Subretinal scar found in otherwise healthy 10-year-old

The patient came in for a routine exam and was asymptomatic.

Jeffrey S. Wigton

Jeffrey S. Dreier

A 10-year-old female patient presented to our office for an annual vision examination. It was her first visit to our office and she was completely asymptomatic. Her visual acuity with her habitual glasses was 20/25 in the right eye and 20/40 in the left eye. Best corrected visual acuity was 20/20 -0 in the right eye with –2.00 –0.50 x 102 and 20/20 -0 in the left eye with –3.00 –0.25 x 095.

The cover test showed orthophoria at distance and near. Color vision testing with pseudoisochromatic plates was 11/11 correct OD and 11/11 correct OS. Stereopsis was measured to 30 seconds of arc stereoacuity, and accommodative and convergence skills were normal.

Figures 1 and 2. The Optomap (Optos, Marlborough, Mass.) images taken on the patient’s initial visit reveal a normal fundus in the left eye (right). In the right eye (left), notice the subretinal scar extending from the superior retina to the posterior pole and then to the temporal retina. Just temporal to the macula there is a pigmented, elevated lesion surrounded by a hypopigmented area. Images: Wigton J

Pupil reflexes were normal and there was no afferent pupillary defect. Ocular motilities were full, and fields were full to confrontation. Tonometry was 11 mm Hg OD and OS by applanation. Slit lamp examination of the anterior segment was unremarkable in both eyes with deep and quiet anterior chambers and clear media.

Fundus examination through dilated pupils was unremarkable in the left eye. Fundus examination of the right eye revealed an unusual circumlinear subretinal scar extending from the superior retina to the posterior pole and then to the temporal retina. There was a 1+ DD hypopigmented area surrounding a pigmented, elevated lesion just temporal to the macula and contiguous with the scar.

What is your diagnosis?

This young patient and her mother denied any recent illness, but did mention that they have a pet dog. Possible differential diagnoses that were initially considered included toxoplasmosis and Toxocara canis. A consult with a pediatric ophthalmologist and retinal specialist was ordered, and serological tests for toxoplasmosis and toxocariasis were obtained. These lab tests were negative.

After careful fundus examination including high magnification photographs of the lesion, the retinal specialist identified the presence of a nematode (silver lesion) at the inferotemporal edge of the scar within an area of focal inflammation. A subsequent conformational change and confirmed movement of the silver lesion further established the lesion as a nematode.

Figure 3. A high magnification photograph of the lesion was helpful in identifying the presence of a nematode/maggot. Note the slight movement of the maggot when this photo by the retinal specialist is compared to the original Optomap photo from our office. There was some discussion regarding whether the nematode was along the track or the silver lesion to the left of the track. Image: Rosenberg P

In collaboration with experts in the field of intraocular nematodes, the retinal specialist made the diagnosis of ophthalmomyiasis interna based on the clinical morphology of the nematode and the pathognomonic subretinal tracks. Lima and colleagues point out that these subretinal tracks are peculiar to dipterous larvae. They note that the only other worm to produce a subretinal track in the United States is Alaria americanus, but this nematode has significantly different morphological characteristics, and its tracks may be continuous but often irregular in course.

Myiasis is a term coined by Reverend Frederick William Hope in 1840 to refer to diseases resulting from infestation of fly larvae (maggots) of the order Diptera as opposed to those caused by other insect larvae. When the infestation involves the eye, the condition is called ophthalmomyiasis.

Can involve external, internal eye

Ophthalmomyiasis can involve the external eye and adnexa (ophthalmomyiasis externa) or the internal ocular structures (ophthalmomyiasis interna anterior and ophthalmomyiasis interna posterior).

The first stage larvae of these flies are obligate parasites that burrow their way into host tissues. Adult flies, vectors (such as ticks and mosquitoes) and patients’ hands may transport eggs and larvae to the surface of the eye. Kearney and colleagues note that in humans, eggs of the Hypoderma (Edemagena) tarandi fly are usually found on the hair of the scalp and may be transferred to the eye.

The sheep nose bot fly (Oestrus ovis), whose eggs hatch while inside the female fly, can deposit larvae while still in flight by flying close to the eyes and ejecting a stream of larvae into the target area. No history of insect bite is usually present in myiasis. Such was the case with our patient, who denied insect bites or ticks.

Diagnosis made by clinical exam

Kearney and colleagues note that ophthalmomyiasis is usually diagnosed by clinical examination. In ophthalmomyiasis interna, the diagnosis is made by finding a larva in the anterior or posterior segments or by the characteristic linear tracks made by the larva beneath the retina. The specific species can be determined by its external morphology.

Gregory and colleagues, in their article on ophthalmomyiasis externa, report the following genus/species of dipterous larva as possible etiological agents in ophthalmomyiasis: O. ovis, Hypoderma bovis, H. lineatum, H. tarandi, Cuterebra sp., Gasterophilus intestinalis, Cochliomyia macellaria and Rhinoestrus purpureus.

