So for late antiquity, though trade was obviously mediterranean wide, there were primarily two main spines around which the bulk of trade in the Roman Empire revolved around: the carthage-rome route, and the alexandria-constantinople route (in high imperial times, it would simply be carthage AND alexandria to Rome).

As I'm sure you can surmise, these two routes also reflect the movement of grain doles to supply the two imperial capitals. However, the grain dole was more than just a one way supply toward the capital, it also included ancillary trade goods (like african red slip dishware) that were carried along with the grain to be shipped to the capitals and the northern mediterranean, as well as goods needed by the southern mediterranean on return trips by the same ships, now no longer ladened with grain.

This bulk grain shipment was vital to the empire not only because of food, but because it provided a massive degree of relatively expense-free shipping, as the bulk of it was paid for by the government. Apparently no other bulk movement of goods was as large until modern times as the trade between these two spines. This is simply because of the ease of transport of sea trade, made possible by Roman hegemony of the mediterranean (i.e. pirate free), which if you think about it, was also not possible again until relatively modern times. Though there was consistent land transport elsewhere, like that from southern Gaul to northern Gaul to supply the legions on the Rhine, the size of this trade paled in comparison, and was also far more expensive, done only because of the necessity of maintaining the limes for Roman security.

So what does this mean for the empire during a civil war? Well it meant that so long as these trade spines were maintained, the empire can manage. You'll notice even in the Crisis of the Third Century, Gallenius' section of the empire still included Carthage/Africa, which was just enough to keep his part of the empire going even without Egypt (which had it lasted, would have supplied the Palmyrean empire), and even then Constantinople had not yet emerged as an alternate capital to be a rival consumption center.

And you'll notice, that it's not a coincidence that the rapid decline of hegemony in both empires, can be tied to periods shortly after the loss of these trade spines. Western Rome ending roughly 40 years after the loss of Roman Africa/Carthage, Eastern Rome being forced to completely reorganize itself in the roughly 80 years after the loss of Byzantine Egypt. It simply wasn't in the interest of whoever was in charge to mess with these key trade spines, unless they were in possession of a capital and NOT at least one of those key trade areas. This is also why so much energy was expended in Majorian's last ditch effort to try to retake Carthage from the Vandals, and why the failure to do so ultimately cemented the western empire's demise.

Now as I'm sure someone like /u/tiako will point out, what I lay out here does not include foreign trade, which I believe the current research says precipitated downward sharply after the 2nd century. Unfortunately, that is outside of my knowledge base, but I will say the bulk of what I mention in here is true for the internal trade of the empire, to the point of its final fragmentation.