Let me begin by giving the obligatory disclaimer: if, when, and how to treat your AN is a totally personal choice. Your decision will be the right decision.
With that out of the way, here's what I've learned. First of all, mild or severe symptoms can appear suddenly. I woke up from a nap with pronounced hearing loss, severe vertigo, nausea, vomiting and more. My balance never completely recovered (although it improved dramatically over the course of several months) and my hearing got progressively worse on my AN side while my tumor remained undiagnosed for 7 years. You're lucky you have been diagnosed before symptoms have presented themselves. Research your options thoroughly and now, so that if you decide to take quick action at some point in the future, all your decisions and plans will already be in place. You don't want to have to do a bunch of research if and when you become very ill.
When should you seek treatment? In a telephone conversation with Dr. Derald E. Brackmann (famous neurosurgeon at House Ear Clinic in L.A.), Dr. Brackmann told me that he's seen some ANs stay the same size for 20 years. After my first follow-up MRI revealed my tumor was growing -- and faster than the average AN -- Dr. Brackmann said I "should probably seek treatment." Note that at this point my tumor was 2.1 cm along the oblique transverse axis and just beginning to press against my brain stem. Dr. Brackmann added, however, that in Sweden it is routinely advised to all patients that they wait until they are deaf on the AN side before seeking treatment, at which time surgery is performed.
Sounds radical, huh? But the reasoning is that both surgery and radiosurgery often have negative, lifelong consequences (which are too numerous to list here but are detailed in many other posts on this forum). Dr. Brackmann told me that, in rare cases, someone can suddenly go completely deaf on the affected side (this usually happens more gradually, although it can happen in big, progressive steps downward in hearing function, so I don't wish to understate the risk). In this case, quick treatment with steroids will bring back hearing in 50% of the cases, according to Dr. Brackmann. Note that although Dr. Brackmann was aware of Swedish doctors' emphasis on watch-and-wait, he thought I should probably seek treatment, which in my case he advised should be GammaKnife (GK) or CyberKnife (CK) treatment to help preserve my hearing, which although damaged is still remarkably good on my AN side for a tumor my size.
Now for the flip side: the doctor I chose to treat me, Dr. Steven D. Chang (Stanford University Medical Center), when he looked at my followup MRI and discovered my tumor was growing, pointed out that the fact the tumor was now pressing against my brain stem was significant (in a bad way). While not recommending I seek treatment immediately or at any point, for that matter -- out of respect for a patient's right and need to make decisions for themself -- when pressed he said the longer I wait the greater the chance was that my current symptoms would increase and that new symptoms (such as facial numbness and other symptoms related to facial-nerve damage) would appear. Based on his assessment, I decided to schedule treatment immediately.
There are other, more pressing reasons to consider getting treatment sooner rather than later, however. First of all, the larger the tumor gets, the harder it will be to treat without repurcussions. For example, if you decide to get radiosurgery, any structures that the tumor is pressing into or wrapped around become more likely to receive collateral damage. That is, your brainstem, for example, gets increasingly in the way of applied radiation as the tumor pushes into it more and more, so it takes a radiation hit. Also, most institutions won't treat a tumor that is over 3 cm in size with GK or CK. So if you wait too long and your tumor grows to over 3 cm, you eliminate two very important options for treatment and are left with either resection (surgery) or standard fractionated stereotactic radiosurgery (FSR) at a place such as Staten Island Hospital (they will treat tumors greater than 3 cm in size).
So, to recap, you shouldn't panic and seek treatment before you've had a chance to research and consider your options carefully. But thinking you can wait forever to get treated may be a dangerous attitude, too. It might be good to know whether the tumor is still growing, as determined by a followup MRI, before seeking treatment for an asymptomatic tumor. But then again, it might grow quite a bit larger in the interim period and eliminate some treatment options for you. It would be unusual for your tumor to grow to over 3 cm in size in the six-month period between initial and followup MRI, as the average growth rate is only 1 or 2 mm per year. But there are many stories on this forum of accelerated growth rates. I'm one of them. My tumor grew 2 to 3 mm along each of the 3 axes of measurement in just 5.5 months. One other poster reported their tumor fully doubling in size in a 6-month period.
It's a tough decision, ColoradoGal. But you're not alone. This board has tons of useful information and also offers emotional support for fellow ANers. Welcome and best wishes.