Addiction Biology · 4 authors, 3 centres
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This systematic review of 20 studies found that acute cannabis use selectively impairs oculomotor control, increasing saccadic latency and inaccuracy and impairing inhibitory control. Chronic cannabis users, especially those with early onset of use, show enduring deficits in visual scanning efficiency. Eyelid tremors may serve as a reliable indicator of recent cannabis consumption, though they are not directly linked to behavioral impairment.
**Background:** The increasing availability of cannabis for recreational and medicinal purposes raises concerns about its impact on safety-sensitive activities such as driving. Current roadside tests rely on biological markers of Δ9-tetrahydrocannabinol (THC) consumption, which do not necessarily indicate impairment. Characterizing THC-specific alterations to oculomotor behavior may offer a more sensitive measure for indexing drug-related impairment. This systematic review aimed to synthesize current evidence on the acute and chronic effects of cannabis/THC on driving-relevant oculomotor behavior, including saccadic activity, gaze behavior, nystagmus, smooth pursuit, and eyelid/blink characteristics.
**Methods:** The review was prospectively registered (DOI 10.17605/OSF.IO/A4H9W) and followed PRISMA guidelines. A search string was developed with a research librarian and applied to EBSCOhost, PsycNet, PubMed, Scopus, and Web of Science for articles published up to November 2022, with a forward search in July 2023. Two reviewers independently screened 1388 citations after deduplication; 59 underwent full-text assessment, and 20 studies met eligibility criteria. Included studies comprised 12 experimental acute dosing trials (9 randomized controlled trials and 3 nonrandomized/pilot studies) and 8 observational studies (5 cross-sectional and 3 epidemiological studies). Risk of bias was assessed using the Cochrane RoB 2 tool for randomized crossover studies, ROBINS-I for nonrandomized studies, and the Critical Appraisal Skills Programme checklist for epidemiological studies. Due to significant heterogeneity, meta-analysis was not performed.
**Key Results:** A total of 2699 participants were included across studies, with ages ranging from 15 to 65 years. Acute THC administration (doses ranging from 5.9 mg to 89.49 mg) selectively impaired oculomotor control. Saccadic latency increased post-THC relative to placebo in some studies (e.g., one study using consecutive doses up to 8 mg THC), and saccadic accuracy significantly decreased compared with placebo (e.g., after 2, 4, and 6 mg THC). Acute THC also increased anticipatory memory-guided saccades. Saccadic peak velocity and amplitude constants were not significantly altered. Gaze behavior studies showed that acute THC (5 mg) increased fixation duration compared with placebo, but visual search characteristics were not consistently affected. Among chronic cannabis users with cannabis use disorder (CUD), increased saccade amplitudes, antisaccade errors, fixation duration, and number of visual regressions were observed, particularly in those with early age of onset (14–16 years). Horizontal gaze nystagmus (HGN) impairment was significantly increased after acute THC inhalation in some studies (e.g., at 5 and 105 minutes after smoking a 1.776 g cigarette containing 2.93% THC) and in CUD groups, but not in roadside epidemiological studies using Drug Evaluation and Classification (DEC) tests. Vertical gaze nystagmus was not reliably produced. Smooth pursuit results were inconsistent: two studies showed no differences, while two others showed decreased smooth pursuit (e.g., a decrease in the percentage of time in smooth pursuit after consecutive THC doses, and decreased pursuit speeds after 25.1 mg THC). Eyelid tremors were observed in 57% to 58% of cannabis users in roadside studies, with good sensitivity (86%) and specificity (94%) for identifying recent cannabis use. Blink duration decreased after acute oral THC (5 mg) in one study.
**Clinical Implications:** Acute THC consumption selectively impairs oculomotor control, particularly increasing saccadic latency and inaccuracy and impairing inhibitory control, which may disrupt response selection and efficient processing of environmental changes during driving. Chronic cannabis users, especially those with early onset of use, exhibit enduring oculomotor deficits that reduce visual scanning efficiency and increase the likelihood that visual events go unattended. The sensitivity of field sobriety tests (e.g., HGN) may vary with cannabis use chronicity, questioning their efficacy across diverse user populations. Eyelid tremors appear to be a reliable indicator of recent cannabis consumption but are not directly linked to behavioral impairment. Defining THC-specific changes in oculomotor control may enhance the precision of roadside impairment assessments and vehicle safety systems.