applied for this tumor by Virchow, because of the macroscopic appearance and the histologic structure that showed many parallel fibers thought to be axons. Because of the hearing loss associated with these tumors, the origin of the tumor was thought to be the cochlear nerve, whereafter the term acoustic neuroma emerged and was widely used for many years. However, the name is a misnomer, as the cells of origin were identified by Murrey & Stout to be Schwann cells (termed after the German physiologist and histologist Theodor Schwann who originally described the nerve sheath cells in the 1800s). In 1975, Steward and Schuknecht found that the neurilemmal–glial junction was the site of predilection of the tumor. Moreover, the tumor is usually confined to the vestibular component of the eighth cranial nerve, although invasion of the cochlear nerve and facial nerve has been described. The term vestibular schwannoma was first recommended by Eldridge and Parry at a consensus meeting in 1992, followed by wide acceptance.
Based on the finding that the response to the caloric stimulation test was reduced in most cases of VS, an assumption was made that the tumor originated from the superior vestibular nerve. However, in a recent prospective study, the tumor originated from the inferior vestibular nerve in more than 90% of the cases.
Epidemiology of vestibular schwannomas
Incidence
Several centers have reported an increasing number of diagnosed vestibular schwannomas during the last several years. In the period from January 1957 to July 1976 the incidence of VS in Denmark was estimated to be 5.4 vestibular schwannomas per 1 million population per year. This estimate was based on publications from Danish neurosurgical departments.
Since 1976, data for all diagnosed VS in Denmark, with a population of 5.4 million people, have been entered prospectively into a national database, which by the end of 2010 includes more than 2500 patients. The data and the results from the observed patients (the wait-and-scan strategy) from this database form the basis for the present epidemiologic results, as well as the results of the natural course of the disease, including growth and hearing data.
Since 1976, the number of diagnosed vestibular schwannomas has been constantly increasing from 7.8 vestibular shwannomas per 1million population per year, until an apparent peak in 2004, with 123 diagnosed tumors (23 vestibular schwannomas per 1 million population per year). From 2004, the incidence decreased to 19 vestibular schwannomas per 1 million population per year in 2008 ( Fig. 1 ). Thus, after several decades with an increasing incidence of diagnosed tumors, a peak seems to have been reached. This leveling may reflect an approximation to the true incidence.
The increase in the number of diagnosed vestibular schwannomas has plausibly been caused by several factors, the most important being continuously improving access to continuously improving diagnostic equipment (ie, magnetic resonance imaging [MRI]). Another cause may be heightened symptom awareness among the general population and especially among the elderly, as a longer and healthier lifespan may be expected in developed countries. Better and more widespread audiological testing equipment, as well as the easier access to diagnostic imaging may in addition have heightened the awareness of a possible VS among general practitioners and otolaryngologists.
Tumor Size at Diagnosis
From a mean extrameatal size of about 30 mm in the mid-1970s, the tumor size at diagnosis has decreased continuously over the decades, to a mean size of 10 mm in the most recent period from 2003 to 2008. In the 1970s, no purely intrameatal tumors were diagnosed, whereas the large and giant tumors constituted about 40% of all the tumors. Now, the intrameatal tumors constitute 33% of tumors diagnosed, whereas the large and giant tumors constitute a mere 6% of the diagnosed tumors ( Fig. 2 ).
Age at Diagnosis
The age of the patient at the time of diagnosis of the VS has been slowly increasing from 49 years in 1976 to 58 years in 2008 ( Fig. 3 ).
Analyzing the diagnostic age distribution throughout the 33-year period, the number of patients aged 40 years or less has remained almost unchanged. Thus, the increasing number of diagnosed VS is primarily constituted by patients belonging to the age group older than 50 years.
In the beginning of the period covered, 81% of the patients were 60 years or younger, and only 4% were older than 70 years. At the end of the period, 59% of the patients were younger than 60 years, and 12% were older than 70 years.
In the age group of 40 years or younger, the mean tumor size at diagnosis was 23 mm. The size decreased with increasing age, and was 13 mm in the group of patients 70 years or older, even though the mean tumor size decreased throughout the period.
Epidemiology of vestibular schwannomas
Incidence
Several centers have reported an increasing number of diagnosed vestibular schwannomas during the last several years. In the period from January 1957 to July 1976 the incidence of VS in Denmark was estimated to be 5.4 vestibular schwannomas per 1 million population per year. This estimate was based on publications from Danish neurosurgical departments.
Since 1976, data for all diagnosed VS in Denmark, with a population of 5.4 million people, have been entered prospectively into a national database, which by the end of 2010 includes more than 2500 patients. The data and the results from the observed patients (the wait-and-scan strategy) from this database form the basis for the present epidemiologic results, as well as the results of the natural course of the disease, including growth and hearing data.
