Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

Thyroid dysfunction

Adrenocortical hyperactivity and corticosteroid

Liver dysfunction

Duchenne and Becker muscular dystrophies

Myotonic dystrophy

Albinism

NON-ORGANIC VISUAL LOSS

Non-organic or functional visual loss occurs when the nature or the amount of visual impairment is incompatible with objective clinical findings. Non-organic visual loss has been classified into two categories. In the ‘‘malingering’’ type, willful pretension or exaggeration of symptoms is consciously made for personal gains. In the so-called ‘‘hysteria’’ type, non-organic symptoms are the result of subconscious process. Because differentiation between the two types depends on knowing the psychological origin of the symptoms, determining whether the non-organic visual loss is strictly due to ‘‘malingering’’ or ‘‘hysteria’’ is not always possible. Thompson (1) further categorized patients with non-organic visual loss into four groups. The ‘‘deliberate malingerer’’ purposely feigns visual loss. The ‘‘worried imposter’’ willingly exaggerates visual loss but worries that there may be serious disease. The ‘‘impressionable exaggerator’’ believes disease is present and is determined not to hide his disease. The ‘‘suggestible innocent’’ is convinced the symptoms are real but remains inappropriately complacent. The diagnosis of nonorganic visual loss is made on the basis of excluding organic diseases, which requires thorough clinical examination and appropriate work-up. Therefore, patients with occult or early organic disease may be erroneously diagnosed as having nonorganic visual loss. Further, non-organic visual loss may occur in the presence of concurrent unrelated organic ocular disease as well as in visually asymptomatic patients who have non-physiological test results such as tunneling of visual fields. On the other hand, patients with mild vague complaints and normal clinical examinations may not necessarily

be given a diagnosis of non-organic visual loss. The prognosis of non-organic visual loss is variable and unpredictable (2,3).

Many techniques and maneuvers are available to elicit non-physiological subjective visual acuity and field results in patients suspected of non-organic visual loss (4,5). However, with minimal coaching, some normal persons can easily imitate reproducible quadrantic, hemianopic, and altitudinal visual fields with automated or manual perimetry (6). In cases of equivocal non-organic visual loss, electrophysiological tests are helpful to exclude organic diseases. Diffuse retinal dysfunction can be determined by full-field ERG. Focal retinal dysfunction may be detected by focal and multifocal ERG, but the patient should be monitored for intentional or unintentional poor fixation that may produce falsely impaired results. Suppressed multifocal ERG responses have been reported in patients with non-organic visual loss as well as healthy volunteers who attempted suppression with inattention, poor fixation, and by adjusting the focus to the greatest blur (7).

The use of pattern and flash VEP in diagnosing nonorganic visual loss has been advocated by several investigators (8–12). Steele et al. (10) found that objective estimate of visual acuity based on pattern VEP is useful in diagnosing non-organic visual loss. Subsequently, Xu et al. (12) reached the same conclusion in a study of 72 patients. In addition, Towle et al. (11) found that the P300 component of the VEP, whose amplitude is not influenced by stimulus pattern size, can also be helpful in determining non-organic visual loss.

Voluntary suppression of the VEP response is possible in some normal persons so an abnormal result in patients suspected of non-organic visual loss should not be attributed automatically to organic disease. Pattern onset=offset and flash VEP are less susceptible to this effect than pattern reversal VEP. Uren et al. (13) found that both the amplitude and the latency of the VEP response may be impaired in normal persons by deliberate poor fixation and defocusing. Therefore, direct observation of the patient during VEP is essential to reduce poor fixation. Bumgartner and Epstein (14) noted that of the 15 normal subjects they studied, one-third was capable of altering or obliterating the pattern

reversal VEP responses by maneuvers such as meditation, daydreaming, and convergence. Likewise, Morgan et al. (15) found approximately 20% of their 42 normal subjects were able to consciously extinguish or alter the pattern reversal VEP, but only about 10% could extinguish the flash VEP. In the same study, no significant change in pattern reversal VEP implicit time and no change in flash VEP amplitude were found in the normal subjects who tried but failed to extinguish their VEP.

