This study will have a trial phase, extension phase, and a long-term extension phase. The primary objectives of the trial phase are to assess the pharmacodynamics (PD) of eladocagene exuparvovec treatment by evaluation of homovanillic acid (HVA) levels and to assess the safety of the SmartFlow® magnetic resonance (MR) Compatible Ventricular Cannula for administering eladocagene exuparvovec to pediatric participants with aromatic L-amino acid decarboxylase (AADC) deficiency. The extension phase is designed to capture additional clinical information for eladocagene exuparvovec through study evaluations, changes in motor development, AADC-specific symptoms, and other PD measures. The long-term extension phase is designed to capture long-term safety and efficacy data from participants treated with eladocagene exuparvovec.

Inclusion Criteria:

• Pediatric participants must have genetically-confirmed AADC deficiency with typical clinical characteristics and decreased AADC enzyme activity in plasma.
• Cranium sufficiently developed to allow placement of ClearPoint® system for stereotactic surgery.
• Persistent neurological defects secondary to AADC deficiency despite standard medical therapy (dopamine agonists, monoamine oxidase inhibitor, pyridoxine, or other forms of vitamin B6) in the opinion of the investigator.
• Unable to ambulate independently (with or without assistive device).
• Baseline hematology, chemistry, and coagulation values within the normal pediatric laboratory value ranges, unless in the investigator's opinion the out of range values are not clinically significant with respect to the participant's suitability for surgery.
• Participant must test negative for coronavirus disease of 2019 (COVID-19) a maximum of 72 hours prior to receiving gene therapy.
• Participant must be on stable dosage for 3 months prior to baseline for all medications related to treatment of AADC deficiency, including dopamine agonists, monoamine oxidase inhibitors, anticholinergic drugs, and vitamin B6.
• Females of childbearing potential must have a negative pregnancy test at screening and baseline and agree to abstinence or double-barrier form of contraception for the duration of the study following discharge from the hospital (acceptable methods will be determined by the site).
• Males sexually active with females of childbearing potential must agree to use a barrier method of birth control during the study following discharge from the hospital.
• Parent(s)/legal guardian(s) of the participant must agree to comply with the requirements of the study, including the need for frequent and prolonged follow up.
• Parent(s)/legal guardian(s) with custody of the participant must give their consent for the participant to enroll in the study.

Exclusion Criteria:

• The participant has presence of other significant medical or neurological conditions that would create an unacceptable operative or anesthetic risk.
• Participants with pyridoxine 5'-phosphate oxidase or tetrahydrobiopterin (BH4) deficiency.
• Contraindication for imaging studies (computed tomography [CT] scan, PET or magnetic resonance imaging [MRI]), including sedation limitations or metal that would interfere with a brain MRI.
• Anti-adeno-associated virus, serotype 2 (anti-AAV2) antibody titer higher than 1:1200 or >1 optical density value by enzyme-linked immunosorbent assay.
• Participants who have received treatment with other experimental therapies within the last 24 weeks prior to planned gene therapy administration, or any treatment ever with a gene therapy.
• Evidence of a clinically active infection.
• Females who are pregnant or breast feeding.