Human ocular angiostrongyliasis: a literature review

Introduction

Human ocular angiostrongyliasis, caused by Angiostrongylus cantonensis, is a non-fatal disease. However, it usually causes permanent damage to the affected eye and sometimes even blindness. Prommindaroj et al. reported the first case of A. cantonensis infection involving the eye in 1962 ¹ and, since then, cases have occasionally been reported in tropical countries. So far, 35 cases were found by our review of the literature. However, because this disease is so rare, many physicians may be unaware of it and a number of cases may have gone unreported or unrecognized. It is, therefore, important that this condition be brought to the attention of both physicians and the general public.

Methods

Data for this study were identified by searches of Medline (PubMed), ScienceDirect, Google Scholar and ISI Web of Knowledge. Search terms included ‘ocular angiostrongyliasis’, ‘ocular Angiostrongylus cantonensis’, ‘eye angiostrongyliasis’, ‘eye Angiostrongylus cantonensis’, ‘ocular guangzhouguangyuanxianchong’ (Mandarin for ocular A. cantonensis) and ‘ocular guangzhouguangyuanxianchongbing’ (Mandarin for human ocular angiostrongyliasis). Relevant articles or book chapters in English and Chinese were consulted. These databases were searched up until the end of 2009 when this article was prepared.

Results

In this study, 26 references which reported 35 patients with human ocular angiostrongyliasis were found by searching the internet.

Epidemiology

The first case of human ocular angiostrongyliasis was reported in 1962 ² and, to date, 35 more cases from 10 countries have been reported (Table 1). The ages of the patients ranged from 1–72 years, although 66% of the cases occurred in patients aged 20–40. Men were affected more frequently than women (22 versus 13).

Aetiology and pathology

In 32 of the 35 cases, only a single larva was found in the eye, 19 in a left eye and 13 in a right eye, and all except one worm were still alive at the time of diagnosis. ⁴ One probable dead larva was found between the optic nerve and the sheath and no larvae were found in two of the reported cases. Ocular angiostrongyliasis can be found in different parts of the eye including the anterior chamber and vitreous chamber (Table 2).

Clinical manifestation

The most common symptom was loss of vision. The clinical features reported in the 35 patients with human ocular angiostrongyliasis are summarized in Table 3.

Diagnosis

Human ocular angiostrongyliasis can be definitively diagnosed if the worm is detected in the eye of the patient. Worms were found in 32 (91.4%) of the 35 cases. In addition, a diagnosis of human ocular angiostrongyliasis can be based on clinical symptoms, medical history, laboratory findings in the blood and serological tests. A history of eating an intermediate host, such as snails, transport hosts, such as frogs and fish, or contaminated vegetables is highly supportive for the diagnosis of human ocular angiostrongyliasis.

Eosinophilia may also be a helpful indication for positive diagnosis. Eosinophilia of between 14–34% (normal range 0.5–5%), was found in 12 of 15 cases who had routine peripheral blood examination. ^11,15

Treatment

Surgery was performed to remove a worm in 31 cases. Prior to surgery, laser immobilization was carried out in 11 cases and retinal cryopexy in two cases. In some cases intravenous methylprednisolone was administered to lessen the intraocular inflammation. In addition, larvicidal drugs and steroid treatment were given in three cases but the method of treatment in one case was unknown. However, regardless of treatment method, the visual outcome of all 35 cases was not markedly improved from the presenting condition.

Discussion

Of the 2827 cases of human angiostrongyliasis that have been documented worldwide, ²⁷ 1.2% (35/2827) were diagnosed with ocular angiostrongyliasis. How the worm enters the eye chamber is not known, but the following route may be the most likely. After reaching the brain tissue, larvae may migrate along the surface of the brain, particularly at the base of the brain, where they may transverse the optic nerve, travelling between the nerve and sheath until they reach the eye itself.

Kanchanaranya et al. found that fifth stage A. cantonensis larvae were trapped between the optic nerve and nerve sheath and in the periorbital tissue in the rat. ¹¹ Furthermore, we found a report of ocular angiostrongyliasis in which there was a nodule between the optic nerve and the sheath on orbital CT scan. ¹⁴ Although it was not confirmed by pathology, it is highly probable that the nodule was an A. cantonensis larva. Maybe, during the process of migration to the eye between the optic nerve and the sheath, the larva died and was trapped in the affected eye.

Eosinophilic meningitis was diagnosed in half of the human ocular angiostrongyliasis cases reviewed and common symptoms included headache, neck, paraesthesia and fever. This may indicate that ocular angiostrongyliasis can occur when a worm moves randomly from the bloodstream to the eye without invading the brain or meninges. On ophthalmic examination, all cases presented with diminished vision in the affected eye and the visual acuity varied from floaters to perception of light only. ⁹ The visual acuity in the non-affected eye was normal.

If the worm is found in the eye, this disease can be diagnosed. The worm in the eye can be detected by slit lamp or funduscopic examination and it is recommended that this examination be performed in any individual presenting with a history of eating raw intermediate hosts and visual loss, with or without eosinophilic meningitis. Immunodiagnosis, such as ELISA, western blot and specific monoclonal antibody detection, may help in the diagnosis, although their sensitivity and specificity varies. ^28,29 In addition, eosinophilia was found in most cases where routine blood examination was undertaken. It is helpful for diagnosis.

Several treatments, including corticosteroid, laser and surgical removal of the parasite or a combination of therapies, were used in human ocular angiostrongyliasis. Anthelmintic drugs, such as albendazole, are not usually recommended for the treatment because dead worms may evoke severe inflammatory responses in the eye.

Although there is no evidence that surgical interventions improve the course of the disease, surgical removal is still recommended in order to prevent further ocular damage. As the actively mobile worm is difficult to remove without causing tissue damage, ^10,13 it is recommended that immobilization by laser treatment is carried out prior to surgical intervention. ¹⁵

Although several treatments have been used, the visual outcome of all 35 cases was not markedly improved. Therapeutic success depends upon early and complete surgical removal of the parasite in order to maintain the initial visual acuity.