Frequency and Correlates of Involuntary Emotional Expression Disorder in Parkinson’s Disease

Involuntary emotional expression disorder (IEED) is characterized by sudden episodes of laughing or crying that either occur spontaneously or are out of proportion to the stimuli that provoke them. A variety of terms have been used to refer to this disorder, including pseudobulbar affect, pathological crying and laughing, emotional incontinence or lability, and pathologic affect.1,2 Recently, Cummings et al.3 proposed the term involuntary emotional expression disorder (IEED) as an inclusive nosological concept to characterize patients with neurological disease or injury who experience episodic and involuntary bouts of uncontrollable emotional expression. 

IEED has been observed in patients with stroke (10-20%),4-9 traumatic brain injury (5-11%),4,5 Alzheimer's disease (39%),10 multiple sclerosis (10%),11 amyotrophic lateral sclerosis (19-49%),12,13 seizure disorders, 14 multiple system atrophy-cerebellar type,15 and corticobasal degeneration.16 However, to ourknowledge there are no previous reports on the frequency and clinical correlates of IEED in Parkinson's disease.

Classic pathophysiological theories of IEED are based on the assumptions of serial processing and hierarchical control. According to these assumptions, IEED results from the release of cortical inhibition of brainstem centers that integrate the motor activation patterns involved in laughing and crying. Thus, IEED is an essential part of the pseudobulbar palsy syndrome associated with bilateral lesions in corticobulbar pathways. However, IEED may also be seen in patients with unilateral lesions that do not involve motor or premotor areas. For instance, Ross and Rush17 reported that IEED may result from lesions of the right inferior frontal lobe in association with a major depressive disorder. 

More recently, Parvizi et al.15 suggested that the critical lesion or dysfunction eliciting IEED is located along frontoponto-cerebellar pathways.18 IEED can be seen in neurological disorders without any demonstrable lesions but with disruptions in fronto-subcortical circuits. For instance, McCullagh et al.13 studied amyotrophic lateral sclerosis patients and implicated the prefrontal cortex in the pathophysiology of IEED.

There is also evidence that IEED is related to disruption of monoaminergic modulation of both limbic structures and bulbar motor networks.19 If this hypothesis is correct, IEED should be a relatively frequent finding among patients with Parkinson's disease, who undergo a selective degeneration of aminergic pathways and brainstem nuclei.6,7,18,20 In spite of Parkinson's diease being the second most common neurodegenerative disease, little is known about IEED in Parkinson's disease. Thus, the aim of the present study was to examine the frequency and clinical correlates of IEED in Parkinson's disease. We hypothesized that IEED would be relatively common among Parkinson's disease patients, IEED would occur without comorbid depression, and IEED would be associated with excess disability.