Acoustic Neuroma Size and Growth
Facts about AN Growth
Great pictures of AN growth
Here are great pictures of the AN as it's growing:
Note: The sizes of the ANs in that picture are mislabeled; the AN labeled 1cm is actually 2cm, etc. To figure out the correct size (in cm), measure both the AN (out from inside the ear) and the head (going across horizontally) right on the picture, in any units. Then divide the first by the second, and multiply by 20 (the actual size of an average head, in cm).
AN growth and the facial nerve
A good illustration of the interaction between the AN as it grows and the facial nerve is at:
How are AN's measured?
The classical AN shape is a pear, with a tail in the ear canal and a roughly spherical part extending into the brain cavity. One dimension is read along the pear's axis, parallel to the canal. The second dimension is usually the width of the horizontal slice through the spherical part. When the third dimension is present, it is also for the horizontal slice through the spherical part, but perpendicular to the previous one. The second and third dimension are often close, but will not be the same unless the slice is completely spherical.
How often do AN's occur?
John Hopkins University reports the incidence rate of unilateral ANs to be 1 in 100,000. 5% of these patients turn out to have NF2.
Some technical stuff on AN growth
by Bederson JB, von Ammon K, Wichmann WW, Yasargil MG,
Department of Neurosurgery, University Hospital of Zurich, Switzerland.
Seventy of 178 patients with acoustic tumors initially were treated conservatively and have been followed up for an average of 26 +/- 2 months. The tumor size was determined by the mean maximum anteroposterior and mediolateral diameters, using computed tomographic or magnetic resonance imaging scans obtained sequentially throughout the follow-up period. The average tumor growth was 1.6 +/- 0.4 mm the 1st year, and 1.9 +/- 1.0 mm the 2nd year (range, -2 to 17 mm/y): 4 tumors showed apparent regression, 28 (40%) had no detectable growth, and 37 (53%) exhibited growth (average, 3.8 +/- 1.2 mm/y). Within individual patients, the tumor growth rate determined during the 1st year of follow-up was predictive of tumor growth rate determined during the following year. Rapid tumor growth or clinical deterioration in 9 of the 70 patients (13%) who initially were treated conservatively necessitated subsequent surgery an average of 14 +/- 5 months after the patient was initially seen. This group had a larger initial tumor size (27.0 +/- 3.4 mm vs. 21.3 +/- 0.9 mm, P less than 0.05), and a faster 1-year growth rate (7.9 +/- 2.3 mm/y vs. 1.3 +/- 0.3 mm/y, P less than 0.05) than the 61 patients who did not require surgery. Two patients, however, experienced neurological deterioration that required surgery, even though there was no tumor growth. The high incidence of acoustic tumors with no detectable growth or apparent spontaneous regression must be taken into account when evaluating the indications for surgery and the efficacy of radiotherapy.
Surprizing AN facts
"Silent ANs" are all around us
There are many more ANs around than we know about, since the ones that do not cause any symptoms are almost never diagnosed. In the Scandinavian countries, every death results in an autopsy, and these "silent ANs" are found and recorded. Their statistics indicate that about 1% of all people have ANs, at least at the time of their death.
Your tumor may not be as big as they say
I had three neurosurgeons tell me mine was a 2 cm because they tend to ballpark the dimensions. A much more experienced neurotologist told me mine was "what the radiologist had written on the report - a 1.4 cm." Doctors may round it up to make numbers simpler for the patient, and we need to watch out... Did you radiology report say 2 cm (or 3 or 4) or did your surgeon call it that size?
Make sure your Drs. look carefully at your expensive MRI's. I was told that I had significant growth because my tumor had developed two parts from Dec '99 to April '00. Last week, while sorting my MRI films, my girlfriend noticed that the exact same two parts were indeed on one of the Dec. '99 films and are still the same size.
Two types of ANs
ANs never "engulf" nearby tissues, and do not send tentacles into them. They just push against them. While some ANs easily peel away from neighboring tissues during surgery, others are "sticky", adhering to nearby tissues like a band-aid. This is what makes total surgical removal so risky -- try to get the band-aid off without pulling at a single hair!
Some ANs shrink spontaneously
There are a few people whose ANs have actually declined over time with no treatment. They are rare, but you may be only of the lucky ones.
(by a patient) "I know one of these individuals and over a total period of 10 years, that person's AN has gone down from around 1.4cm to 8 mm and then back up to 1.2 and is still 1.2 (for the past 3 years)."
