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what is it?
Atypical can often be the way Meniere's is described in the earlier stages of the disease, prior to all the classic symptoms being presented. The "a" in atypical means not i.e. Not typical. Usually symptoms fluctuate or are episodic as they are with classic Meniere's.
In atypical Meniere's a patient may suffer the vertigo but not the deafness/tinnitus/fullness. Or deafness, fullness and tinnitus are present but without the vertigo. Or perhaps in some other varying form. This makes a Meniere's diagnosis less likely and many people will go for years before they become "classic" and finally get a proper diagnosis.
Most believe the above will go on to become classic Meniere's and all classic symptoms will appear. Others believe these are not true Meniere's.