The role of visually evoked potentials in themanagement of hemispheric arachnoid cystcompressing the posterior visual pathwaysVignesh Raja, MRCOphth,a Anupma Kumar, MRCOphth,a Jon Durnian, MRCOphth,a

Richard Hagan, PhD,b Neil Buxton, FRCS,c and William Newman, FRCOphtha

We report a case of an occipital arachnoid cyst in an infant, man-aged on the basis of changes in visually evoked potentials(VEPs). A significant asymmetry of VEP responses prompted neuro-surgical intervention, which improved visual behavior and electricalresponse to both pattern and flash stimuli.

Case Report

A5-month-old girl with a confirmed antenatal diag-nosis of an occipital arachnoid cyst measuring 3.3� 2.8 � 3.9 cm (Figure 1A) was referred for oph-

thalmological evaluation. The cyst was located betweenthe occipital poles, distorting the left side more than theright, with suspected compression of the left occipital cor-tex and optic radiations.

On examination, she appeared to have good central fix-ation but was less interested in objects presented fromthe right side. Anterior segment and fundus examinationof both eyes were unremarkable. No significant refractiveerror was noted. Pupillary responses were normal. Therewas no evidence of nystagmus or strabismus.

On 3-channel flash visually evoked potential (VEP) test-ing (2 cds/m2 flash intensity delivered via a Ganzfeld bowl),binocular responses showed a consistent asymmetry withright occiput response significantly greater than left occi-put recordings (Figure 2); the right occiput recording wasalso larger than the midline response. Binocular pattern re-versal VEP showed a possible response to 500 checks anda repeatable response to 1000 checks (Figure 3), with theright occiput response greater than the left. The asymme-try in VEPs strongly suggested that the arachnoid cyst wascompressing the optic radiations passing toward the left

Author affiliations: aDepartment of Ophthalmology, Alder Hey Children’s Hospital,Liverpool, United Kingdom; bDepartment of Clinical Engineering, Royal LiverpoolUniversity Hospital, Liverpool; cDepartment of Paediatric Neurosurgery, Alder HeyHospital, Liverpool

Institution of study: The Royal Liverpool Children’s NHS Trust, Alder Hey Children’sHospital, Eaton Road Liverpool, L12 2AP, United Kingdom.

This work was presented as a poster at the 2009 Annual Congress of the Royal College ofOphthalmologists, Birmingham, May 19-21.

The authors have no conflict of interests to disclose.Submitted June 13, 2009.Revision accepted October 11, 2009.Reprint requests: Vignesh Raja, MRCOphth, MRCS (Edin), Specialist Registrar—

Ophthalmology, Alder Hey Hospital, Eaton Road, Liverpool, United Kingdom, L12 2AP(email: drvigneshraja@gmail.com).J AAPOS 2010;14:85-87.

Copyright � 2010 by the American Association for Pediatric Ophthalmology andStrabismus.

1091-8531/2010/$36.00 1 0doi:10.1016/j.jaapos.2009.10.010

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occipital cortex. Based on these findings, the patient under-went an uneventful open marsupialization of the occipitalarachnoid cyst. Postoperative scans showed improvementin the cerebrospinal fluid collection.

Six months postoperatively, fixation was steady, and thepatient seemed equally interested in objects presented fromeither side. On 3-channel flash VEP (F-VEP) testing, bin-ocular responses were much larger on both right and leftocciput, with improved latency on the left occiput(Table 1), although a noticeable asymmetry was still pres-ent (Figure 2). On monocular stimulation, there was no ob-vious lateralization. Pattern reversal VEP (PR-VEP)showed no obvious asymmetry on the lateral channels.The midline channel response increased in amplitude anddecreased in latency (Figure 3), although responses toboth F-VEP and PR-VEP could still be considered some-what delayed (Table 1). Repeat computed tomographic(CT) imaging showed reduction in the size of the arach-noid cyst, to 1.3 � 1.5 � 1.2 cm (Figure 1B).

