Charles Bonnet syndrome in patient with impaired visual field and good visual acuity

Dear Editor:

Charles Bonnet syndrome (CBS) is a condition characterised by the appearance of simple or complex visual hallucinations in patients with impaired vision and conserved cognitive status.^1–3 Increased life expectancy and quality of life in these patients, many of whom have age-related eye disease, such as age-related macular degeneration, is causing an increased incidence of CBS, which can affect up to 60% of the patients with low visual acuity (VA).^1–4 However, CBS can also manifest in patients with good VA but with visual field alterations secondary to such neurological disorders as those caused by certain cerebrovascular accidents (CVA).

The patient was a 75-year old woman assessed in the neuro-ophthalmology unit of our hospital due to stable visual field alterations consisting of right homonymous hemianopsia secondary to CVA. She had suffered an atherothrombotic infarct of the left posterior cerebral artery a year before. She reported sudden onset of hallucinations one month before. Hallucinations consisted of distorted faces in colour and movement which lasted for approximately 30 minutes and predominantly occurring when waking up in the morning. Although hallucinations affected the whole visual field, the patient reported that they manifested predominantly on her left visual hemifield. She reported no other types of hallucinations. Personal history included medically treated arterial hypertension and diabetes mellitus. The patient's personal and family history did not reveal any other relevant finding or known allergy.

Examination revealed a VA of 0.8 in both eyes (OU) but anterior pole, intraocular pressure, and eye fundus were normal in OU. Campimetry study (OCTOPUS 1-2-3, programme G1X) showed stable left homonymous hemianopsia since the CVA. The patient was assessed in our neuro-opthalmology multidisciplinary unit, which included ophthalmology, neurology, and psychiatry consultations. Imaging studies showed that cerebrovascular involvement was stable and trigger factors for hallucinations were ruled out. Our patient was diagnosed with CBS secondary to visual field alterations despite having good VA.

CBS is characterised by the presence of complex visual hallucinations which are usually repetitive, persistent, and appear suddenly. The patient did not exhibit cognitive deficit or any other sensory hallucinations. They typically manifest as images of people, animals, or trees, and they do not make sounds. Images can also be in black or white or in colour, and they may be static or appear to move.⁵ In most cases, these hallucinations last less than 10 minutes and are usually repetitive. The course of the disease can be episodic or cyclical with a duration generally lasting less than 18 months, although progression had lasted several years in some published cases.^3,4

The cause originating these hallucinations is still unknown, but neuronal deafferentation is thought to be responsible for CBS development. According to this theory, loss of stimulation of retinal nerve cells caused by any eye disease decreases stimulation of the occipital cortex, but unlike in amaurosis, stimulation does not disappear completely. Residual afferent signals are thought to trigger the deafferentation phenomenon by causing histological, biochemical, and anatomical changes to synapses in an attempt to compensate for the limited stimulation. This transforms neurons into hyperexcitable cells.^3,5 These changes were evidenced both in the presynaptic and postsynaptic terminal, as well as in the dysfunction in primary and secondary areas of visual cortex. Although the cause of CBS is still unknown, there are some trigger factors such as fatigue, stress, dim lighting, and flash blindness.^3,5,6

Our patient did not show any eye disease that could justify development of CBS. However, cerebral impairment with lesion of the optic pathway and associated visual field alteration would be responsible for neuronal deafferentation, which, as a compensatory phenomenon, would trigger the development of visual hallucinations. This has happened in cases of visual field alterations^7,8 and visual field alterations after surgery of the central nervous system.^9–12 Deficit of afferent nerve fibres typical of left homonymous hemianopsia would lead to the development of hallucinations in the whole visual field but predominantly on the left side.

We do not know the cause of CBS in our patient, who was ophthalmologically stable and showed no decreases in VA. However, progressive neuronal deafferentation caused by decreased afferent fibres to the occipital cortex would be responsible for the development of hallucinations, also favoured by the trigger factor of morning flash blindness.

As conclusion, CBS can develop in patients with good VA; therefore, ophthalmologists, neurologists, and psychiatrists should be aware of this process to avoid incorrect diagnoses and treatments.^13,14 Progressive development of neuro-ophthalmology multidisciplinary units by the 3 specialties would promote a better knowledge of CBS, which will surely improve quality of life of our patients.^13,14