Optokinetic nystagmus cannot be generated without functioning visual afferents reaching the cortex, and following movements evoked by a tilted mirror likewise require intact vision. Together with normal pupils, normal fundi and normal imaging, these constitute positive evidence that the visual pathways work, which is the definition of functional visual loss. The correct next step is a positive explanation, not further testing.
The distinction from cortical blindness matters. Cortical blindness spares the pupillary light reflex because that pathway leaves before the lateral geniculate nucleus, but it abolishes optokinetic nystagmus, and in Anton syndrome the patient confabulates vision while having a demonstrable structural lesion on imaging. Visual evoked potentials may be added if doubt persists, but they can be degraded by poor fixation and defocusing, so a normal response is reassuring while an abnormal one is not conclusive. Treatment strategies reported in functional visual loss include transparent discussion of the positive signs with the patient and family, acknowledging everyday behaviours that demonstrate visual ability, occipital transcranial magnetic stimulation to induce phosphenes, and hypnotherapy.
On the ABPN blueprint this item maps to the Diagnostic procedures axis, because the competency is knowing which bedside tests establish the diagnosis and when further investigation adds nothing. Demonstrating the positive signs to the patient in a non-accusatory way is the entry point to treatment.
Incorrect Answers
- A. Cortical blindness abolishes optokinetic nystagmus and would be accompanied by an occipital abnormality on magnetic resonance imaging.
- B. Visual evoked potentials are not required when optokinetic nystagmus and mirror tracking are unequivocally present, and delay in explanation harms outcome.
- C. Feigning requires demonstrable deliberate deception for external reward and cannot be inferred from a positive bedside sign; confrontation is harmful.
- E. A retinal cause would produce abnormal fundi or an afferent pupillary defect and could not permit optokinetic responses if vision were truly absent.
Testing Pearls
- Optokinetic nystagmus requires intact visual afferents and cannot be voluntarily suppressed easily.
- Cortical blindness spares pupils but abolishes optokinetic nystagmus.
- The mirror test elicits following movements in patients claiming total blindness.
- Visual evoked potentials can be degraded by deliberate defocusing, so abnormal results are not conclusive.
- Explain the positive signs transparently rather than confronting the patient with accusations.
References
- Jankovic J, Mazziotta JC, Newman NJ, Pomeroy SL, editors. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022. p. 2084.
- Yeo JM, Carson A, Stone J. Seeing again: treatment of functional visual loss. Pract Neurol. 2019;19(2):168-172.
- Stone J. Functional neurological disorders: the neurological assessment as treatment. Pract Neurol. 2016;16(1):7-17.