Discussion

Arachnoid cysts are intra-arachnoid fluid collections thataccount for about 1% of all intracranial space-occupyinglesions.1 Although most arachnoid cysts are static through-out life, some grow and exert a mass effect on adjacentstructures. Arachnoid cysts can cause a variety of clinicalsigns and symptoms, including headache, weakness, sei-zure, hydrocephalous, scoliosis, cognitive decline, andvision loss.2 Nonsurgical treatment options for asymptom-atic patients include observation through serial radio-imaging for asymptomatic patients. Surgical options forsymptomatic patients include shunt placements, craniot-omy, endoscopic fenestration, and stereotactic aspiration.2

VEPs offer an objective noninvasive method to assess thevision of infants and young children. The electrode mon-tage used at our center is ground at Fpz, common referenceat Fz, and recording electrodes at Oz (midline), O1 (leftocciput), and O2 (right occiput) (Jasper 10-20 system).The F-VEP is seen as the most robust stimulus3 and isoften easier to perform in less cooperative children.4 Whileintersubject variability is high for the F-VEP, the intertrialvariability for the same subject is good, with 15% variabil-ity of latency.5 PR-VEPs are much less variable4 and inter-subject standard deviation between trials for the samesubject has been reported as 1.51 ms.6 This low variabilitymakes PR-VEP a more sensitive and specific tool forassessing visual pathway dysfunction. The use of pattern

85

FIG 2. Binocular FVEP responses before and after surgery. Figure shows clear repeatable asymmetry, with improved response from each occiput,although a noticeable asymmetry is still present.

FIG 1. A, CT scan at presentation: parafalcine arachnoid cyst, measuring 3.3 � 2.8 � 3.9 cm, between the occipital poles compressing left side.B, CT scan 6 months after surgery: cyst reduced in size, measuring 1.3 � 1.5 � 1.2 cm.

86 Raja et al Volume 14 Number 1 / February 2010

reversal with variable check sizes has been shown to be bet-ter than flash stimulus in patients in detecting latency shiftsand hints for visual function correlating to visual acuity.7

The usual waveform is an initial negative peak (N1 orN75) followed by a large positive peak (P1 or P100) andthen another negative peak (N2 or N145). The P100 is

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FIG 3. Left, the midline response to a 500 and 1000 check pattern reversal presurgical intervention (2 runs). Right, the midline response after surgicalintervention (2 runs).

Table 1. Response amplitudes and latencies before and aftersurgical intervention

FVEPPRVEP to 100’

Visit Rocc Locc Mocc

Preop 18.6uV@ 153ms 9.4uV@ 183ms 4.5uV @199msPostop 53uV@ 158ms 19.3uV@ 155ms 7.8uV @155ms

Rocc, right occipital lobe; Locc, left occipital lobe; Mocc, monocular.

Volume 14 Number 1 / February 2010 Raja et al 87

generated largely in the striate cortex as a response to thecentral region of the visual field. The latency and ampli-tude of the P100 are the main parameters of study. Al-though VEPs are more sensitive for patients withanterior visual pathway lesions, their role in assessingoccipital lobe lesions has been established.8

In our patient, the role of the VEP was to identify signif-icant loss of function from the left occiput, suggesting poorvisual response in the right visual field: these results seemedto correlate with the clinical findings. The F-VEPs rein-forced clinical findings and proved a physiological deficitover and above that identified by the CT scan. There wasa remarkably asymmetric VEP to flash (rudimentary) andpattern (more complex visual processing) stimuli. Aftersurgery, the PR-VEP improved on all 3 recorded channels,with no obvious asymmetry; amplitude became larger, andwaveform became symmetrical.

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It is possible that VEPs improved in part with age fromthe time of presentation: latency of VEPs tends to improvewith myelination and age. In our case, the pre- and postin-tervention latencies from both sides were still delayed,most likely because of the cyst and its effect on the develop-ment of the posterior visual pathway.

References

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