235 Infective Causes of Epilepsy M. Bonello, MBBS, MRCP1 B.D. Michael, MBChB, MRCP, PhD1,2 T. Solomon, MBBS, FRCP, PhD, DTM&H1,2 1 The Walton Centre NHS FoundationTrust, Liverpool, United Kingdom Address for correspondence B.D. Michael, The Institute of Infection 2 The Institute of Infection and Global Health, University of Liverpool, and Global Health, University of Liverpool, 1st Floor Ronald Ross Liverpool, United Kingdom Building, 8 West Derby Street, Liverpool L69 7BE, United Kingdom (e-mail: [email protected]). Semin Neurol 2015;35:235–244. Abstract A wide range of infections of the central nervous system are responsible for both acute seizures and epilepsy. The pathogenesis and clinical semiology of the seizure disorders vary widely between the infective pathogens. The exact mechanisms underlying this are poorly understood, but appear, at least in part, to relate to the pathogen; the degree of cortical involvement; delays in treatment; and the host inflammatory response. The treatment of infective causes of seizures involves both symptomatic treatment with antiepileptic drugs and direct treatment of the underlying condition. In many cases, early treatment of the infection may affect the prognosis of the epilepsy syndrome. The greatest burden of acute and long-term infection-related seizures occurs in resource- Keywords poor settings, where both clinical and research facilities are often lacking to manage ► infection such patients adequately. Nevertheless, education programs may go a long way toward ► encephalitis addressing the stigma, leading to improved diagnosis, management, and ultimately to ► epilepsy better quality of life. Infectious Agents Causing Epilepsy used to make the diagnosis. These include elevated cerebro- spinal fluid (CSF) leukocyte counts, raised protein, evidence of Any infection of the cortex can potentially result in seizures. viral nucleic acid in the CSF by polymerase chain reaction This primarily relies upon the structural damage occurring (PCR), or intrathecal synthesis of antiviral antibody. Neuro- during the infection, although secondary inflammatory pro- imaging can also help (►Fig. 1).1,2 Encephalitis usually cesses may also provoke seizures. Certain conditions, includ- presents with an encephalopathy syndrome for which there ing viral encephalitis and parasitic infections—such as is often a broad differential diagnosis.3 Features raising “cerebral” malaria and neurocysticercosis—are inherently suspicion for infective encephalitis include a history of fever associated with seizures and the risk of developing epilepsy. or coryzal illness, recent travel, and inadequate vaccination.1 Other conditions, such as bacterial meningitis, can cause A wide range of pathogens can cause encephalitis. Antibody- seizures in the acute setting, although this is less frequent associated encephalitis, which may be paraneoplastic or de and they rarely result in long-term epilepsy. Here we will novo, is of increasingly recognized importance, but it is review the most common central nervous system (CNS) beyond the scope of this article and is covered elsewhere in infectious disorders causing epilepsy, specifically focusing this themed edition. This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. on viral encephalitis and parasitic infections. Encephalitis results in roughly 19,000 hospitalizations and 1,400 deaths per year in the United States4 with a reported Viral Encephalitis incidence between 0.7 to 13.8 per 100,000 per year.1,3,5 It is Encephalitis encompasses a broad range of pathophysiologi- an important condition as delays in diagnosis and in starting cal process that result in inflammation of brain parenchyma treatment can result in significant morbidity and mortality. and therefore the diagnosis is fundamentally a histopatho- Acute symptomatic seizures occur in 2 to 67% of patients with logical one.1,2 In view of the impracticality of obtaining brain a diagnosis of encephalitis overall.4 The incidence of acute tissue, surrogate markers of brain inflammation are routinely symptomatic seizures and the subsequent development of Issue Theme Etiology of Epilepsy; Guest Copyright © 2015 by Thieme Medical DOI http://dx.doi.org/ Editors: Philip Smith, MD, FRCP, Publishers, Inc., 333 Seventh Avenue, 10.1055/s-0035-1552619. FAcadMEd, and Rhys Thomas, BSc, MRCP, New York, NY 10001, USA. ISSN 0271-8235. MSc, PhD Tel: +1(212) 584-4662. 