Herpes Zoster-induced Optic Neuritis – Reporting a Rare Complication

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A 33-year-old male presented with sudden onset vision loss in his right eye for 10 days. Two weeks before the presentation, the patient had developed pain with tingling sensation over the right side of the scalp, forehead and eyelid. He was posted at high altitude area at the time of onset of his symptoms. It was followed by vesicular eruptions over the same area [Figure 1a]. He was diagnosed as a case of herpes zoster and was started on oral acyclovir 800 mg 5 times a day for 10 days. Hutchinson’s sign was negative with no involvement of ocular surface. On 7^th day of medication, the patient developed rapid onset diminution of vision in his right eye. His ophthalmic history was unremarkable with no such previous episodes. On evaluation, his best-corrected visual acuity had dropped to 6/36 in his right eye. There was no evidence of keratitis or uveitis. Intraocular pressure was within normal limits in both eyes. Fundus evaluation revealed a healthy disc with an unremarkable fundus picture. The only positive finding was presence of relative afferent pupillary defect grade II and defective colour perception in his right eye. A clinical diagnosis of atypical optic neuritis was made.

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Figure 1:

(a) Grouped vesicles seen in the distribution of the right ophthalmic branch of the trigeminal nerve suggestive of herpes zoster, (b) Magnetic resonance imaging brain and orbits with contrast revealed that the right optic nerve was thickened and revealed diffuse patch heterogenous hyperintense signal. Retrobulbar portion of the optic nerve and optic nerve sheath diameter measured 2.1 mm and 5.2 mm, respectively, on the right side while it was 1.4 mm and 4.0 mm, respectively, on the left side.

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An urgent contrast-enhanced magnetic resonance imaging (CE-MRI) of brain, spine and orbit was done. The right optic nerve was thickened in comparison to the left side and revealed diffuse patchy heterogenous hyperintense signal [Figure 1b]. Retrobulbar portion of the optic nerve and optic nerve sheath diameter measured 2.1 mm and 5.2 mm, respectively, on the right side while it was 1.4 mm and 4.0 mm, respectively, on the left side. Visually evoked potential confirmed anterior visual pathway lesion with an increased latency on the right side. These findings confirmed the diagnosis of retrobulbar neuritis. Serological tests for hepatitis B and C, human immunodeficiency virus (HIV), tuberculosis and syphilis were negative. His biochemical parameters, antinuclear antibody and angiotensin-converting enzyme were within normal limits. He received intravenous methylprednisolone 1 g daily for 3 days followed by 1 mg/kg/day for 14 days with rapid taper over the next 3 days (as per optic neuritis treatment trial). He was shifted from oral to intravenous acyclovir 500 mg 3 times a day for 15 days. After completion of the 14-day treatment, visual acuity had recovered to 6/9. However, there was a persistent inferotemporal field defect. Repeat CE-MRI of orbit after 1 month of stopping treatment revealed comparable optic nerve diameters on the right and left side (3.4 and 3.1 mm, respectively).

Herpes zoster ophthalmicus (HZO) infection is caused by the varicella zoster virus. The virus lies dormant in the trigeminal ganglion till its reactivation, usually triggered due to weakened immune function which can be caused by old age, previous viral illness, immunosuppressive drugs, HIV or past stressors.^[1] In our case, an aggravating factor could be the patient working at high altitudes with extreme cold temperatures. The appearance of a vesicular rash in the ophthalmic region of the trigeminal nerve usually precedes eye lesions, affecting the eye in 17% of cases.^[2] Ophthalmic manifestations include vesicular eyelid rash, dendritic, interstitial or macular keratitis, uveitis, secondary glaucoma, retinal necrosis, vasculitis or optic neuritis. Optic neuritis is an extremely rare complication occurring in only 0.4% of HZO.^[2,3]

The mechanism of optic nerve involvement is unknown. One of the proposed mechanisms is the direct extension of the virus to the optic nerves through the cavernous sinus whereas another suggested that postulate is the local extension causing damage to the optic nerve leading to meningoencephalitis. A more likely mechanism is ischaemia of the posterior ciliary vessels and nerves, causing significant inflammation and swelling of the nerve.^[3,4]

The close temporal association of optic neuropathy after the onset of cutaneous zoster provided the basis for the diagnosis of herpes zoster optic neuritis (HZON). Brain magnetic resonance imaging is a useful investigation for demyelinating optic findings. Erythrocyte sedimentation rate and C-reactive protein are useful in ruling out giant cell arteritis. Levels of these non-specific inflammatory markers have previously been reported to be normal in HZON.^[4,5] Exclusion of other infectious causes of optic neuropathy may require additional serum and cerebrospinal fluid (CSF) studies. CSF analysis in HZON may show mild lymphocytic pleocytosis.^[3-5] However, it was not done in our case as all other blood parameters and investigations were normal and did not point towards any other infectious aetiology. The mainstay of HZON treatment remains intravenous acyclovir at a recommended dosage of 10 mg/kg 3 times daily for 10–14 days.^[4] Data on the supporting role of systemic corticosteroids in this condition are scarce. Concomitant use of systemic corticosteroids and antivirals remains controversial due to the risk of host immunosuppression. A study done by Tunis and Tapert reported only 25% of HZON patients responding to systemic corticosteroids.^[5]

We administered intravenous methylprednisolone with intravenous acyclovir pertaining to the fact that the ocular symptoms are caused by inflammation and swelling rather than active replication of the virus itself.^[5] A significant increase in the diameter of the optic nerve sheath on CEMRI justified the presence of severe inflammation.

We report this case as isolated optic neuritis is a rare ocular complication following HZO and high clinical suspicion is warranted for better visual recovery.