Visual hallucinations in dementia with Lewy bodies: transcranial magnetic stimulation study

Abstract

Background: The aetiology of visual hallucinations is poorly understood in dementia with Lewy bodies. Pathological alterations in visual cortical excitability may be one contributory mechanism.

Aims: To determine visual cortical excitability in people with dementia with Lewy bodies compared with aged-matched controls and also the relationship between visual cortical excitability and visual hallucinations in dementia with Lewy bodies.

Method: Visual cortical excitability was determined by using transcranial magnetic stimulation (TMS) applied to the occiput to elicit phosphenes (transient subjective visual responses) in 21 patients with dementia with Lewy bodies and 19 age-matched controls.

Results: Phosphene parameters were similar between both groups. However, in the patients with dementia with Lewy bodies, TMS measures of visual cortical excitability correlated strongly with the severity of visual hallucinations (P = 0.005). Six patients with dementia with Lewy bodies experienced visual hallucination-like phosphenes (for example, seeing people or figures on stimulation) compared with none of the controls (P = 0.02).

Conclusions: Increased visual cortical excitability in dementia with Lewy bodies does not appear to explain visual hallucinations but it may be a marker for their severity.

Fig. 1

Boxplots showing phosphene parameters in all participants (minimum, first quartile, median, third quartile and maximum displayed). (a) Phosphene threshold boxplot – there were no significant differences between the controls and patients for phosphene threshold (P = 0.56). (b) Phosphene response rate boxplot – there were no significant differences between controls and patients for phosphene response rate (P = 0.57).

Fig. 2

Stimulus response plot averaged across subgroups of controls (n = 10) and patients with dementia with Lewy bodies (n = 9) who underwent the stimulus response paradigm. The Y-axis shows response rate expressed as a percentage of responses (out of 10) at a given stimulation intensity. The x-axis shows stimulation intensity expressed as a % of stimulator output relative to phosphene threshold.

Fig. 3

Scatter plots showing phosphene parameters in patients with dementia with Lewy bodies (DLB) against Neuropsychiatric Inventory (NPI^hall) score ((a) and (b)). (c) and (d) show phosphene parameters in all participants against phosphene complexity scores. (a) NPI v. phosphene threshold: significant negative correlation (τ = –0.49, P = 0.005); (b) NPI v. phosphene response rate: significant positive correlation (τ = 0.61, P = 0.001). (c) Complexity score v. phosphene threshold: controls and patients showed a negative correlation with phosphene threshold (controls: τ =–0.54, P = 0.003; patients: τ =–0.67, P<0.001); (d) complexity score v. phosphene response rate: controls and patients showed positive correlation with phosphene response rate (controls: τ = 0.49, P = 0.009; patients: τ = 0.55, P = 0.002). Vertical line in (a) demarcates maximum stimulation intensity; four patients had thresholds >100%. Lines in (c) and (d) show linear fits to data (dark blue: controls; light blue: patients).

Fig. 4

Histograms showing frequency distribution of (a) phosphene threshold and (b) phosphene response rate in controls and patients.