Clinical electrophysiological measurements have a long history in the assessment of visual disorders caused by ocular pathology. Starting from the seminal studies of Halliday and his group of visual electrophysiological studies in multiple sclerosis (MS),⁹⁰ other neurological disorders were investigated, using techniques of electroretinogram (ERG) and visual evoked potentials (VEPs) (figure 79.1). Surprisingly, visual studies not only in MS but also in Parkinson's disease (PD) have allowed new insights into the pathophysiology of these neurological diseases and have revealed hitherto unknown aspects of visual system organization.

Figure 79.1.  

Pattern visual evoked potentials, recorded from a midline occipital electrode from the left and right eyes of a healthy subject (A) and two patients who were recovering from acute attacks of optic neuritis in the right eye with onset four weeks (B) and three weeks (C) previously. (From Halliday AM, McDonald WI, Mushin J: Visual evoked response in diagnosis of multiple sclerosis. Br Med J 1973; 4:661–664; with permission of Lancet.)

PD is generally known as a movement disorder, neuropharmacologically as a dopaminergic deficiency syndrome affecting the basal ganglia, and anatomically as loss of dopaminergic neurons of these structures. However, in the last two decades, anatomical, biochemical, neurophysiological, and clinical studies have demonstrated involvement of the central nervous system (CNS) beyond the basal ganglia. One of the affected areas is the visual system from the lowest level, from the retina up to the frontoparietal cognitive centers of the brain.

Because PD is predominantly a disease of the elderly, it is not surprising that many patients have visual complaints, such as tired eyes, blurred vision, and difficulty in reading. They may represent various etiologies and clinicians do not relate these nonspecific complaints to a disease known to be a “movement disorder.” Visual abnormalities specific to PD are usually hidden and not likely to be uncovered during a routine neurological examination or by ordinary high-contrast visual acuity (VA) testing. Contrast sensitivity (CS), a measure that can be affected independently from VA, provides a sensitive test for vision impairment in neurodegenerative diseases.^26,28 Nonspecific visual complaints may, however, be related to impaired CS. Intact CS is very important for most visual functions,¹³⁹ for example, for the normal perception and discrimination of depth.¹⁶⁰ It is determined by the inverse of the minimal contrast necessary to distinguish objects of patterns presented at a given spatial frequency (SF). CS is abnormal in PD.^{24,30,39,58,60,105,157} However, reduced CS in PD goes undocumented in the majority of patients, as many vision care specialists are not aware of testing for a potentially profound CS deficit in a patient with near normal VA. In CNS lesions, a VA score no worse than 20/40 may go along with a 20-fold reduction in CS to size of targets of considerable practical significance for everyday vision.^26,28 The spatial and temporal selectivity of visual losses detected with CS in PD is consistent with the results of electrophysiological tests (electroretinogram [ERG] and visual evoked potentials [VEP]). Additionally, it has been shown that idiopathic PD patients, subjects with drug-induced parkinsonism,¹⁰⁹ and animals with experimentally induced parkinsonism^81,82,84,110 exhibit similar visual impairments. The specificity of the common visual loss is likely to be due to dopaminergic deficiency. In all of these conditions, the visual impairment has been established with psychophysical and electrophysiological measures and supported by the results of neuropharmacological and histochemical studies.

MS commonly involves the visual pathways, as was noted as early as 1890 by Uhthoff.¹⁶⁷ A spectrum of visual complaints exists, ranging from the manifestations of acute temporary demyelination of the optic nerve, resulting in sudden visual loss, to subtle clinical disturbance, which could be discovered only with neurophysiological or psychophysical testing. One of the breakthroughs in establishing the clinical value of VEP occurred when Halliday et al.⁹⁰ first described that in carefully examined MS patients who have never suffered optic neuritis, commonly over 90% of subjects had abnormal, delayed VEPs. The original results, with slightly different percentages, were proven many times (see, for example, Logi et al.¹¹⁴). It is an established interpretation of visual deficits in MS that many patients may have suffered an asymptomatic involvement of the visual pathway.^71,115 An understanding of this type of subclinical disease has been much aided by the availability of magnetic resonance imaging (MRI). Nearly all of the studies provide evidence of more or less continuous disease activity,^111,125,200 suggesting that even acute MS lesions do not give rise to symptoms or clinical signs. Recent data suggest that the damage caused by progressive subclinical lesions have a larger impact on the visual pathways than the damage caused by acute optic neuritis itself.¹¹⁵ Besides simple visual loss, MS patients are known to have heterogeneous neurocognitive disturbances, such as dysfunction of attention, visuospatial perception, memory, and executive mechanisms.^141,153

In this chapter, electrophysiological studies of visual impairment in PD and MS are supplemented by psychophysical and imaging data when appropriate. We shall focus our discussion on the known physiology of neuronal receptive fields in the retina and cortex and on the relationship between the physiology of the visual pathways and the known or putative pathogenesis of PD and of MS. For each disease, we first discuss retinal, optic nerve, and primary visual cortical causes of visual impairment. Second, we discuss visual electrophysiological measurements that address cognitive aspects of visual processing, most likely involving extrastriate and nonoccipital cortices. We emphasize the clinical importance of new or newer versions of electrophysiological techniques that have emerged in the last decades as the result of physiological and pathophysiological studies of the visual system.