Documenta Ophthalmologica 73: 139-148, 1989. �9 1990 Kluwer Academic Publishers. Printed in the Netherlands.

Visual evoked potential abnormalities in chiasmal lesions

JELKA BRECELJ, MIRO DENISLIC, MIHA SKRBEC 1 University Institute of Clinical Neurophysiology; 1Institute of Roentgenology, University Medical Centre, 61105 Ljubljana, Zalo~ka cesta 7, Yugoslavia

Accepted 28 July 1989

Key words: pattern visual evoked potential, full-field and half-field stimulation, chiasmal lesions

Abstract. Pattern reversal visual evoked potentials (VEPs) were recorded from 38 patients with lesions affecting the chiasmal area. Lesions were confirmed by computer tomography and all patients had ophthalmologic examination. VEPs to full-field stimulation (0-16 ~ r) were com- pared with those obtained with half-field stimulation.

Changes in VEPs were seen as a nonrecordable or attenuated P 100 (abnormal amplitude ratio) or as a prolonged P 100. Analysis of the records showed that temporal half-field stimulation (crossed fibers) yielded a higher rate of abnormal responses (80%) than full-field stimulation (66%). The most frequent abnormality in the former stimulation was a nonrecord- able P 100 (42%) and in the latter an abnormal amplitude ratio of P 100 (41%). When the uncrossed fibers were stimulated with the nasal half-field, abnormalities were detected in 32% of responses.

Lesions in the region of the sella turcica were also associated with a high incidence of delayed responses (39% of patients when crossed fibers were stimulated). However, the magnitude of the delays was smaller (1-32 ms) compared with delays in patients with demyeli- nating disease.

Findings of this study show that half-field stimulation assists in the interpretation of responses to full-field stimulation. In addition, half-field stimulation can reveal abnormalities that are not detected with full-field stimulation.

Introduction

It has been observed by Halliday and associates [1] that lesions affecting the visual pathway in the chiasmal area show significant asymmetries in distri- bution of responses to pattern reversal full-field stimulation. This distri- bution, where the P 100 in the left eye response is seen over the right hemisphere and the P 100 in the right eye response is seen over the left hemisphere, has been termed "crossed asymmetry." Subsequent publi- cations have confirmed these observations [2, 3, 4]. Altered visual evoked potential (VEP) waveform and amplitude attenuation have also been de- scribed [1, 5]. Gott and associates [6] have found that the most frequent

140

abnormality was a delayed P 100. VEP abnormalities have been detected in the absence of any demonstrable visual field loss, and on the contrary abnormalities have not been detected even when a bitemporal hemianopsia has been clearly recordable [1, 3, 6, 7].

Half-field stimulation in the evaluation of chiasmal lesions has also been introduced [2, 3, 4, 7, 8, 9]. In comparing full with half-field stimulation an increased percentage of abnormal responses has been demonstrated in the latter stimulation [9].

In spite of the evidence that VEPs are sensitive in detecting chiasmal lesions, different studies did not yield similar conclusions of VEP changes. This indicates the need for a more systematic study of various VEP abnor- malities that occur in chiasmal lesions. Further research of half-field stimulation in the evaluation of crossed or uncrossed visual fibers damage is also necessary.

This study describes VEP changes in chiasmal lesions as well as a com- parison between full-field and half-field stimulation.

Method

The study group consisted of 38 patients (22 female, 16 male, ranging in age from 15 to 63 years, mean age 39 years) with a lesion affecting the visual pathway in the chiasmal area. The lesions were confirmed by computer tomography. In 27 patients the operative clinical material showed that two had a meningioma, four had a craniopharyngioma, and 21 had pituitary tumors.

Visual field defects were revealed by Goldmann perimetry in 22 patients. From 16 patients with normal visual fields, the visual acuity test was done in 12 patients: 8 had normal acuities, 3 had acuity reduced in one eye (6/8 or 6/10), and 1 had acuity reduced in both eyes (6/10 and 6/8).

