Pain is the commonest symptom and its severity ranges from a feeling of eye strain (30 %) or foreign body sensation (3 %) to a retro-orbital pain (20 %) [9]. In the majority of patients, it is a non-specific and benign symptom. However, in certain patients, ocular pain in dengue may represent a sight-threatening event such as acute angle closure [40]. In this particular patient though, who was female with fairly short axial length of 22 mm and normal posterior segments on B scan ultrasonography, it is uncertain if the angle closure was a direct complication of dengue infection or a coincidental occurrence. Of greater significance is a 6-year-old child who presented with what was thought to be acute angle closure initially and was subsequently diagnosed as panophthalmitis which resulted in permanent visual loss [41]. Hence, although pain in dengue often has no detrimental outcome, a high index of suspicion needs to be maintained for potentially blinding causes.

Other symptoms commonly reported include blurring of vision (10 %), diplopia (3 %) and floaters and flashes (3 %) [9]. However, a significant proportion of patients with maculopathy may be asymptomatic, and although an abnormal Amsler chart test has high specificity of 95 % for maculopathy, nonetheless its sensitivity is only 30 % [6, 8]. Therefore, the absence of visual symptoms does not exclude the presence of ocular involvement, and systematic screening especially in patients with severe dengue remains the main means of detection.

Although macula changes such as cotton wool spots, oedema and haemorrhages may be observed together in association with major changes in the peripheral retina or with an optic neuritis [3, 18, 19, 21, 22, 31, 37, 45, 53], a predominant involvement of the macula with none or minimal peripheral changes is increasingly being reported as a cause of visual symptoms in patients with dengue, occurring in up to 10 % of patients hospitalised with dengue. It is usually a bilateral condition (73 to 80 %), although it may be asymmetrical, manifesting about 1 week after the onset of fever, when the platelet counts are generally at their lowest. Blurring of vision (18 to 100 %) and central scotoma (7 to 90 %) are the most common complaints. Less commonly, the patients may complain of floaters or metamorphopsia (5 %). The presenting visual acuity varied from 20/25 to count fingers closely, corresponding to the extent of macula oedema. In addition to oedema, other features of dengue maculopathy include foveolitis, small white or yellow subretinal dots, intraretinal haemorrhages, retinal epithelial swelling, vascular sheathing, optic disc hyperaemia and vitritis [6, 19, 21, 22, 35, 47, 54, 55] (Figs. 13.1, 13.2 and 13.3).

The term foveolitis is used to describe a small, well-circumscribed yellow-orange subretinal lesion localised to the fovea (Fig. 13.4a, b). These lesions measure between 0.2 and 0.5 mm and may be seen as an isolated lesion or in combination with the other findings described above such as retinal haemorrhages and vascular sheathing. Optical coherence tomography (OCT) of these lesions shows a disruption of the outer sensory retina with or without any accompanying elevation of adjacent retina [54, 55]. In addition to foveolitis, other OCT changes that have been seen in eyes with dengue maculopathy include diffuse retinal thickening, cystoid macular oedema and exudative detachment [19, 22, 56]. All these changes resolved within a month, but the visual function was not always similarly restored. Teoh et al. showed that the different types of OCT changes were useful predictors of visual outcome at 2 years. Patients with diffuse retinal thickening had the least visual disturbance at presentation with more than 85 % having 20/40 or better vision and the best visual outcome. Only 30 % had a residual scotoma and all had 20/40 or better vision. In contrast, the majority of eyes with foveolitis (96 %) presented with 20/80 or worse vision, and although the Snellen acuity improved to 20/40 or better in at least 60 %, all eyes still had a visually disturbing scotoma. While 80 % of eyes with cystoid oedema also had poor visual at presentation and extremely swollen maculae, 81 % regained 20/40 or better vision with only 56 % still experiencing a residual scotoma [56].

Fundus fluorescein angiography (FFA) may be normal in up to 70 % of eyes with dengue maculopathy. FFA changes that have been described include the presence of early hyperfluorescence that persists to the late phase, blocked fluorescence, small vessel occlusion or leakage, capillary non-perfusion, knobbly hyperfluorescence of perifoveal arterioles and early pinpoint hyperfluorescence.

Similarly, indocyanine green angiography (ICGA) may be normal in about 60 % of eyes or show mid- or late phase hypofluorescent spots or large vessel hyperfluorescence suggesting that there may be an underlying choroidopathy as well. The predictive value of angiography is uncertain as in the series from Loh et al., although FFA changes were seen only in eyes that had poor presenting visual acuity of 20/400 or worse, all recovered to 20/40 or better vision. Similarly, other authors also found that despite the presence of severe leakage or non-perfusion on angiography, the retinal changes resolved with visual acuity of 20/40 or better in the majority of eyes [19, 22, 47, 54].

The vasculopathy in dengue affects mainly the venules and arterioles and is usually evident clinically as sheathing of the involved vessels. In a few cases, however, the vasculopathy may only be obvious on FFA (Fig. 13.5a, b and c) [22]. Occlusion of the main retinal arteries have also been described with the patients presenting clinically as a branch or central retinal arterial occlusion with corresponding changes FFA [57, 58].

In addition to OCT, FFA and ICGA, ancillary tests that may be required include perimetry and electrophysiology.

The laboratory tests used to confirm a diagnosis of dengue fever are determined by the timing as well as local availability of the tests. Prior to day 5, when the patient is still febrile, the virus may be detected by means of polymerase chain reaction for viral RNA, ELISA or rapid tests for viral antigen, in particular the nonstructural protein 1 (NS1) antigen, or virus cell culture. Subsequently, as the fever settles when the viral load decreases in tandem with an increase in antibodies production, immunoglobulin (Ig)M or a rise in paired sera IgG immunoassays is preferred. Although NS1 antigen levels are highest during the first week, it may still be detectable in some patients even up to the second week of onset [63, 64].