Dear Editor

We read with interest Cohen and associates’ report on the onset of Charles Bonnet syndrome (CBS) following photodynamic therapy (PDT) for choroidal neovascularization (CNV).[1] An earlier study reported that 16.6% of patients developed CBS following macular photocoagulation for CNV.[2] Holroyd and co-workers reported a patient with sudden onset of visual hallucinations after laser treatment, which ceased after a second laser treatment one year later.[3] Two other patients’ visual hallucinations ceased acutely after laser therapy for macular degeneration.[4]

We recently reported a case of CBS following bilateral sequential Argon-Nd:YAG laser peripheral iridotomies.[5] This case differs from the others noted above in that the laser treatment involved the anterior segment of the eye and was not specifically targeted on the retina although it is possible that some laser energy may have been transmitted to the posterior pole during the iridotomies. It is therefore interesting to note that various ophthalmic laser procedures may precipitate as well as terminate the symptoms of CBS.

Several different mechanisms may be involved in the pathogenesis of CBS.[6,7] The sensory deprivation theory[8] may explain its onset following certain ophthalmic laser procedures. An acute reduction in visual acuity may occur following destruction of foveal tissue during macular photocoagulation [2,3] and secondary to anterior segment inflammation and corneal changes after laser peripheral iridotomy.[5] Cohen et al. suggested that acute anatomical changes at the fovea following PDT could have triggered visual hallucinations in some patients.[1] To explain cessation of hallucinations after laser procedures, Holroyd et al. postulated that laser photocoagulation may have destroyed the neurons causing abnormal sensory signals, thereby terminating the hallucinations.[4]

Regardless of the mechanism resulting in hallucinations, it appears that CBS is a possible sequelae of many ophthalmic laser procedures and these symptoms can be quite distressful to some patients. It may be useful to ask specifically for symptoms of CBS following laser procedures as some patients who have been unwilling to discuss their symptoms have subsequently expressed relief upon learning that CBS does not imply a psychiatric illness.

References

(1) Cohen SY, Bulik A, Tadayoni R, et al. Visual hallucinations and Charles Bonnet syndrome after photodynamic therapy for age related macular degeneration. Br J Ophthalmol 2003;87:977-9.

(2) Cohen SY, Safran AB, Tadayoni R, et al. Visual hallucinations immediately after macular photocoagulation. Am.J Ophthalmol. 2000;129:815- 6.

(3) Holroyd S, Rabins PV, Finkelstein D, et al. Visual hallucinations in patients from an ophthalmology clinic and medical clinic population. J Nerv.Ment.Dis. 1994;182:273-6.

(4)Holroyd S, Rabins PV. A three-year follow-up study of visual hallucinations in patients with macular degeneration. J Nerv.Ment.Dis. 1996;184:188-9.

(5) Tan CSH, Yong VKY, KG Au Eong. Onset of Charles Bonnet Syndrome (Formed Visual Hallucinations) Following Bilateral Laser Peripheral Iridotomies. Eye, in press.

(6) Fernandez A, Lichtshein G, Vieweg WV. The Charles Bonnet syndrome: a review. J Nerv.Ment.Dis. 1997;185:195-200.

(7) Menon GJ, Rahman I, Menon SJ, et al. Complex visual hallucinations in the visually impaired: the Charles Bonnet Syndrome. Surv.Ophthalmol 2003;48:58-72.

(8) Berrios GE, Brook P. The Charles Bonnet syndrome and the problem of visual perceptual disorders in the elderly. Age Ageing 1982;11:17-23.