The recording montage consisted of five electrodes placed 5 cm above the inion and 5 cm apart so that the two lateral electrodes were located 5 cm and 10cm to either side of the midline electrode. All occipital electrodes were referred to a common midfrontal electrode (Fz, 10-20 system).

Each eye was stimulated with full, nasal, and temporal half-field (0-16 ~ r). A slide of black and white checkerboard pattern reversing at 1.6 Hz was projected onto a translucent screen. Check size was 502 The luminance of white checks was 300cd/m a, that of black l l .5cd /m 2. The recording amplifiers had a bandwidth of 0.8 to 800 Hz. One hundred responses were averaged. Each stimulus was repeated at least twice. Analysis time was 250 ms, interstimulus interval 0.5 ms.

141

Table 1. Mean amplitude ratios and latencies of P100 in 34 healthy subjects. (19 females, 15 males, mean age 32 years, range 20-47 years.)

Stimulation P 100 amplitude ratio P 100 latency

mean _+ S.D. Upper limit mean _+ S.D. Upper limit (mean + 2.5 S.D.) (mean + 2.5S.D.)

Full-field 1.57 _+ 0.75 3.44 97.2 _+ 4.4 108 Left half-field 1.28 _+ 0.27 1.95 96.1 _+ 5.0 109 Right half-field 1.27 _+ 0.25 1.89 97.0 -+- 4.9 109

In the patients' records the absence of P 100, P 100 amplitude ratio, and latency of P 100 were evaluated.

P 100 amplitude was measured as peak-to-peak difference between N 70 and P 100.

In full-field responses the interhemisphere P 100 amplitude ratio was used as an estimate of asymmetry. In each eye full-field response, the P 100 amplitude was measured over both hemispheres. Thus the P 100 amplitude ratio was calculated between the electrodes 5 cm lateral to the midline on each side. In patient records with abnormal full-field ratio, P 100 was absent or significantly attenuated over one hemisphere.

In half-field responses the interocular P 100 amplitude ratio was cal- culated in order to estimate the amplitude reduction. The P 100 amplitude ratio was calculated from left half-field responses of both eyes as well as from right half-field responses of both eyes. The P 100 amplitude was measured over the 5 cm ipsilateral channels. In the patient records the P 100 amplitude ratio was calculated when the nasal half-field responses were within normal limits. Therefore in the records with abnormal half-field ratio, P 100 was absent or attenuated in responses to temporal half-field stimula- tion and P 100 was normal in responses to nasal half-field stimulation.

The latency of P 100 in full-field responses was measured over the midline channel and in the half-field responses over the 5 cm ipsilateral channel.

The control group consisted of 34 healthy subjects with normal visual acuities and visual fields. Mean values for P 100 amplitude ratio and latency and upper limits are presented in Table 1.

Results

In patients with chiasmal lesions VEP changes could be seen as nonrecord- able or significantly attenuated P 100 or as delayed P 100. The VEPs of two patients are presented in Figs. 1-4.

142

LEFT EYE 616 HIGHT EYE 6/6

:!:3

5 ~ 3 P I

Fig. 1. Visual evoked potentials to full-field stimulation in a patient with temporal loss of the visual field,

Distribution of P 100 (marked by arrow) is asymmetric. In responses to stimulation of the left eye P 100 is distributed over the opposite channels compared to responses from the right eye. Such distribution is called crossed asymmetry.

Figure 1 shows a crossed asymmetry distribution in the responses to full-field stimulation. The P 100 (marked by arrow) is lateralized over the right hemisphere in responses to left eye stimulation and over the left hemisphere in responses to right eye stimulation. This patient had temporal loss of the visual field. The distribution features of full-field responses in fact indicated that these were responses to nasal half-field stimulation. This was confirmed with separate half-field stimulation. Figure 2 shows normal VEPs (P 100 is marked by arrow) with nasal half-field stimulation and undetect- able VEPs with temporal half-field stimulation.

