How is treatment of long qt syndrome different from treatment of arrhythmias?

May be same. Long qt syndrome is a spectrum of genetic conditions that result from gene mutations affecting how electrolytes are handled in heart muscle cells( there are >8 types of these ) . Most of these mutations predispose to potentially life threating arhrythmias occuring with mild provocation. Some of these may be preventable with arrhythmia medications and some require electrical cardioversion (icd).
Long qt. you need to use a med all the time, sometimes an ICD or sympathectomy is necessary,
More focused. Long qt syndrome, especially if famiial or genetic, has grave inplications vis a vis risk of sudden cardiac death from high grade ventricular arrhythmias. More routine arrhythmias run the gamet from equally serious to simply benign or bothersome, often not requiring treatment or simpler agents. Long qt syndrome may require potent agents with major adverse effects, or an implantable defibrillator.