The goal of this clinical trial is to learn if external trigeminal nerve stimulation (eTNS) works to treat ADHD symptoms in children on the autism spectrum (ASD). It will also learn about the efficacy and tolerability of the eTNS device. The main questions it aims to answer are:

• Does eTNS reduce ADHD symptoms?
• Does eTNS improve core and associated features of ASD?

Participation spans 8-12 weeks and includes:

• 4-5 in-person visits
• 4 brief virtual check-ins
• Nightly use of the eTNS device with a small sticky patch applied to child's forehead
• Randomized assignment (those who start with the sham device may try the active device later)

Inclusion Criteria:

• Confirmed clinician diagnosis of ASD according to DSM-5 criteria, corroborated by prior testing (or obtained in study screening) with the Autism Diagnostic Observation Schedule (ADOS) with or without the Autism Diagnostic Interview-Revised (ADI-R)
• IQ > 70 as corroborated by prior testing (or obtained in study screening) with the Wechsler Abbreviated Scale of Intelligence Scale (WASI)
• Confirmed diagnosis of ADHD according to DSM-5 criteria with minimum ADHD-RS score of > 24
• Stable on current medications for a minimum of 4 weeks before baseline
• Ability to complete protocol testing
• Both the child participant and their primary caregiver must be fluent in English (speaking, reading, and understanding), as the questionnaires and assessment tools used in this study have been validated only in English.

Exclusion Criteria:

• Current major depression, history of psychosis, bipolar disorder, elevated risk of self-harm
• History of moderate to severe coarse brain injury
• Active medical illness expected to interfere with study assessments
• Presence of implanted stimulator (e.g., vagal nerve stimulator)
• Active dermatologic condition likely to interfere with eTNS electrode wearability
• Sleep disorder likely to interfere with nightly eTNS in the opinion of the study physician
• Inability to communicate discomfort or pain
• Current and anticipated continued use of antipsychotic or stimulant medication