The responses to full-field stimulation from the second patient are within normal limits (Fig. 3). P 100 is distributed over both hemispheres in re- sponses to left eye and right eye stimulation. In this patient visual field loss was not detected. The abnormality was detected with half-field stimulation. In Fig. 4, P 100 is significantly attenuated in responses to temporal half-field stimulation of the left eye. The P 100 amplitude ratio, which was calculated

143

L E F T EYE RIGHT EYE

Fig. 2. VEPs to half-field stimulation from the same patient as in Fig. 1. On stimulation of the temporal half-field of both eyes, VEPs are absent. Stimulation of the nasal half-field yields normal responses (the wave P 100 is marked by

arrow).

between left half-field responses of the left and right eye, exceeded normal

limits (3.5). The dominant feature of the full-field responses was the asymmetric

distribution of the P 100. The asymmetry was caused by attenuation or absence of the P 100 over one hemisphere, which resulted in domination of the P 100 over the other hemisphere. The interhemisphere P 100 amplitude ratio was used as an estimate of asymmetry. In 26 of 38 patients (68%) the amplitude ratio was abnormal. In the majority of patients (19) the asym- metry occurred in responses from one eye (Table 2). When both eyes showed asymmetries (seven patients), these could be characterized either as crossed in six patients or uncrossed in one patient. Perimetry did not reveal field defects in all of these patients. In two of seven patients with binocular asymmetry and in six of 19 patients with monocular asymmetry, field defects

were not detected. Our results showed that delayed P 100 was also the abnormality observed

in patients with chiasmal lesions (Table 3). Delays were recorded in full-field

144

LEFT EYE RIGHT EYE

0 50 100 15o 2oo 250~m 0 '50 i oo 150 2oo aso. ,s ] ~ , u v i l I I I I I I [ I I I ]~J+ 5 ~ 3 2 I

Fig. 3. VEPs to full-field stimulation are within normal limits. Perimetry did not show any field defect.

responses of 16 patients (42%). The latency of P 100 was 1 to 32 ms beyond the upper limit of the normal (mean + 2.5 SD). In responses to half-field stimulation P 100 delays were recorded from 20 eyes of 15 patients (39%) when crossed fibers were stimulated, and from 13 eyes of 10 patients (26%) when uncrossed fibers were stimulated. The range of delays was similar to that recorded with full-field stimulation.

VEP abnormalities in responses to full-field stimulation were compared with those to half-field stimulation (Table 3). The results show that a higher rate of abnormal responses was recorded when mainly crossed fibers were stimulated with the temporal half-field (80%) than when both crossed and uncrossed fibers were stimulated with full-field (66%) or when only un- crossed fibers were stimulated with the nasal half-field (32%). In addition, full-field and half-field stimulation differ in the type of detected abnor- malities. In responses to full-field stimulation the most frequent abnormality was an abnormal P 100 amplitude ratio. This was recorded in 31 of 76 eyes (41%). On the contrary, when crossed fibers were stimulated with the temporal half-field, VEPs were not recordable in most cases. P 100 activity was absent in 32 of 76 eyes (42%).

145

LEFT EYE RIGHT EYE

3

"1-- o 50 l oo 150 200 25oms o 50 lOO 15o 2'00 25oiLs ]sA iv i I [ I I ; I J I I / I ,J+'

5 tl 3 2 1

Fig. 4. VEPs to half-field stimulation from the same patient as in Fig. 2. The abnormality is seen on stimulation of the left half-field. P 100 (arrow) is of reduced amplitude from the left eye compared with that from the right eye.

Discussion

The results o f this s tudy conf i rm prev ious obse rva t ions tha t VEPs are

sensit ive in de tec t ing visual p a t h w a y lesions in the ch iasmal area [1]. The

Table 2. P 100 amplitude ratio abnormalities in full field responses (n = 38 patients).