Taken together, a diagnosis of non-organic visual loss is well supported when unequivocal normal responses are obtained from a combination of retinal and cortical electrophysiological tests that may include the full-field ERG, multifocal ERG, pattern ERG, pattern VEP, and flash VEP. For instance, Ro¨ver and Bach (9) pointed out that in the presence of an impaired VEP in suspected non-organic visual loss patients, a normal pattern ERG is helpful to confirm the diagnosis. Similarly, the diagnosis of non-organic visual loss is reasonably founded when the full-field ERG, multifocal ERG, and pattern VEP are all well within normal limits. On the other hand, impaired results in tests that require good fixation or attention or both such as the multifocal ERG and pattern VEP, may be due to voluntary maneuvers and may not necessarily be signs of organic disease.

OCULAR DISORDERS

Hyperopia, Myopia, and Myopic

Retinal Degeneration

The ERG responses are impaired in moderate and high myopia even in the absence of myopic retinal degeneration. The reduction in both scotopic and photopic ERG amplitudes correlates directly with increased axial eye length rather than directly with refractive error (16–19). The ERG amplitude reductions generally are more likely to reach clinical significance for myopia of more than 5 diopters. Compared to ERG amplitudes, the b-wave to a-wave amplitude ratios, implicit times, and retinal sensitivity, as measured by

increased stimulus intensity ERG responses, are less likely to be affected (17–19). Reduced and delayed multifocal ERG is also found in myopia and is primarily the result of cone function loss (20). In addition, EOG reductions are associated with myopia but a clear correlation between axial eye length and EOG light-peak to dark-trough amplitude ratios was not found (16,21).

Myopic retinal degeneration refers to progressive retinal thinning and chorioretinal atrophy that occur in some highly myopic persons. In contrast to myopia without retinal degeneration, myopic retinal degeneration causes notable reduction in EOG that is somewhat related to the extent of the chorioretinal atrophy (21,22). Scotopic and photopic full-field ERG responses are generally moderate to severe impaired with the b-wave being more likely to be affected than the a-wave (Fig. 16.1) (22).

Electrophysiological responses in hyperopia are less well studied. In a study of 31 patients with refractive errors of greater than þ5 diopters, Perlman et al. (17) noted no consistent full-field ERG abnormalities nor a correlation between ERG responses and axial eye length.

Cataract and Media Opacities

Cataract and other media opacities affect electrophysiological responses by absorbing, reflecting, and scattering the incoming light stimulus. Dense opacities such as brunescent cataract and dense vitreous hemorrhage that absorb light are likely to result in reduced and prolonged full-field ERG responses, but even an extremely dense media opacity is unlikely to completely extinguish standard full-field ERG responses in an otherwise normal eye. Opacities such as mature white cataract that scatters light are unlikely to diminish the full-field ERG responses. On the contrary, a unilateral dense white mature cataract produces a modest increase in the pupil–light reaction indicating greater visual afferent input (23).

When the view to the retina and the optic nerve head is obscured by a dense media opacity, the integrity of the visual

Figure 16.1 Full-field ERG responses of an 11-year-old boy with high myopia ( 14 diopters) and myopic retinal degeneration. Visual acuity was 20=200. Note the reduced and prolonged cone and rod responses.

pathway can still be assessed by a combination of diagnostic tests such as afferent pupillary defect testing, ultrasound, full-field ERG, and flash VEP. A visual pathway defect should be suspected if an afferent pupillary defect is seen in an eye with a cataract, even a very dense one (23). Ultrasound is helpful to detect anatomical alterations behind the media opacity. Several investigators have demonstrated better visual outcome after cataract extraction in patients with dense cataracts and normal or near normal preoperative fullfield ERG or VEP, or both (24–28). Because full-field ERG is a measure of retinal function and VEP tests the entire visual

system, VEP testing is helpful to determine whether optic neuropathy or more posterior visual pathway defect is present. In patients with unilateral media opacity, a comparison of the full-field ERG and VEP responses with those of the fellow eye is important to minimize misinterpretation of test results due to intersubject variability (29,30). Unfortunately, none of the diagnostic tests mentioned can consistently detect amblyopia, which may be confirmed only with an accurate past ocular history (31).

High intensity bright-flash ERG has been advocated to assess visual prognosis in patients with media opacity and preexisting ocular conditions such as diabetic retinopathy (32). Because ERG response may be affected by the media opacity or the retinal condition or both, the results should be interpreted cautiously. For instance, visual improvement may occur after vitrectomy for dense vitreous hemorrhage even when the preoperative ERG responses are nondetectable (33,34).

