antiarrhythmic agents. Some SSRI drugs are also occasionally tried, as are treatments more commonly used to treat postural orthostatic tachycardia syndrome, such as fludrocortisone. Invasive treatments include forms of catheter ablation such as sinus node modification (selective ablation of the sinus node), complete sinus node ablation (with associated implantation of a permanent artificial pacemaker), and AV node ablation in very resistant cases (creation of iatrogenic complete heart block, necessitating implantation of a permanent artificial pacemaker). However, invasive treatments can also make the symptoms worse. Differential diagnoses IST is primarily a diagnosis of exclusion. Upon exertion, an inappropriate heart rate response of sinus tachycardia can be seen in some Inborn Errors of Metabolism that result in metabolic myopathies, such as McArdle Disease (GSD-V). Rare diseases are more likely to be misdiagnosed, indeed, "Ninety percent of people with GSD V received a misdiagnosis before a corrected diagnosis (GSD VII unknown), resulting in a median diagnostic delay of 29 years, which can seriously affect QoL [Quality of Life]… Of those who are misdiagnosed, 62% report being misdiagnosed more than once." See also Supraventricular tachycardia Sinus tachycardia Postural orthostatic tachycardia syndrome Dysautonomia Metabolic myopathies References Further reading Cardiac arrhythmia