Since 1976, the number of diagnosed vestibular schwannomas has been constantly increasing from 7.8 vestibular shwannomas per 1million population per year, until an apparent peak in 2004, with 123 diagnosed tumors (23 vestibular schwannomas per 1 million population per year). From 2004, the incidence decreased to 19 vestibular schwannomas per 1 million population per year in 2008 ( Fig. 1 ). Thus, after several decades with an increasing incidence of diagnosed tumors, a peak seems to have been reached. This leveling may reflect an approximation to the true incidence.
The increase in the number of diagnosed vestibular schwannomas has plausibly been caused by several factors, the most important being continuously improving access to continuously improving diagnostic equipment (ie, magnetic resonance imaging [MRI]). Another cause may be heightened symptom awareness among the general population and especially among the elderly, as a longer and healthier lifespan may be expected in developed countries. Better and more widespread audiological testing equipment, as well as the easier access to diagnostic imaging may in addition have heightened the awareness of a possible VS among general practitioners and otolaryngologists.
Tumor Size at Diagnosis
From a mean extrameatal size of about 30 mm in the mid-1970s, the tumor size at diagnosis has decreased continuously over the decades, to a mean size of 10 mm in the most recent period from 2003 to 2008. In the 1970s, no purely intrameatal tumors were diagnosed, whereas the large and giant tumors constituted about 40% of all the tumors. Now, the intrameatal tumors constitute 33% of tumors diagnosed, whereas the large and giant tumors constitute a mere 6% of the diagnosed tumors ( Fig. 2 ).
Age at Diagnosis
The age of the patient at the time of diagnosis of the VS has been slowly increasing from 49 years in 1976 to 58 years in 2008 ( Fig. 3 ).
Analyzing the diagnostic age distribution throughout the 33-year period, the number of patients aged 40 years or less has remained almost unchanged. Thus, the increasing number of diagnosed VS is primarily constituted by patients belonging to the age group older than 50 years.
In the beginning of the period covered, 81% of the patients were 60 years or younger, and only 4% were older than 70 years. At the end of the period, 59% of the patients were younger than 60 years, and 12% were older than 70 years.
In the age group of 40 years or younger, the mean tumor size at diagnosis was 23 mm. The size decreased with increasing age, and was 13 mm in the group of patients 70 years or older, even though the mean tumor size decreased throughout the period.
Natural history of VS (growth pattern and hearing)
The natural history of VS growth is enigmatic. The tumor may grow continuously or only to a certain size, followed by stagnation or even shrinkage. Progressive growth in the cerebellopontine angle will eventually lead to compression of the brain stem and/or the cerebellum, occlusion of the fourth ventricle, and subsequently incarceration.
The percentage of growing tumors has been reported to vary from 30% to 90%, depending at least in part on the length of the observation period. Most of growth observation studies have, however, surveyed a relatively small number of patients and have further been subject to considerable referral bias and patient selection bias, by only including very old patients, patients unwilling to undergo surgery, or patients not eligible for surgery due to significant concurrent disease. The growth and hearing data presented are based on the Danish patients from 1976 to 2008, with a diagnostic tumor size less than 20 mm extrameatal and allocated primarily to observation by wait-and-scan.
Measurement of Tumor Size and Determination of Growth Rate
According to the consensus meeting in Japan in 2003, a tumor should be defined as either purely intrameatal or intra-extrameatal, in case of tumor extension into the cerebello-pontine angle. The size of an intra- and extrameatal tumor is determined by the largest extrameatal diameter, excluding the intrameatal portion.
Determined tumor growth rate may depend on the diagnostic tool (computed tomography [CT] vs MRI), the method of measurement (number or plane of dimensions assessed), and criteria for the determination for growth (number of millimeters).
The present criterium for growth of a purely intrameatal tumor was growth to extrameatal extension.
For intra- and extrameatal tumors, growth was defined as an increase of at least 3 mm in the largest extrameatal diameter, to rule out interindividual measuring variability and error due to unaligned scanning images. Largest diameter measurement is adequate when merely questioning absolute growth, which is the parameter relevant for a clinical assessment and decision making, as it is the absolute size that determines the risk of brain stem or adjacent cranial nerve compression. The adequacy of largest diameter measurement, however, has been questioned by 1 group of investigators, advocating Bayesian tissue classification and partial tumor volume segmentation on MR images for control of tumor growth. Volumetric determination of relative growth rate is definitely mandatory when addressing basic science issues, as a tumor may grow in only 1 or 2 dimensions and (eg, 2 mm growth in a 6 mm tumor is dramatically different from 2 mm growth in a 26 mm tumor, considering the rate of cellular proliferation).
Growth of Intrameatal Tumors
Of the intrameatal tumors, 83% remained purely intrameatal during the observation. In 17% of tumors, the intrameatal tumor increased in size to extrameatal extension ( Fig. 4 ). Of the tumors that grew over the years of observation the following was noted:
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64% grew during the first year
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23% grew during the second year
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5% grew during the third year
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8% grew during the fourth year
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No tumor growth occurred after the fifth year of observation (see Fig. 4 ).
