Agoraphobia in panic disorder (PD) has been related to abnormal balance system function. Vision influences balance and behavioural adaptations; peripheral vision influences orienting and fast defensive reactions whereas central vision analyzes details of objects. We have hypothesized that the abnormal balance function in PD could be mainly related to peripheral vision as part of a defensive alarm system in the brain. In 25 patients with PD and agoraphobia and 31 healthy controls we assessed, by posturography, balance system reactivity to video-films projected in peripheral and central visual fields (randomized sequence). Length, velocity and surface of body sway were calculated. Patients increased their body sway during peripheral stimulation, whereas controls did not; the two groups showed a similar increase of body sway during central stimulation. Anxiety levels during peripheral stimulation significantly influenced the postural response in the group of patients. These preliminary results suggest that the higher visual sensitivity to peripheral stimulation in patients with PD and agoraphobia may be linked to a more active “visual alarm system” involving visual, vestibular and limbic areas that might influence the development of agoraphobia in situations where environmental stimuli are uncertain.
Hypersensitive alarm systems have been postulated in panic disorder (PD). Klein's suffocation false alarm theory postulates that panic attacks occur when a suffocation alarm system is erroneously triggered (Klein, 1993); Gorman's neuroanatomical model postulates panic attacks as conditioned fear responses mediated by an overly sensitive fear network (Gorman et al., 2000); the three-alarms (true, false, learned) theory postulates panic attacks as the results of both spontaneous firing of fear system and conditioning processes to internal or external cues (Bouton et al., 2001). Although these theories do not completely overlap, they share the idea that hypersensitivity in alarm systems triggered by stimuli plays a key role in the pathogenesis of PD and related phobic conditions.
Patients with PD and agoraphobia often show a high sensitivity to complex sensorial environments (shopping malls, traffic, crowds) where they experience dizziness and discomfort (Jacob et al., 1993 and Asmundson et al., 1998). Most studies showed that these patients do not have specific vestibular diseases but have subclinical abnormalities of the balance system and that their balance control relies mainly on non-vestibular cues, propioceptive or mostly visual (visual dependence) (Jacob et al., 1995, Jacob et al., 1997, Asmundson et al., 1998, Perna et al., 2001 and Staab, 2006).
Overall, vision provides important information for balance adaptations and behavioural responses to surrounding stimuli; visual information from central visual fields and that from peripheral visual fields have complementary roles with specific functions and might have different effects on postural control (Berencsi et al., 2005). Keeping in mind that there are partial overlap and significant cross-talk among the areas comprising the two visual streams, preferred pathways have been proposed. Central vision involves mainly parvocellular cells and the ventral stream visual areas (V1-V2-V4, inferior occipital-temporal cortex) and analyzes details of objects near the focus of attention (Kandell, 1991, Stephen et al., 2002 and Palmer & Rosa, 2006). Peripheral vision involves mainly magnocellular retinal cells, lateral geniculate nucleus and the dorsal stream visual areas (occipital -V1-V2-V3-V5/MT- and parietal cortices); this is a fast pathway, with shorter latency and more sustained activation than the central vision pathway, and has connections with areas of limbic cortex; thus, peripheral vision scans surroundings for changing conditions and is involved in fast orienting and postural adaptations and in defensive reactions to potentially dangerous stimuli.
On this basis, we hypothesized that the patients with PD who develop agoraphobia might be hypersensitive to the influence on balance of moving visual stimuli, resembling the everyday-life environment, localized in the peripheral visual field.
