There seems to be a common assumption (among those not familiar with the genres) that brutal death metal is just death metal with the "brutality" cranked up, death metal but somehow moreso. I think, however, that this is not the case. And BDM fans on music forums often criticize certain death metal bands being put forward as BDM, saying things like "This is about the style BDM, not about death metal that is (or that you think is) brutal". Similarly, those disparaging BDM will say things like "slams and pig squeals and stupid constant blasting - these things aren't brutal at all". So "brutal" as descriptor is not exactly the same (though quite similar) to the "brutal" in "brutal death metal". But what is this style? It is totally undertheorized, barely even written about to the point that even finding a description of the genre is difficult, coming mainly through forum posts as far as written material goes. Nothing official/academic, at any rate. Finding a history of the genre, for example, is very difficult.
Suffocation seem to be considered the founders - Effigy of the Forgotten came out as early as 1991! Sometimes early Cryptopsy is considered BDM, but I'm not so sure about that. The major early-mid 2000s BDM bands seem to be Disgorge (US) above all, and then (the following are bands I like from the period, since it's so damn hard to find anything about influence...) Disavowed, Pyaemia, and Deeds of Flesh. I would probably also consider Cannibal Corpse's Butchered At Birth a major influence. In all honesty, this style of BDM is probably exemplified by Disgorge's Consume the Forsaken (2002), but their two previous albums had all the basic elements there.
And there is a new style. While we might say the above bands are "old school BDM", there is also inevitably a "modern BDM" setting itself apart with different production, technical abilities, and of course influences both "inside" and "outside". If old school BDM was already an event, was there another with modern BDM? I don't think so, but there may be one with slam. Modern BDM (non-slam) is exemplified by (I want to say) Guttural Secrete, newer Deeds of Flesh (which I don't like), Benighted (considered paradigmatic), etc. Slam is of course exemplified by Cephalotripsy's Uterovaginal Insertion of Extirpated Anomalies, an album that (as metal-archives user MutantClannfear remarked, is not even death metal anymore; I'm inclined to agree). Abominable Putridity, Epicardiectomy, and (for the technical brand) 7 H. Target are other slammy BDM bands.
Slam and regular old BDM exist in a very weird relationship. I actually don't think they are the same at all except for that they seem to go pretty well side-by-side in a track, and they have the same superficial elements (production jobs, vocal styles, guitar tones and drum styles). Basically, what the hell is their relation? I would like to find this out.
And another thing: if I theorize war metal as really-damn-close to absolute negation in all its aspects, what is the relationship of BDM to negation? Slam is clearly negative in some aspects but has a more... disgustifying tendency since it utilizes catchy hip-hop rhythms in a disgusting abrasive manner. But non-slam BDM? They definitely don't negate everything, for example technical ability (as do war metal bands), but this is perhaps all the better to pulverize and negate things a hundred different and varied ways (war metal tracks seem to hit you a ton in one single way, over and over again). This relation must be better worked out.
So the final exploration of this post: BDM is a real qualitative shift from regular old death metal. The relation to rhythm and... just sheer sound is different. See Defeated Sanity's Chapters of Repugnance, where the instruments simply do not play notes. They are textures. (But compare this to Conqueror, the war metal band with the closest use of the guitar. The comparison would be fruitful). BDM is a sonic hammer - these weird pure relations (don't tell me BDM uses notes in the normal sense!) are combined and recombined and dashed against the listener. So many variations, yet the music can start to sound "same-y" after only a brief listen (especially to those uninitiated; veteran listeners can of course pick out many more subtleties). Extreme metal represented a break with rock/heavy metal music - maybe BDM represented a break with death metal of the same variety. Hence why death metal fans and BDM fans (and especially slam fans!) are often so at odds. It is a mistake to consider BDM a subspecies of death metal (even more so for slam).