Patients may also experience abnormal eye movements ("oculogyric crises"), Parkinsonism, upper body weakness, muscular pains, tremors, neck rigidity, and behavioral changes including psychosis. Klazomania (a vocal tic) is sometimes present. Cause
The causes of encephalitis lethargica are uncertain. Some studies have explored its origins in an autoimmune response, and, separately or in relation to an immune response, links to pathologies of infectious disease—viral and bacterial, such as in the case of influenza, where a link with encephalitis is clear. Postencephalitic Parkinsonism was clearly documented to have followed an outbreak of encephalitis lethargica following the 1918 influenza pandemic; evidence for viral causation of the Parkinsons symptoms is circumstantial (epidemiologic, and finding influenza antigens in encephalitis lethargica patients), while evidence arguing against this cause is of the negative sort (for example, lack of viral RNA in postencephalitic Parkinsonian brain material). In reviewing the relationship between influenza and encephalitis lethargica (EL), McCall and coworkers conclude, as of 2008, that "the case against influenza [is] less decisive than currently perceived… there is little direct evidence supporting influenza in the etiology of EL," and that "[a]lmost 100 years after the EL epidemic, its etiology remains enigmatic." Hence, while opinions on the relationship of encephalitis lethargica to influenza remain divided, the preponderance of literature appears skeptical.The German neurologist Felix Stern, who examined hundreds of encephalitis lethargica patients during the 1920s, noted that their encephalitis lethargica typically evolved over time. The early symptoms would be dominated by sleepiness or wakefulness. A second symptom would lead to an oculogyric crisis.
Just 10 days before von Economos breakthrough in Vienna, Jean-René Cruchet described 40 cases of "subacute encephalomyelitis" in France.The number of people infected during the ten years of the pandemic is unknown, but it is estimated that more than 1 million people contracted the disease, which directly caused more than 500,000 deaths. Encephalitis lethargica assumed its most virulent form between October 1918 and January 1919. The pandemic disappeared in 1927, as abruptly and mysteriously as it first appeared. The great encephalitis pandemic coincided with the 1918 influenza pandemic, and it is likely that the influenza virus potentiated the effects of the causative agent of the encephalitis or lowered resistance to it in a catastrophic way. Aftermath
Many surviving patients of the 1915–1926 pandemic seemed to make a complete recovery and return to their normal lives. However, the majority of survivors subsequently developed neurological or psychiatric disorders, often after years or decades of seemingly perfect health. Post-encephalitic syndromes varied widely: sometimes they proceeded rapidly, leading to profound disability or death; sometimes very slowly; sometimes they progressed to a certain point and then stayed at this point for years or decades; and sometimes, following their initial onslaught, they remitted and disappeared. Postencephalitic Parkinsonism is perhaps the most widely recognized of such syndromes. Notable cases
Notable cases include:

Muriel "Kit" Richardson (née Hewitt), first wife of actor Sir Ralph Richardson, died of the condition in October 1942, having first shown symptoms in 1927–28.