FP940 : Multifocal Electroretinogram in Eyes Undergoing Removal of Intraocular Iron Foreign Body

Dr. Pranita Sahay, S16689, Dr.Mahesh Chandra, Dr. Devesh Kumawat, Dr. Pranav Ranjan

PURPOSE: Multifocal electroretinogram (mfERG) in eyes undergoing pars plana vitrectomy (PPV) with removal of intraocular iron foreign body (IOFB).

METHOD: 20 eyes with IOFB were evaluated with mfERG at baseline; postoperative 1 week, 1 month, 3 months and 6 months. Fellow eyes served as control.

RESULTS: All patients were male with median age 25 years (range 18-55 years) and hammer-chisel injury being the most common mode of trauma (95%). Baseline mean amplitude of P1 wave (595.55 µV) was significantly reduced (p<0.01) in <2° compared to controls (1167.25 µV) which after 6 months of surgery increased significantly (776.89µV) (p=0.01), though it was still significantly worse (p<0.01) than the control. Significant delay in the baseline P1 wave mean implicit time was noted (p=.02) which did not decrease significantly at 6 month (p=.65).

CONCLUSION: mfERG P1 wave implicit time may serve as a permanent marker for retinal damage due to iron IOFB.

Introduction

Siderosis bulbi is caused by deposition of iron due to retained iron intraocular foreign body, leading to degeneration of the retina, development of cataract and secondary glaucoma in the late stages. Siderotic changes of retina become evident after a period of few months to years. Clinically it is characterized by pigmentary changes of the peripheral retina, pigmented speckling of the macula, loss of foveal reflex, altered color of the perimacular region, obliterative changes of the arteries and rusty discoloration of the optic disc.

Till date full-field electroretinogram (ERG) remains the gold standard investigative modality to assess the retinal dysfunction in patients with iron intraocular foreign body (IOFB). The development of changes in ERG has important prognostic implications and is an indication for surgical removal of the IOFB.

While ERG measures the mass potential that is the summed electrophysiological activity of the retina, multifocal electroretinogram (mfERG) is a novel electrophysiological test developed to give a topographic measure of the electrophysiological activity at the macula.

Though ERG changes in eyes with retained iron IOFB are already documented in literature, there is paucity of data regarding mfERG changes in eyes with iron IOFB. This study illustrates the changes in P1 and N1 waves of mfERG in eyes with iron IOFB at baseline and after surgical removal of the foreign body.

Material and methods

Subjects

A prospective comparative interventional study undertaken after obtaining approval from Ethics Committee, All India Institute of Medical Sciences (AIIMS), New Delhi, India. The study adhered to the tenets of the Declaration of Helsinki. Twenty eyes of twenty consecutive patients presenting to the outpatient department of Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS with antecedent history of trauma and evidence of retained iron IOFB on non-contrast computed tomography (CT) orbit were enrolled. Fellow eye of the same patients served as control. A written informed consent was obtained from all patients before enrollment into the study.

The inclusion criteria for enrollment included retained IOFB, clear ocular media, ability to fixate, normal intraocular pressure (≤21 mmHg) and patients willing to participate and available for all subsequent follow ups.

The exclusion criteria included corneal opacity, traumatic cataract, vitreous hemorrhage, endophthalmitis, retinal detachment, macular scarring, any preexisting retinal disorder before the trauma and patients not willing to participate in the study.

Parameter assessed

The preoperative clinical assessment included recording the corrected distance visual acuity (CDVA) on Snellen chart, anterior segment slit lamp examination, intraocular pressure recording (NT-530P Non-contact tonometer, Nidek Technologies), fundus and vitreous evaluation (slit lamp biomicroscopy with +90D noncontact lens) and dilated peripheral retinal examination (Indirect Ophthalmoscopy with +20D noncontact lens). The investigations conducted included photopic and scotopic full-field electroretinogram (Vision monitor Mon 2012H Metrovision, France), multifocal electroretinogram (Vision monitor, Monpack 3, Metrovision, France) and non-contrast CT scan of the orbit. The site of injury to globe was noted and classified as per ocular trauma classification (OTC) group for open globe injury.

Surgery

All affected eyes underwent 25 gauge pars plana vitrectomy with IOFB removal with help of either intraocular forceps or magnet. Isoexpansile gas (18 % sulfur hexafluoride or 14% perfluoropropane) was used for endotamponade if there was no retinal tear/ detachment and 1000 centistokes silicone oil (Aurosil, Aurolab, India) was inserted in the event of any retinal tear/ detachment. Silicone oil was removed after 3 months of surgery in eyes in which it was used.

Follow up schedule

Follow-up of patients was done at 1 day, 1 week, 1 month, 3 months and 6 months after the surgery.  The outcome measures at follow-up included CDVA, IOP, posterior segment evaluation (slit lamp biomicroscopy and indirect ophthalmoscopy) and recording mfERG for P1 and N1 wave amplitude and latency.

Statistical analysis

Analysis of the data was done using IBM SPSS 20 software. CDVA was converted to logarithm of minimal angle of resolution (Log MAR). Group averages of P1 and N1 wave amplitude and implicit time were analyzed in each retinal ring. Groups with non-parametric data were subjected to ‘Mann Whitney test’ and to assess the changes in these groups over a period of time ‘Friedman test’ was used. Groups with parametric data were subjected to ‘Two sample t test’ and to see the changes in these groups over a period of time ‘repeated measure analysis’ was done. A ‘p value’ of ≤ 0.05 was considered statistically significant.

