Introduction
Chloroquine and hydroxychloroquine are antimalarial agents widely prescribed for autoimmune conditions such as rheumatoid arthritis and systemic lupus erythematosus (Yam and Kwok, 2006). However, their long-term use is associated with ocular toxicity, particularly retinopathy, which can lead to irreversible vision loss. This essay explores the mechanisms, risk factors, clinical features, and management of this toxicity from an ophthalmology student’s perspective, drawing on evidence from clinical guidelines and studies. By examining these aspects, the discussion highlights the importance of vigilant monitoring to balance therapeutic benefits against potential harm, while noting limitations in current screening protocols.
Mechanisms of Toxicity
The exact mechanisms underlying chloroquine and hydroxychloroquine-induced ocular toxicity remain incompletely understood, though they are thought to involve lysosomal dysfunction in retinal cells. These drugs, being weak bases, accumulate in lysosomes, disrupting enzyme activity and leading to the buildup of intralysosomal debris (Marmor et al., 2016). This primarily affects the retinal pigment epithelium (RPE) and photoreceptors, with chloroquine exhibiting greater toxicity than hydroxychloroquine due to its higher tissue affinity (Bernstein, 1991). Indeed, animal models suggest that high doses cause RPE degeneration and photoreceptor loss, arguably mirroring human pathology (Rosenthal et al., 1978). However, translating these findings to clinical practice is challenging, as human studies are limited by ethical constraints on controlled toxicity induction. Furthermore, genetic factors may influence susceptibility, though evidence here is sparse and requires further research.
Risk Factors
Several factors increase the risk of developing retinopathy. Daily dosage is critical; for hydroxychloroquine, exceeding 5 mg/kg of real body weight heightens risk, while chloroquine’s threshold is lower at 2.3 mg/kg (Marmor et al., 2016). Duration of therapy also matters—toxicity is rare before five years but rises thereafter, reaching 20% after 20 years for hydroxychloroquine (Melles and Marmor, 2014). Other contributors include renal impairment, which impairs drug clearance, and concomitant tamoxifen use, which synergistically damages the retina (Royal College of Ophthalmologists, 2018). Age and pre-existing macular disease may exacerbate vulnerability, though evidence is mixed; for instance, older patients often have comorbidities complicating attribution of visual changes. Generally, these factors underscore the need for personalised risk assessment, yet guidelines sometimes overlook individual variability, such as in obese patients where ideal weight calculations can be imprecise.
Clinical Manifestations
Ocular toxicity typically manifests as bilateral bull’s-eye maculopathy, characterised by a ring of depigmentation surrounding a central foveal sparing. Early symptoms are subtle, including paracentral scotomas and colour vision deficits, progressing to nyctalopia and visual field loss if undetected (Marmor et al., 2016). Fundus examination may reveal RPE mottling, while advanced imaging like optical coherence tomography (OCT) shows ellipsoid zone disruption, and multifocal electroretinography (mfERG) detects functional abnormalities before structural changes (Royal College of Ophthalmologists, 2018). However, symptoms often appear late, and toxicity can progress post-discontinuation, highlighting the insidious nature of the condition. In severe cases, peripheral field constriction mimics retinitis pigmentosa, though this is less common with hydroxychloroquine. These features, while distinctive, can overlap with age-related macular degeneration, posing diagnostic challenges in elderly users.
Screening and Management
Effective management relies on regular screening. The American Academy of Ophthalmology recommends baseline fundus examination within the first year, followed by annual assessments using OCT and Humphrey visual fields for high-risk patients (Marmor et al., 2016). The Royal College of Ophthalmologists echoes this, advocating risk stratification and early cessation upon toxicity detection (Royal College of Ophthalmologists, 2018). There is no specific treatment; discontinuation halts progression but does not reverse damage, emphasising prevention. Challenges include resource limitations in screening, particularly in low-income settings where these drugs are used for malaria. Therefore, education on self-monitoring for visual changes is vital, though its efficacy is unproven.
Conclusion
In summary, chloroquine and hydroxychloroquine induce ocular toxicity through lysosomal disruption, with risks amplified by dose, duration, and comorbidities. Clinical signs like bull’s-eye maculopathy necessitate prompt screening and drug cessation to avert irreversible harm. From an ophthalmology standpoint, this underscores the need for multidisciplinary collaboration in monitoring, while acknowledging gaps in mechanistic understanding and global access to advanced diagnostics. Ultimately, refined guidelines could enhance patient safety, balancing these drugs’ efficacy against their ocular perils.
References
- Bernstein, H.N. (1991) ‘Ocular safety of hydroxychloroquine sulfate (Plaquenil)’, Southern Medical Journal, 84(2), pp. 274-279.
- Marmor, M.F., Kellner, U., Lai, T.Y., Melles, R.B. and Mieler, W.F. (2016) Recommendations on Screening for Chloroquine and Hydroxychloroquine Retinopathy (2016 Revision). Ophthalmology, 123(6), pp. 1386-1394.
- Melles, R.B. and Marmor, M.F. (2014) ‘The risk of toxic retinopathy in patients on long-term hydroxychloroquine therapy’, JAMA Ophthalmology, 132(12), pp. 1453-1460.
- Rosenthal, A.R., Kolb, H., Bergsma, D., Huxsoll, D. and Hopkins, J.L. (1978) ‘Chloroquine retinopathy in the rhesus monkey’, Investigative Ophthalmology & Visual Science, 17(12), pp. 1158-1175.
- Royal College of Ophthalmologists (2018) Hydroxychloroquine and Chloroquine Retinopathy: Recommendations on Screening. Royal College of Ophthalmologists.
- Yam, J.C. and Kwok, A.K. (2006) ‘Ocular toxicity of hydroxychloroquine’, Hong Kong Medical Journal, 12(4), pp. 294-304.
