Question: I have a patient that an EEG was performed under general anesthesia due to behavioral issues and autism. I'm not sure what I use for the CPT® or ASA Crosswalk code for billing. Whatever code is correct, I think I should also use the 23 modifier. Is this right? Nevada Subscriber Answer: CPT® does not include a specific code for this situation, so you should report unlisted code 01999 (Unlisted anesthesia procedure[s]). Also append modifier 23 (Unusual anesthesia) and include documented behavioral issues and autism diagnosis codes to explain why anesthesia was necessary.