ADHD is characterized by an impaired dopamine system. Dopamine is usually reduced in ADHD.
Dopamine is also involved in the visual system.
Reduced visual performance was found in 76% of children with ADHD studied. A meta-analysis of 35 studies with 3,250,905 participants found a 94% increased risk of unspecified visual problems in ADHD (OR = 1.94).
Young adults with ADHD showed more problems with depth perception, peripheral vision, and color perception, especially in the blue spectrum, compared with unaffected individuals.
ADHD medication improved visual field abnormalities and visual acuity in children with ADHD. Another study found no significant improvements. In one small study, surgical treatment of strabismus improved ADHD symptoms in 7 of 8 subjects in the parent report.
A small study was able to distinguish ADHD sufferers from non-affected people quite well using eye movement tracking.
It is therefore conceivable that eye problems may contribute to or exacerbate ADHD symptoms, just as ADHD may contribute to or exacerbate eye problems.
1. Visuoperceptive problems in ADHD (+ 1000 %)
One study found visuoperceptual problems in 21% of children with ADHD, compared with only 2% of those not affected.
2. Convergence and ADHD (+ 200 % to + 400 %)
One study found convergence insufficiency in 24% of children with ADHD, compared with only 6% of unaffected children.
Several other studies found ADHD three times more common in children with convergence insufficiency than in those not affected.
A meta-analysis of 35 studies with 3,250,905 participants found a fivefold risk of reduced near convergence point (OR = 5.02) in ADHD.
3. Stereoacuity (stereotactic vision, depth perception) in ADHD (+ 333 %)
Stereoacuity is the ability of a person to see objects as separate entities along different distances.
Several studies report impaired stereoacuity in children with ADHD. Stereoacuity (depth perception) was impaired in 26% of children with ADHD, compared to 6% of those not affected.
4. Strabismus / heterophoria (strabismus / latent strabismus) in ADHD (+ 93% to + 700%)
One study found in children with ADHD
- More strabismus (squinting) (17% vs. 2% in unaffected)
- More heterophoria (latent strabismus) (27% vs. 10% in unaffected individuals)
One cohort study found a 15% increased ADHD risk with strabismus. Another cohort study found a doubled ADHD risk in strabismus convergens (internal strabismus, esotropia) and a 44% increased ADHD risk in strabismus divergens (external strabismus, exotropia).
A meta-analysis of 35 studies with 3,250,905 participants found ADHD to increase the risk of strabismus by 93% (OR = 1.93) and to increase the risk of hyperopia and hypermetropia by 79% (OR = 1.79).
5. Astigmatism in ADHD (+ 79 % to + 300 %)
One study found astigmatism in 24% of children with ADHD, compared with only 6% of those not affected.
A meta-analysis of 35 studies with 3,250,905 participants found a 79% increased risk of astigmatism in ADHD (OR = 1.79).
6. Eye movements (saccades) and ADHD
Several studies found slower and more variable saccadic reaction times in children with ADHD using the gap/overlap test. One study was able to eliminate this biomarker by using warning signals during the test.
One experimental study (n = 16) reported significant abnormalities of eye movements in ADHD as assessed by electrooculography (EOG).
Another study found that in children, visual field shifts moderated the relationship between hyperactivity/impulsivity on the one hand and problems with attentional focusing and information intake on the other. As performance accuracy decreased, visual field shifts increased, although this correlation did not reflect symptom severity.
Another study found significant abnormalities in ADHD sufferers in the modulation of the eye vergence response (eye vergence = opposite / disjugated / disjunctive eye movements) during attention tasks. The diagnostic test accuracy was 79%.
One large study found a correlation between premature anticipatory eye movements and inattention, but not between directional errors and ADHD symptoms.
A meta-study found evidence that children with ADHD
- made more directional errors in an antisaccade task
- were slower and performed worse on oculomotor tasks
- Eye movements performed less precisely
Saccadic eye movements (gaze jumps) are strongly influenced by factors such as attention and inhibition. Since attention and inhibition are impaired in ADHD, it seems plausible that saccadic eye movements show abnormalities in ADHD.
Voluntary eye movements are controlled by the dlPFC: Voluntary control of eye movements is closely related to attentional control. The dlPFC also houses working memory, which is typically impaired in ADHD.
One study found significantly higher pupillary velocity scores in children with ADHD, which correlated positively with RNFL measurements of their right eyes.
The abnormalities of saccadic eye movements in ADHD are thought to be improvable with computer training.
7. Macular thickness in ADHD
A small study hypothesized that increased macular thickness in children with AD(H)D may represent the increased ratio of right frontal lobe thickness to parietal cortex thickness in ADHD.
8. Accommodation in ADHD
Children with ADHD showed a reduced accommodation response that was not influenced by the accommodation stimulus. There was no clear effect of medication in ADHD on accommodation accuracy. Accommodation refers to the ability of the eye to focus/focus on objects at different distances.
A meta-analysis of 35 studies with 3,250,905 participants found ADHD to be at increased risk for increased delay (Hedge’s g = 0.63 [CI: 0.30, 0.96]) and variability (Hedge’s g = 0.40 [CI: 0.17, 0.64]) of the accommodative response.
9. Neurophysiological changes of the optic nerves in ADHD
One study found smaller optic nerves, smaller neuroretinal rim areas, or reduced retinal artery tortuosity more often in children with ADHD.
10. DRD4-7R and vision in ADHD
The D4 dopamine receptor gene, DRD4, is instrumental in converting light into electrical signals in the retina. The transcription of DRD4 shows a strong circadian pattern.
The DRD4 7R variant is one of the strongest single gene risks for ADHD. For more information, see ⇒ Candidate genes in ADHD In the chapter ⇒ Emergence.
DRD4-7R correlates with a lower ability to reduce the light-sensitive second messenger cyclic adenosine monophosphate (cAMP) upon illumination.
Besides, DRD4-7R correlates with higher daytime sleepiness, which could be a consequence of the seesaw between dopamine and melatonin.
11. (Asymmetric) pupil diameter in ADHD
Pupillary dilation is a physiological index of increased arousal and noradrenergic activity of the locus coeruleus.
One study found significantly larger tonic pupil diameters and suppressed stimulus-evoked phasic pupil dilation in ADHD subjects. This supports the notion that pupil diameter reflects firing of noradrenergic neurons in the locus coeruleus and that the neuromodulatory noradrenergic system of the locus coeruleus may be dysfunctional in regulating vigilance in ADHD.
ADHD correlates with overactivity of the locus coeruleus, particularly in the right hemisphere. The kinetics of pupil diameter and reflects the neuronal activity of the locus coeruleua in connection with cognitive functions such as attention and arousal. In this context, temporal patterns of pupil diameter have intrinsic significance. An asymmetric pupil diameter correlating with the severity of inattention, impulsivity, and hyperactivity in ADHD could be attributed to a left-right imbalance of locus coeruleus activity.
12. Refractive errors and ADHD (?)
One study found refractive errors in 83% of children with ADHD examined. A meta-analysis of 35 studies with 3,250,905 participants found no clustering of refractive errors in ADHD (Hedge’s g = 0.08 [CI: -0.26, 0.42]) .
Dysfunction of retinal dopamine could affect neurodevelopmental growth of the eye, leading to refractive errors. This could help explain the frequency of refractive errors in ADHD.
13. Retinal nerve fiber layer thickness unchanged
A meta-analysis of 35 studies with 3,250,905 participants found no change in retinal nerve fiber layer thickness in ADHD (Hedge’s g = -0.19 [CI: -0.41, 0.02]) .