Binocular Monocular

Abnormal P 100 7 19 amplitude ratio Field defects 5 13

Table 3. P 100 delay in 38 patients with chiasmal lesions.

Stimulation Range Mean No. of No. of (ms) (ms) eyes patients

Full-field 1-32 8.1 19 16 Temporal half-field 1-33 9.6 20 15 Nasal half-field 3-22 5.6 13 10

146

Table 4. VEP abnormalities in 76 eyes of 38 patients with chiasmal lesions.

VEP abnormalities Full-field stimulation Half-field stimulation

Temporal Nasal

Absent P 100 9 32 9 Abnormal amplitude ratio 22 9 1 Prolonged P 100 10 14 13 Abnormal amplitude ratio 9 6 1

+ prolonged P 100

Total 50 (66%) 61 (80%) 24 (32%)

observed changes in VEPs included a nonrecordable, attenuated P 100 or prolonged P 100. These changes have also been reported by others [2, 3, 4, 5, 6].

Abnormal distribution of P 100 in full-field responses was observed in 68 % of our patients. However, asymmetries were rarely observed in both eyes (18 %). Uncrossed asymmetry was observed in one patient and crossed asymmetry in six patients. Crossed asymmetry distribution has been as- sociated with chiasmal lesions [1, 2, 3, 4]. This pattern has never been described in normal subjects; therefore it can be said that crossed asymmetry is a sign of a chiasmal lesion. However, our results show that crossed asymmetry is not a very frequent chiasmal abnormality. This can be expec- ted, as the decussating fibers in the optic chiasm can rarely be equally affected.

Gott and associates [6] reported that a delayed P 100 was the most prominent change of VEPs in chiasmal lesions. This was observed with full-field stimulation. However, it is well known that delays in full-field responses can be apparent, and therefore an increase in latency should be clarified with half-field stimulation [10, 11]. In our group of patients delays were not uncommon. In multiple sclerosis, delays of full-field responses range from l0 to lOOms, with mean values of about 35 to 40ms [11]. In our patients with chiasmal lesions delays were less marked (from 1 to 32 ms, with a mean value of 8 ms). However, half-field recordings permit distinction between P 100 "true" delays and P 135 "apparent" delays. When P 100 latency was measured in responses to crossed visual fiber activity, P 100 delays from 1 to 33 ms were identified in 39% of patients.

The value of half-field stimulation in clinical VEP testing has been stressed by Halliday [10]. It is supposed that full-field stimulation reflects activity of both crossed and uncrossed visual fibers, while half-field stimulation reflects activity of chiefly crossed or chiefly uncrossed fibers. Since this methodology was used in our study, it is not surprising that the sensitivity of half-field

147

responses was better than that of full-field responses. A greater percentage of abnormal responses were recorded with temporal half-field stimulation, as compared with full-field stimulation or nasal half-field stimulation.

In comparing full-field with half-field responses we found differences in VEP abnormalities. The most frequent abnormality seen in the full-field responses was an abnormal P 100 amplitude ratio. In responses to stimu- lation of crossed visual fibers with the temporal half-field the most frequent abnormality was a nonrecordable P 100. The asymmetric distribution of P 100 in full-field responses resembled the response distribution to nasal half-field stimulation. This was demonstrated with separate recording of each half-field response. In responses to temporal half-field stimulation P 100 was significantly reduced or nonrecordable. The nasal half-field response was normal and thus constituted the full-field response. Our findings confirm that half-field stimulation enables the interpretation of abnormalities found in full-field responses. The abnormalities seen in full- field responses could be differentiated with half-field stimulation as lesions in crossed or uncrossed fibers (Figs. 1, 2). In addition, half-field stimulation revealed abnormalities not seen in full-field stimulation (Figs. 3, 4).

