This disorder is most commonly seen in fancy goldfish like orandas.
In the case of Orandas, I think it's due to the fact that they are abnormally shaped.
Some fish live with the condition for years, probably due to the tender loving care provided by their owners.
You should be aware that it's normal for a small percentage of animals to get sick, if this affliction is affecting more than 20 %
of the population of your aquatic pets then something is seriously wrong. If it's just one fish, sadly, I would not spend time worrying. Also you should consider the age of your fish. An 8 year old angelfish is a great great grandfather.
The main thing to check for are environmental causes, namely poisoning, food poisoning in particular.
Frozen food needs to stay frozen 'til fed.
Dry food needs to stay dry 'til fed.
Live food needs to stay live 'til fed.
Another way fish can get damage is from fighting. More so with other cichlids, but it is possible with angelfish, that a hurt fish took a beating from a more aggressive fish that they live with.
I don't think this condition of your angelfish is curable....but, you could try treating it with Metronidazole, as for "Malawi Bloat".
I wrote a short instruction for bloat treatment at
http://unclenedsfishfactory.com/Library ... Bloat.html
This might work if the cause of your angel's problem is internal protozoans.
Whether you treat it with medication or not, I would strongly suggest isolating it in a seperate tank.
If it has a contagious disease, it can't spread any disease if it lives alone for a while.
If it's getting picked on by other fish, they can't beat it up if it's in a different tank.
If it does need medicine, it's much easier to flush the tank and re-dose in a small seperate tank.