Nonconvulsive status epilepticus: The diagnostic dilemma
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.107914
Source of Support: None, Conflict of Interest: None
Nonconvulsive status epilepticus (NCSE) represents a diagnostic dilemma, and so far several different definitions and diagnostic criteria have been proposed in the literature order to address and try to solve the diagnostic problems related to this condition. NCSE is practically diagnosed as an enduring epileptic condition with reduced or altered consciousness, but without major convulsive movements together with epileptiform discharges on EEG. The diagnosis of NCSE is therefore electro-clinical, and diagnostic criteria for this epileptic condition should take into account both clinical and EEG features. Based on a comprehensive search of the literature, this brief review critically appraises the electro-clinical diagnostic criteria for NCSE present in the Literature, with particular emphasis on EEG features encountered in NCSE.
Keywords: Coma, diagnosis, EEG, non convulsive status epilepticus
Non convulsive status epilepticus (NCSE) represents a diagnostic dilemma, and to date several different definitions and diagnostic criteria have been proposed in order to address and try to solve the diagnostic problems related to this condition.
The current International League Against Epilepsy (ILAE) classifications of seizures and epileptic syndromes do not encompass the status epilepticus. , Nevertheless, the NCSE is practically diagnosed as an enduring epileptic condition in which continuous or recurrent electrographic seizure activity is responsible for diverse clinical symptoms including altered mental state, behavioral and perception abnormalities like auras, vegetative disturbances or reduced or altered consciousness, but without major convulsive movements. 
This definition implicitly includes the presence of epileptiform discharges on the EEG, which proves therefore essential in the diagnostic workup of NCSE.
As the diagnosis of NCSE is electro-clinical, diagnostic criteria for this epileptic condition should take into account both clinical and EEG features. However, differential diagnosis and confirmation of NCSE can be difficult and depends mostly on EEG. In other terms, a clinical suspicion of NCSE cannot be definitely confirmed without performing an EEG.
In the first part, this concise review briefly describes and critically appraises the electro-clinical diagnostic criteria for NCSE available in the literature. The second part focuses on EEG patterns encountered in NCSE, with particular emphasis on its dynamic EEG features.
A comprehensive search of the literature on electro-clinical diagnostic criteria for NCSE was conducted. The MEDLINE (accessed by Pubmed; 1966-December 2011) electronic database was searched using the medical subject headings (MeSH): "Diagnosis", "Status epilepticus" and following free terms, combined in multiple search strategies with Boolean operators in order to find relevant articles: "non convulsive status epilepticus", "non convulsive status epilepticus", "diagnostic criteria". Only papers explicitly reporting electro-clinical (i.e., EEG and clinical) diagnostic criteria for NCSE were included. Titles and abstracts of the initially identified studies were screened to determine if they satisfied the selection criteria. Full text articles were retrieved for the selected titles. Reference lists of the retrieved articles were searched for additional publications. No language restrictions were applied.
Six articles were identified through the search strategy described above. ,,,,, It is noteworthy to consider that the most recent reports of the International League Against Epilepsy dealing with NCSE did not take into account EEG features of such an epileptic condition. In fact, the new ILAE report on terminology and concepts for organization of epileptic seizures does not consider status epilepticus at all. Status epilepticus has been considered the most conspicuous omission in this new ILAE report, , especially considering the great advances of our understanding of this condition, its various types, and their impact and relevance on the classification, pathophysiology, and management of epilepsies.  The definite diagnosis of NCSE relies on the performing of an EEG recording, so that without such a diagnostic test, NCSE may be clinically suspected, but non confirmed. As a consequence, each discussion on diagnostic criteria of NCSE should deal with EEG features of this epileptic condition. It is therefore surprising to consider that despite a pletora of published studies dealing with NCSE; only a few articles explicitly focus on EEG activity. As an emblematic example, it is worth reporting that in the most recent years (between 2005 and 2011), only two papers explicitly dealing with EEG features of NCSE were published in Epilepsia (the ufficial journal of the ILAE). ,
Furthermore, a recent, comprehensive review published in Lancet Neurology deserves to EEG only a few sentences,  whereas the most recent ufficial proposal of SE classification (including NCSE) made by the ILAE is based exclusively on clinical and aetiological characteristics, without reporting EEG features. 
All diagnostic criteria for NCSE available in the literature focus on electro-clinical features which characterize this epileptic condition.