Small ANs cannot push on the brainstem
There seems to be a very common minsunderstanding among AN patients when told that their tumor is "impacting the brainstem". They think that the AN is physically "pushing on the brainstem" and endangering their life, and they panic unnecessarily.
The difference between "impacting" and "pushing" is like the difference between seeing someone and touching them. "Impacting" means that the brainstem is feeling the increased pressure resulting from AN's presence. Basically, this happens as soon as the AN leaves the ear canal and starts pushing against the brain, which lies between the AN and the brainstem. But this is not dangerous, in and of itself! As all live tissues, the brainstem has the capacity to do a lot of bending.
The bending of the brainstem is dangerous only when the blood flow between the brain and the body through the brainstem is interrupted. This happens when the brainstem has been squashed, rather than just bent. For a good illustration, you can see http://itsa.ucsf.edu/~rkj/Illustrations/ANillus.html Only in the last of the pictures is the situation dangerous to the brainstem.
Another cause of danger to the brainstem is actually from the surgery rather than the AN itself. When the tumor touches the brainstem, it is more likely that a surgeon will accidentally damage the brainstem when removing the tumor, a big no-no.
In either case, the tumor typically has to be very large before there is danger to the brainstem. So the next time your doctor talks about the brainstem, make sure there is really cause before you panic!
A great web site for cranial nerves
Did you know that all cranial nerves have both a number and a name. The best place we could find for learning about the numbers, names and location of various cranial nerves is here. It's interactive, so you can see just where the nerve is when you select its name.
Anatomy lesson for dummies
(by a patient)
Try to imagine the nerves from the inner ear and the face as state highways running east-west (horizontal) to the big north-south (vertical) interstate highway (brain stem). These state highways (nerves) have to get through the mountain (skull) to get to the interstate on the other side. The highways go through tunnels to get to the interstate. The facial nerve (highway #7) goes through the inner auditory canal (IAC) tunnel. Going through this same tunnel is highway #8, it is one of those combination highways that is really three merged highways called the superior vestibular nerve highway, inferior vestibular nerve highway, and cochlear (hearing) nerve highway. Also going through this same tunnel is a railline which we will call the internal auditory artery. It seems to me that is all the Great Engineer could fit into this tunnel, and it is crowded. Try to imagine highway #7 as an elevated highway running through the top of the tunnel right under the ceiling of the tunnel, with highway #8 immediately below it, with the superior vestibular lanes on top, and the inferior vestibular lanes immediately below it, and the cochlear lanes right under them. The internal auditory artery railline is separate from highway #8, but runs right between the lanes. State highway #5 (trigeminal nerve) runs through the mountain in a separate tunnel north (above) the IAC tunnel. As all these highways and railines emerge from their tunnels, they come out into this great open valley (cerebellopontine angle) which they must cross to reach the interstate (brain stem).
In this fairytale land of "nervous highways" occasionally an unexplainable growth (AN) will occur in the asphalt, usually on the superior vestibular branch of highway #8, and usually it starts just inside the tunnel from the valley (cerebellopontine angle). If the growth stays in the tunnel, we call it an intercanalicular AN. Some grow out into the valley and can get so large as to fill the valley and even attach to the interstate (brain stem). These ANs can really mess up traffic (the messages going to the brain) on any highway they come in contact with or cause pressure on.
To remove an AN, usually the entire stretch of highway around where it originates from, must come out, generally that is the superior and inferior branches of highway #8. It is always a challenge to get the entire AN removed without damaging highway #7 or the cochlear branch of highway #8. It is also important not to damage the internal auditory artery railroad, because if it doesn't supply its blood to the cochlea for any length of time, there will no longer be any worthwhile traffic coming down the cochlear branch of highway #8. Another way to stop the AN is to carefully zap it with some radiation to stop its growth, hopefully shrink it, and not hurt any of the neighboring highways, railroads, or other infrastructure in the process.
(by a patient) To really understand the physical relationship between the AN, the nerves and the arteries in and around the inner auditory canal, I strongly recommend studying Figure 1 in the article Microsurgical Anatomy of Acoustic Neuroma by Albert Rhoton and Helder Tedeschi. It is in the journal Otolaryngologic Clinics of North America, Volume 25, Number 2, April 1992, page 257-293. This issue of the journal was dedicated to ANs - Well worth spending an afternoon at the medical school library with.
Last Edited: Monday, November 17, 2003