1. Sildenafil
Sildenafil citrate (Viagra) is an oral treatment for erectile dysfunction. It inhibits phosphodiesterase (PDE) type 5, which degrades cyclic guanosine monophosphate (cGMP) in the corpus cavernosum of the penis. Increased levels of cGMP result in relaxation of smooth muscle within the corpus cavernosum and increased inflow of blood.
Symptoms and signs
Although sildenafil is relatively selective for PDE type 5, it has limited activity on PDE type 6 – an enzyme involved in light excitation within the retina. Approximately 3 percent of men taking the standard 50mg dose experience visual symptoms for up to several hours. These include a bluish tinge to the visual field, hypersensitivity to light, and occasionally blurred vision. Symptoms are more frequent with higher doses.
Numerous cases of non-arteritic ischaemic optic neuropathy (NAION) have been reported within 24 to 36 hours of sildenafil ingestion. As with NAION in general, men with cardiovascular comorbities (e.g., hypertension, diabetes, elevated cholesterol, or dyslipidemia) may be at increased risk of this complication.
Management
Many patients are prepared to accept temporary alterations in their color vision or light sensitivity. Previous episodes of NAION may be considered a relative contraindication.
2. Tamoxifen
Tamoxifen is an oestrogen inhibitor used principally in the adjuvant treatment of selected patients with breast cancer. Retinal toxicity occurs in approximately 1 percent of patients. The incidence correlates with the accumulated dose: clinical signs are rarely seen in patients taking the standard 10mg dose for less than a few years.
Symptoms and signs
The typical crystalline retinopathy presents with bilateral, multiple, crystalline, yellow deposits around the macula. This may be associated with cystoid macular edema and macular or peripheral retinal pigment epithelial changes. Vision may be impaired in severe cases. Other, rare and generally reversible ocular toxicities include vortex keratopathy and optic neuritis.
Management
Baseline ocular examination is recommended within one year of commencing tamoxifen therapy, followed by review at least every 2 years. More frequent review is required if ocular symptoms or signs develop. A decrease or discontinuation of the medication may need consideration if retinal toxicity develops. Retinal lesions usually persist after tamoxifen is discontinued.
3. Vigabatrin
Vigabatrin is an anticonvulsant used in the management of refractory epilepsy.
Symptoms and signs
The main ocular side effect is visual field constriction. This is usually bilateral, and may progress to tunnel vision. Estimates of the incidence of visual field defects vary enormously (i.e., from 0.03 percent to over 20 percent). The time of onset is unpredictable, and the mechanism is currently unknown. Although visual field constriction is irreversible in approximately 80 percent of patients, cessation of vigabatrin usually halts or slows its progression.
Management
Visual field testing is recommended at baseline, then every 6 months or when new visual symptoms develop. Progressive, symptomatic field defects may warrant consideration of discontinuation of vigabatrin.
See Also
Non-Arteritic Ischemic Optic Neuropathy, Cystoid Macular Edema. Toxic retinopathy – Chloroquine and hydroxychloroquine (Aralen, Plaquinel); Phenothiazine toxicity (chlorpromazine, thioridazine, Mellaril), Crystalline maculopathy (Orobronze, canthaxanthine), Talc retinopathy, Toxic and nutritional optic neuropathies
Figure 1
Vigabatrin field constriction.