Results

The median age of the patients was 25 years with the range being 18 to 55 years. Majority of the patients were in the age group of 20-40 years (n=15/20). All patients were male. Hammer chisel injury was the mode of injury in all patients. Fourteen of the patients presented within 30 days of trauma, 5 presented between 1month to 6 months while only 1 presented after 2 years (median time was 20 days).

None of the affected eyes had evidence of siderosis on slit lamp examination. In 80% of the affected eyes (n=16/20), isoexpansile gas was used for endotamponade while in the rest 20% eyes (n=4/20), silicone oil was used. In 3 out of the 4 eyes in which silicone oil was used, the IOFB was found to be incarcerated in the retina while in 1 eye there was localized subretinal fluid around the FB. The baseline median Log MAR CDVA in affected eyes was 0.18 units (range 0.0 to 1.0). Though the CDVA decreased at 1 week follow up, it returned to baseline from 1 month onwards.

With regard to baseline scotopic and photopic full-field ERG, the mean amplitude and latency of both ‘a’ and ‘b’ wave were not significantly different in between the affected and fellow eyes.

MfERG P1 wave

In < 2° ring, the baseline amplitude and implicit time in affected eyes were 595.5 ± 282.7 nV/deg² and 44.5 ± 7.4 msec respectively and that in fellow eyes were 1167.3 ± 464.7 nV/deg² and 41.3 ± 2.3 msec respectively. The decrease in amplitude and delay in implicit time in affected eyes were statistically different from the fellow eye at baseline (p = 0.001 and 0.02 respectively). After removal of IOFB, at 6 months postoperatively, the amplitude in affected eyes improved significantly from baseline (p = 0.010) but still remained significantly lower than that of fellow eyes (p = 0.006) (Table 2). The maximum improvement was noticed in the inner ring (< 2°) in which the P1 wave mean amplitude increased by 30.4% while in the outer ring (> 15°) the P1 wave mean amplitude increased by only 19.4%. The implicit time in affected eyes improved only insignificantly on follow-up (p = 0.65).

Similar changes were seen in P1 wave amplitude and implicit time in 2˚-5˚, 5˚-10˚, 10˚-15˚ and > 15˚ ring at baseline (Table 2 and 3 respectively). The mean amplitude in affected eyes in < 2° ring was 48.9 % less compared to that in fellow eyes (p = 0.001) while in the > 15° ring the mean amplitude in the affected eyes was reduced only by 24% compared to that in fellow eyes (p = 0.02). The mean implicit time in < 2° ring in affected eyes was prolonged by 7.8% compared to that in fellow eyes (p = 0.001) while in > 15° ring the mean implicit time in affected eyes was prolonged by 13.4% compared to that in fellow eyes (p = 0.002). Maximum delay in the mean implicit time was noted in the paracentral 2°-5° ring (18.8%).

On follow-up, variable results were seen in different rings with outer rings showing no significant difference in the amplitude between the affected and fellow eyes at 6 months. Also the implicit time in other rings did not change significantly on follow-up.

Discussion

Till date full-field ERG remains the gold standard test to assess the retinal damage by an iron intraocular foreign body as it can pick up functional abnormalities of the retina before any pathological changes are visible in the fundus or are picked up by fluorescein angiography.

In our study, both the scotopic and photopic electroretinogram were essentially normal in affected eyes in terms of amplitude and latency. Most patients had presented early, within a month after the trauma and had normal fundus thereby indicating that the retinal damage might not have been significant enough to be picked by an ERG.

Full field ERG measures the overall retinal function and often small abnormal areas at macula may remain undetected. Cones are the predominant photoreceptors in the macula. As mfERG selectively tests macular function by measuring local ERG responses from the cone-driven retina under light adapted conditions, it helps in spatial localization of the retinal dysfunction.

In this study, mfERG was affected in all eyes with IOFB in terms of both P1 and N1 amplitude and latency at baseline. This study showed that mfERG has higher sensitivity when compared to ERG in diagnosing ongoing retinal dysfunction secondary to ocular siderosis and thus may be used as a gold standard for the same.

The P1 and N1 wave amplitudes were reduced in all the 5 retinal rings with the inner retinal rings being more affected than the outer retinal rings at baseline. High density of RPE and photoreceptors in the fovea which decreases on moving to the periphery may explain the greater susceptibility of the inner retinal rings in P1 as well as N1 wave.

Reversal of retinal electrophysiological changes on ERG following surgical removal of FB have been previously described.  In the absence of further free ferrous ion release, depending upon the capacity of retina, macrophages may detoxify and store the remnant free ions. Similarly, in this study mfERG changes reversed partially after surgical removal of FB. The mean amplitude of P1 wave showed significant improvement from baseline on follow-up but did not recover completely till 6 months postoperatively.

From this study, it can be suggested that all patients with iron IOFB having a normal ERG should be subjected to mfERG examination (if facility is available). The implicit time of P1 wave may serve as a permanent marker for evidence of past retinal dysfunction due to iron IOFB. The extent of delay of implicit time on mfERG long after the foreign body is removed can help judge the severity of injury by the foreign body.

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