236 Infective Causes of Epilepsy Bonello et al. zoster virus and enteroviruses. Epidemic encephalitis most commonly occurs in geographically restricted seasonal epi- demics and is caused by arthropod-borne viruses (arbovi- ruses), although clinicians need to be vigilant to the potential for advances in the geographical distribution of these patho- gens. Globally, the most important epidemic cause is Japanese encephalitis virus, while others include West Nile virus, tick- borne encephalitis virus and Nipah virus (►Table 1).5 Herpes Simplex Virus Encephalitis Herpes simplex virus (HSV) encephalitis has an annual inci- dence of 1 in 250,000 to 500,000, although this may be an underestimate.8 Herpes simplex virus type 1 is an α herpes double-stranded DNA virus with which most people are infected by adulthood.8 It is transmitted by droplet spread and crosses the mucous membranes of the naso-oral cavity and then travels by retrograde axonal transport to sensory ganglia, predominantly the trigeminal ganglion. There it Fig. 1 Magnetic resonance image of brain (coronal T2-weighted) achieves latency, but reactivates periodically, traveling by showing typical temporal lobe changes in confirmed herpes simplex anterograde axonal transport to the cutaneous nerve end- virus type 1 (HSV1) encephalitis. ings, resulting in spread of the virus, in some cases associated with a localized vesicular eruption, “herpes labialis.” Triggers for viral reactivation include stress, local trauma, fever, epilepsy vary with the cause of encephalitis, the patient’s immunosuppression, menstruation, or hormone imbalance. comorbidities, delays in starting treatment, and the degree of The mechanisms resulting in latency and reactivation are not cortical inflammation.6 Epidemiological studies of seizures fully understood, but toll-like receptors and the type 1 and 2 and the long-term outcome for encephalitis are limited and interferon response probably play a crucial role in initially available data are mainly from passive surveillance; therefore, establishing and subsequently maintaining latency.9 Rarely, they are prone to underestimate, particularly in resource- the virus replicates in the brain, causing encephalitis although poor settings. it is unclear whether this result from further retrograde Patients with encephalitis risk developing seizures in the axonal transport after reactivation in the trigeminal ganglion, acute setting, but are also more likely to develop later or whether this is due to reactivation of HSV already latent unprovoked seizures as described by the International League within the brain (►Fig. 2). Against Epilepsy (ILAE).7 Overall, patients with encephalitis Herpes simplex virus type 1 encephalitis has a bimodal age are around 16 times more likely than the general population distribution, with one-third of cases occurring in young to develop epilepsy.4 Of those who have seizures during the adults and one-half of cases occurring in people older than acute encephalitic illness, the risk is higher at a 22 times more 50 years.10 Seizures occur in approximately 40% of people likely. Even those who do not suffer from acute symptomatic with HSV 1 encephalitis during the acute illness and may be seizures during the acute viral encephalitis are 10 times more the presenting symptom.11 The seizures are typically focal, likely to develop later unprovoked seizures. The annual with and without cognitive involvement, reflecting temporal incidence of epilepsy due to encephalitis in the United States lobe and to a lesser extent frontal lobe infection. Clinical is approximately the same as that due to head injuries, even features that typically precede seizures include alterations in though there are 1.4 million cases of CNS trauma per year, cognition, consciousness, personality, or behavior, in the compared with fewer than 50,000 cases of CNS infection.4 context of an intercurrent or recent febrile illness.3 However, This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. Most commonly, seizures develop within the first 5 years approximately 11% of patients with HSV encephalitis are of the acute encephalitic illness, but they may occur up to afebrile on admission.12 Associated clinical features include 20 years after the acute event. The pathogen may be impor- headache, nausea, vomiting, and features suggesting menin- tant in determining whether a person subsequently develops gism and raised intracranial pressure.12 The presence of acute epilepsy in the long term. For example, encephalitis caused by symptomatic seizures was associated with a worse outcome La Crosse virus has a cumulative incidence of epilepsy of 10 to in a series of patients with HSV encephalitis; furthermore, the 12% while Nipah virus encephalitis has an incidence of development of status epilepticus is associated with a worse approximately 2.2%.4 outcome in encephalitis more broadly.4,13 In addition, a greater impairment in consciousness, older age, and a delay Causes of Infectious Encephalitis in starting acyclovir treatment of > 48 hours