In 1996 Young proposed following diagnostic criteria for a non convulsive seizure (NCS), which may also be adopted to define NCSE.  To define a NCS, at least one of following primary criteria and one or more of secondary criteria, with discharges of >10 seconds are required.
NCSE consists of EEG-ictal episodes that are continuous or recurrent for >30 min without improvement in clinical state or return to baseline pre-ictal EEG pattern between seizures.
Primary criteria provide a morphological description of epileptiform discharges occurring during NCS or NCSE, whereas secondary criteria 1-3 stress the fact that the ictal pattern is a dynamic entity, with epileptiform discharges showing variations in frequency or amplitude. Moreover, secondary criterion 4 points to a clinical or EEG improvement after AED administration, which by itself is another way of stating that in NCSE the ictal pattern is something dynamic.
Litt et al.  defined NCSE as distinct discharges that evolve over time with a change in frequency, amplitude and distribution, and also Chong and Hirsch included "sequential rhythmic, periodic or quasi-periodic waves at ≥1 Hz and unequivocal evolution in frequency, morphology or location" as a primary diagnostic criterion for NCSE.  These Authors specified that neither evolution in amplitude alone nor change in sharpness without other change in morphology are sufficient for the diagnosis.
Similarly to Young's secondary criterion also Chong and Hirsch proposed as secondary criterion a significant improvement in clinical state or the appearance of normal EEG patterns that were previously absent; this improvement needs to be temporally related to an acute administration of rapidly-acting AED.  These Authors specified that resolution of epileptiform discharges leaving diffuse slowing without clinical improvement and without appearance of normal EEG patterns does not satisfy the secondary criterion.
Such a diagnostic criterion raises many questions, mainly because of the conjunction "or" used by the Authors. It is well known that improvement in the EEG pattern following intravenous benzodiazepine administration may occur both in NCSE and in metabolic encephalopathies, , being therefore of limited utility in the differential diagnosis between these conditions: Improvement in the EEG after treatment does not prove that the discharges were ictal and responsible for the patient's impairment of consciousness.  As a matter of fact, instead of considering only EEG changes following benzodiazepine or AED administration, a global electro-clinical response should be considered: after intravenous treatment a marked clinical improvement may occur in NCSE, whereas such a clinical improvement does not occur in metabolic encephalopathies. ,
Evaluating only EEG features without considering clinical aspects may lead to serious consequences, such as misdiagnosis of NCSE and consequent aggressive (and potentially harmful) AED treatment. It has been therefore suggested that NCSE may be regarded as proven, only if both the EEG and the clinical state improve with AED treatment. 
Electro-clinical diagnostic criteria of NCS/NCSE have relevant clinical consequences. Considering that many AED used in NCSE are not without risks for the patients,  only "true" ictal epileptiform activity should be aggressively treated. 
It's like lion hunting: One should use guns against the beast, not against the lion's footprints on the sand. The problem is: What is ictal and what is not? What is the lion and what is footprint?
As evident from daily practice experience, when one considers only EEG, differentiating between true ictal epileptiform activities and epileptiform-like discharges which are an epiphenomenon of a severe structural encephalopathy in comatose patients is an extremely difficult task. Although some EEG activities have a higher ictal significance, each EEG pattern encountered in NCSE is nevertheless part of a so-called "ictal-interictal continuum". ,,
Diagnostic criteria for NCSE available in the literature focus on dynamic and rhythmic features of the ictal pattern, which may support the differential diagnosis between epileptic activity and non-dynamic EEG patterns that may occur in comatose patients with severe and sometimes irreversible cerebral damage. 
Together with the degree of consciousness impairment, , the presence or lack of rhythmic/evolutive EEG features may therefore permit to delineate distinct conditions (NCSE proper, comatose NCSE) different for clinical presentation, response to AEDs, and prognosis,  thus avoiding misdiagnoses and superficial generalizations.
The ictal significance of periodic epileptiform discharges is still matter of debate,  but, according to all diagnostic criteria available in the literature, EEG patterns with evolutive features (in terms of variation in frequency, amplitude and distribution) should be considered ictal.
Diagnostic criteria for NCS/NCSE available in the literature focus on dynamic and rhythmic features of the ictal pattern, which may help in distinguish epileptic activity from static, non-dynamic EEG patterns which may occur in comatose patients with severe and sometimes irreversible cerebral damage. The ictal significance of these non-dynamic EEG patterns, including periodic epileptiform discharges, is still unclear.