Sir,

Following a recent report of monocular Charles Bonnet syndrome (CBS) after enucleation,1 we wish to present a case of complex monocular visual hallucinations in nonarteritic anterior ischaemic optic neuropathy (NA-AION).

Case report

A 68-year-old hypermetropic gentleman with hypertension, diabetes, and hypercholesterolaemia developed crescendo exertional angina over 6 weeks. He underwent coronary artery bypass grafting (CABG) for triple-vessel disease. Previously in 1998, he suffered a left hemisphere transient ischaemic attack. Carotid Doppler ultrasonography had shown an occluded left internal carotid artery (ICA) with 50% right ICA stenosis. The studies repeated preoperatively showed no significant change.

About 6 days postCABG he awoke in hospital with painless left visual loss. His unaided vision was 6/9 OD and only hand movements inferiorly OS. There was a dense left afferent pupillary defect. His left optic disc was swollen and his right had no cup. NA-AION was diagnosed after giant cell arteritis was excluded.

Later that day he began to experience complex visual hallucinations arising solely from his inferior left visual field. The patient soon realised that these were abolished by left eye closure and gaze aversion and, of his own volition, wore a left spectacle occluding patch. He described two young children in black and white, a boy and a girl, aged 5 to 10 years, dressed in Victorian clothing. He also described a bearded doctor. These figures were static, smoothly approached him and were almost constantly appearing except when he abolished them. Each apparition lasted 10 s to 1 min. The hallucinations subsided after 8 days.

Ultrasound demonstrated reversed left ophthalmic artery flow and some anomalous arterial flow within his left postero-superior orbit. Intracranial MR angiography and MR orbits excluded an arteriovenous malformation but did show increased vascularity within this region, consistent with collateral vasculature between the extra- and intra-cranial circulations. MRI brain confirmed an old left hemisphere subcortical infarct and demonstrated no occipital abnormalities.

Comment

Our patient's risk factors for NA-AION included vascular comorbidity, hypermetropic discs, and a state of probable low flow within the left collateralized cerebral circulation.

Monocular visual hallucinations are very infrequent. A retrospective study of phantom eye syndrome found 7 of 112 patients experienced complex visual hallucinations after unilateral enucleation, although it is unclear whether these phenomena were subjectively monocular.2 Uhthoff (1899) probably provided the original description,3 highlighted by the recent case report in Eye.1

CBS varies in semiology. Single hallucinations may last from few seconds to hours. Subjects tend to see people, animals, buildings, and scenery that can be static or moving. When moving, the hallucinations tend to do so en masse.4 Most subjects have reduced vision secondary to ocular disease. Some diagnostic criteria require the absence of central visual or cortical lesions but most require hallucinations with intact sensorium and insight.4

Certain pathophysiological mechanisms have been proposed in order to explain CBS. The perceptual release theory5 postulates the disinhibition of higher cortical activity resulting from a reduction in the afferent stimulus. This releases previously suppressed subconscious perceptual imagery in the form of hallucinations. The phantom vision theory is related to the deafferentation model6, 7 and postulates spontaneous higher visual cortical discharges in response to the loss of visual input.8 It is unclear how these theories apply to monocular complex hallucinations since a cortically generated process should be represented bilaterally, unless there is a mechanism which suppresses the monocular representation for the normal eye.

Hallucination abolition on eye closure is sometimes explained by secondary normalization of sensory inputs. In phantom limb syndromes, for example, the painful symptoms of an amputated arm may diminish when a mirror is placed such that the patient views the existing arm on the other side.9 By covering his affected eye, which had some residual vision, our patient theoretically also normalized his sensory input. This could explain why his hallucinations vanished even with his affected eye open.