VEP abnormalities to full-field or half-field stimulation were found in all our 38 patients, but the perimetry test showed visual loss in only 21 of them. Our results pose a question whether VEP or perimetry are better in detection of chiasmal lesions. Others [1, 7, 8] have reported that VEP abnormalities were seen before visual field defects were demonstrable. On the contrary Maitland and associates [3] have shown that no VEP abnormalities were detected even when a bitemporal hemianopsia was demonstrable. We believe that our results do not permit a conclusive comparison of VEPs with a routine perimetry test. In most patients we tested with Goldmann per- imetry the outer isopter. However, the results do suggest that VEP test is valuable in patients who do not have characteristic loss in the visual field.

Our study shows that recognizing the abnormality in crossed and un- crossed visual fibers can have important implications in evaluation of the visual function. In neurosurgery VEPs are used in evaluating the pre- operative and postoperative lesion [12]. Halliday [10] also underlines that in early signs of chiasmal compression VEPs are important when deciding to treat pituitary adenomas conservatively with bromocriptine.

Acknowledgement

We are grateful to Dr. M. Strucl for valuable comments, to the neurophy- siological assistants L. Kolnik, T. Vidmar, I. Pustotnik, and T. Zakelj who

148

made the recordings, I. Zidar for statistical analysis, and M. Kogej and B. Konec for text processing.

This study was supported by Research Community of SR Slovenia, grant No. C3-0558-306.

References

1. Halliday AM, Halliday E, Kriss A, McDonald WI, Mushin J. The pattern-evoked potential in compression of the anterior visual pathways. Brain 1976; 99: 357-74.

2. Haimovic IC, Pedley TA. Hemi-field pattern reversal visual evoked potentials. II. Lesions of the chiasm and posterior visual pathways. Electroencephalogr Clin Neurophysiol 1982; 54: 121-31.

3. Maitland CG, Aminoff M J, Kennard C, Hoyt WF. Evoked potentials in the evaluation of visual field defects due to chiasmal or retrochiasmal lesions. Neurology 1982; 32: 986-91.

4. Onofrj M, Bodis-Wollner I, Mylin L. Visual evoked potential diagnosis of field defects in patients with chiasmatic and retrochiasmatic lesions. J Neurol Neurosurg Psychiatry 1982; 45: 294-302.

5. Holder GE. The effects of chiasmal compression on the pattern visual evoked potential. Electroencephalogr Clin Neurophysiol 1978; 45: 278-80.

6. Gott PS, Weiss MH, Apuzzo M, Van Der Meulen JP. Checkerboard visual evoked response in evaluation and management of pituitary tumors. Neurosurgery 1979; 5: 553-8.

7. Flanagan JG, Harding GFA. Multi-channel visual evoked potentials in early compressive lesions of the chiasm. Doc Ophthalmol 1988; 69: 271-88.

8. Mfiller-Jensen A, Zschocke S, Dannheim F. VER analysis of the chiasmal syndrome. J Neurol 1981; 225: 334-40.

9. Staudacher Th., Reuther R, Rittmann M, Krastel H. Die Wertigkeit der visuell evozierten Potentiale (VEP) bei Kompression der vorderen Sehbahn, speziell in der Chiasmaregion. Nervenarzt 1985; 56: 560-1.

10. Halliday AM. The value of half-field stimulation in clinical visual evoked potential testing. In: Morocutti C, Rizzo PA, eds. Evoked potentials: Neurophysiological and clinical aspects. Amsterdam: Elsevier Science Publishers BV, 1985; 293-313.

11. Blumhardt LD. The abnormal pattern visual evoked response in neurology. In: Halliday AM, Butler SR, Paul R, eds. A textbook of clinical neurophysiology. Chichester, New York, Brisbane, Toronto, Singapore: John Wiley & Sons, 1987; 307-43.

12. Lennerstrand G. Visual recovery after treatment for pituitary adenoma. Acta Ophthalmol (Copenh) 1983; 61: 1104-17.