The offending organism in our patient was identified as the first instar larva of the genus/species H. tarandi (reindeer warble fly). In the case of ophthalmomyiasis interna, the point of entry into the eye is thought to be the sclera.

Although our patient had a single subretinal track, the more characteristic presentation is one of criss-crossed subretinal tracks across the posterior pole. Ziemianski and colleagues explain that the destruction is caused by a symbiosis between the larva and the associated proteolytic bacteria that it carries. The larva destroys the tissue with toxins and thus makes proteins available for bacterial proteolytic enzymes. Enzymatic digestion, in turn, provides suitable nutrient proteins for the maggot (larva).

Some report significant sequelae, others report none

While some ophthalmomyiasis cases reported in the literature describe significant inflammation and loss of vision from sequelae such as lens dislocation, retinal hemorrhages, exudative uveitis, exudative retinal detachment, total retinal detachment, choroidal neovascularization and fibrovascular scarring, other authors report minimal visual loss or no visual loss at all despite extensive subretinal tracks and retinal pigment epithelium changes. In addition, because vision can remain unaffected, ophthalmomyiasis can be discovered on routine ophthalmic examination, as was the case with our patient.

Interestingly, the retinal specialist indicated that the pigmented elevation with surrounding hypopigmented area noted just temporal to the macula in our patient was very suggestive of previous choroidal neovascularization. Our patient was, therefore, fortunate that this process did not involve the macula.

Ziemianski and colleagues conclude that treatment depends on the location of the nematode/maggot and the presence of inflammation. However, if the maggot is subretinal, they recommend prompt photocoagulation to prevent a decrease in vision due to continued tunneling.

Treatment plan

Our retinal specialist’s treatment plan was to photodocument the intraocular nematode and monitor for any movement. He re-examined the patient in 3 days, and movement was indeed confirmed. Thermal laser photocoagulation was therefore applied.

No post treatment inflammation was noted. However, at the 4-month follow-up visit it was determined that the nematode had moved again. Although the amount of movement was small and the nematode may have already been dead, laser treatment was reapplied to be certain that the nematode was destroyed. There has been no additional movement and the patient continues to be asymptomatic, enjoying 20/20 visual acuity. The retinal specialist is now confident that the nematode is dead.

As Lima and colleagues point out, one of the unique findings with our patient is the single subretinal track in contradistinction to the more characteristic multiple, criss-cross tracks usually seen in ophthalmomyiasis interna. Including our patient, they describe three cases of ophthalmomyiasis interna involving the H. tarandi larva with singular subretinal tracks reported in Pennsylvania during the same time frame, and they speculate that additional cases will be seen.

References:

• Gass JD, Lewis RA. Subretinal tracks in ophthalmomyiasis. Arch Ophthalmol. 1976;94:1500-1505.
• Gregory AR, Schatz S, Laubach H. Ophthalmomyiasis caused by the sheep bot fly Oestrus ovis in Northern Iraq. Optom Vis Sci. 2004;81:586-590.
• Kearney MS, Nilssen AC, Lyslo A, Syrdalen P, Dannevig L. Ophthalmomyiasis caused by the reindeer warble fly larva. J Clin Path. 1991;44:276-284.
• Lima LH, Laud K, Rosenberg P, Tuller R, Olsen K, Yannuzzi L. Ophthalmomyiasis with a singular subretinal track. Am J Ophthalmol. 2010. In press. doi: 10.1016/j.ajo.2010.05.032. Proof accessed online August 24, 2010.
• Phelan MJ, Johnson MW. Acute posterior ophthalmomyiasis interna treated with photocoagulation. Am J Ophthalmol. 1995;119:106-107.
• Syrdalen P, Nitter T, Mehl R. Ophthalmomyiasis interna posterior: report of case caused by the reindeer warble fly larva and review of previous reported cases. Br J Ophthalmol. 1982;66:589-593.
• U.S. Army Center for Health Promotion and Preventive Medicine, Entomological Sciences Program website. http://www.scribd.com/doc/1740120/US-Army-18016064. Accessed October 13, 2010.
• Ziemianski MC, Lee KY, Sabates FN. Ophthalmomyiasis interna. Arch Ophthalmol. 1980;98:1588-1589.

• Jeffrey S. Dreier, OD,can be reached at Wigton Eye Care, 120 Hollywood Drive, Suite 102, Butler, PA 16001; (724) 283-3500; fax: (724) 283-3269.
• Jeffrey S. Wigton, OD, FAAO, can be reached at Wigton Eye Care, 120 Hollywood Drive, Suite 102, Butler, PA 16001; (724) 283-3500; fax: (724) 283-3269; e-mail: jwigton@wigtoneyecare.com.