What is the difference between a depolarising and a non-depolarising NMBA? Do both result in flaccid paralysis?

Depolarising neuromuscular blocking agents (NMBAs) are potent agonists at nicotinic receptors that cause depolarising block (and necessarily also the secondary desensitising/non-depolarising block). In contrast, non-depolarising NMBAs are direct competitive nicotinic receptor antagonists.

An important difference is that non-depolarising NMBAs can be reversed by increasing acetylcholine (ACh) levels by using an acetylcholinesterase inhibitor such as neostigmine. Depolarising NMBAs cannot be reversed in the same way since increasing ACh availability just causes more depolarizing block (and inevitably secondary desensitising/non-depolarising block).

As depolarising NMBAs initially cause activation, they will cause twitching/fasciculation followed by rigid paralysis on onset (although this phase is over quickly) before switching to flaccid paralysis. In contrast, non-depolarising NMBAs will go straight to progressive flaccid paralysis.