The effect of cataract on multifocal ERG responses has not been studied in detail. Reductions of central multifocal ERG responses are found in normal persons when light scatter is produced with liquid crustal diffuser or acrylic sheets (35,36). These findings are relatively consistent with findings of a study comparing multifocal ERG responses before and after removal of mild to moderate cataracts (37).

In terms of other cataract-related electrophysiological findings, reduced oscillatory potentials of the full-field ERG are found in patients with aphakic or pseudophakic cystoid macular edema suggesting inner retinal layer dysfunction not only in the macula but also throughout retina (38). Patients treated for unilateral congenital cataract, who have early surgery and contact lens correction and comply with occlusion therapy, show rapid VEP maturation and have a good visual prognosis (39).

Retinal Detachment

A separation between the retinal pigment epithelium and the photoreceptors, often accompanied by accumulation of serous

fluid in this potential space, is called retinal detachment. In retinal detachment, the amount of ERG deterioration is related to the size of the detachment and the degree of dysfunction of the detached retina. Both rod and cone responses may be reduced and prolonged with similar effects on the a-wave and b-wave. The ERG is likely to be severely reduced in chronic, large retinal detachment. Interestingly, mild reduced ERG may occur in the contralateral normal eye in patients with unilateral retinal detachment (40). In addition, Sasoh et al. (41) using multifocal ERG technique found impaired ERG in both attached and detached retinal areas before and after successful retinal reattachment surgery and the ERG disturbance was more widespread than the visual field abnormality. The short-wavelength cone (S-cone) system may be more prone to damage in retinal detachment than the long-wavelength and medium-wavelength cone systems (42).

The clinical role of ERG in retinal detachment is limited. Some investigators found the amplitude of the ERG response to be of prognostic value for return of visual function after successful retinal reattachment (43,44). However, this finding is not supported by other studies (45). Regardless, retinal reattachment procedures are usually undertaken without the necessity of a preoperative ERG although postoperatively ERG may be helpful to assess retinal function when expected visual improvement fails to occur. The EOG is diminished in retinal detachment, because to maintain a retinal resting potential, the integrity of the retinal pigment epithelium as well as its contact with the photoreceptors must be intact.

Injection of silicone oil or 20% sulfur hexafluoride (SF6) gas into the vitreous chamber as intraocular tamponade is an integral part of a number of retinal reattachment procedures. Frumar et al. (46) found similar impaired ERG responses in patients treated with silicone oil or SF6 gas. A progressive recovery of the ERG was observed over a 6- month follow-up period with accelerated ERG recovery following either the removal of the silicone oil or absorption of the intraocular SF6 gas bubble. The authors proposed that

the insulation effect of the tamponade agents produced the impaired ERG responses. Similar ERG recovery following silicone oil removal was also reported by Foerster et al. (47) and Thaler et al. (48). Further, Foerster found that the EOG also recovered after silicone oil removal but the EOG recovery was not as consistent as the ERG and may reflect the continued effect on the EOG from previous detachment. In contrast, Thaler found EOG fast and slow oscillations non-detectable preoperatively or postoperatively before or after silicone oil removal. Of interest, an ERG model for determining the volume conductor effect of silicone oil on the ERG was developed by Doslek (49). Based on the model, a small reduction in the ERG was not produced until at least 50% of the vitreous was replaced with silicone oil, and the ERG diminished non-linearly as the percentage of silicone oil increased further. Despite the observed ERG impairment due to silicone oil in humans, Meredith et al. (50) did not find any ERG reduction in rabbits following vitrectomy and silicone oil injection.

Pigment Dispersion Syndrome

Pigment dispersion syndrome is characterized by dispersed pigment in the anterior chamber caused by disruption of the iris pigment epithelium from friction with lens zonules during physiological changes in pupil size. The condition is associated with glaucoma, lattice degeneration, and retinal detachment. Abnormal function of the retinal pigment epithelium is suspected. In support of this hypothesis, Scuderi et al. (51) found reduced EOG light-peak to dark-trough amplitude ratios in patients with pigment dispersion syndrome compared with patients with chronic open-angle glaucoma and normal subjects. Likewise, Greenstein et al. (52) noted significantly reduced EOG amplitude ratios in patients with pigment dispersion syndrome (2.00 0.33, mean variance, 14 subjects) and patients with pigmentary glaucoma (1.78 0.29, 11 subjects) as compared to the control group (2.68 0.52).