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Standard EEG and the difficult-to-assess mental status

Kanwaldeep Sidhu, S. MD

Fourth-Year Resident, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, MI, USA

Richard Balon, MD

Professor, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, MI, USA

Victor Ajluni, MD

Assistant Professor, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, MI, USA

NN Boutros, MD

Professor, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, MI, USA

BACKGROUND: Delirium commonly appears on the differential diagnostic list of psychiatric patients in acute care settings. When a patient is unable or unwilling to answer questions about orientation, determination of possible delirium or other probable etiologies becomes difficult. The role of the standard electroencephalogram (SEEG) in evaluating such patients is not known.

METHODS: Exhaustive MEDLINE and PsycInfo searches were performed for the period 1950-2007 for all articles cross-referenced for “delirium” and “EEG.” The focus was on method, comorbid conditions, demographics, and prevalence and nature of reported abnormalities.

RESULTS: We reviewed a total of 45 articles, of which 12 met criteria for more stringent review. All findings are presented in chronological order. Our analysis focuses on SEEG, although we also allude to quantitative EEG when described.

CONCLUSIONS: Diffuse slowing of the EEG is considered one of the hallmarks of an encephalopathic process and is commonly reported in psychiatric patients. The EEG may be helpful in the diagnostic evaluation of patients with a difficult-to-assess mental status.

KEYWORDS: delirium, EEG, psychiatric, slowing, organic, mental status



Assessment of mental status, particularly orientation, is essential in evaluating patients in acute situations. The differential diagnosis of either an altered or a difficult-to-assess mental status (DAMS) ranges from delirium and nonconvulsive status epilepticus to functional disorders like catatonic schizophrenia or elective mutism.

This article focuses on delirium as a cause of DAMS, but other differential diagnoses are briefly addressed as well. Delirium is common in hospitalized medical and psychiatric patients in the United States and is the most common clinical syndrome seen by consulting psychiatrists in general hospitals.1 The impact of delirium on mortality, physical morbidity, length of hospital stay, institutionalization, and health care costs is enormous.2,3 It is estimated that 10% to 15% of patients in general surgical wards and 15% to 25% in general medical wards have delirium during their hospital stays.4 In 1956, Robinson5 suggested that for hospitalized geriatric patients, the prevalence of delirium might be as high as 40%. It is now known that 30% to 40% of hospitalized patients age ≥65 have experienced an episode of delirium during their hospital stay.6 Delirium may be the first indication of serious illness, such as infection, cardiopulmonary disease, metabolic disturbance, or a medication toxicity.7 Hypo-kinetic delirium frequently occurs in psychiatric patients due to a metabolic encephalopathy that develops in the course of neuroleptic treatment.8 Relatively recently, the active introduction of anticonvulsants to the treatment of mood disturbances has resulted in a growing number of cases with delirium, particularly when anticonvulsants are used in combination with various neuroleptics, typical and atypical.8 Most research and literature on delirium focuses on the elderly, although it can occur at any age. The same delirium symptoms appear to occur in adults and children, supporting the clinical practice of making a diagnosis of delirium based on DSM criteria in patients of any age.9-11

The diagnostic criteria for delirium have evolved over time, and the first formal criteria for the term delirium appeared in DSM-III. The first DSM published in 1952 described it under acute brain syndromes, and specific causes for the disturbance were noted. DSM-II maintained the same 5 symptoms as DSM, and described 2 organic brain syndromes, psychotic and nonpsychotic types, each with an acute and chronic delineation.12 Delirium is defined by DSM-IV9 as a disturbance in consciousness characterized by reduced ability to focus, sustain, or shift attention that is accompanied by a change in cognition, or the development of a perceptual disturbance that is not better accounted for by a pre-existing, established, or evolving dementia. This disturbance develops over a short period of time (usually hours to days) and tends to fluctuate during the course of the day. In addition, disturbance of the sleep-wake cycle, incoherent speech, or altered psychomotor activity may be present. Disturbance of the sleep-wake cycle is often the first symptom of an evolving delirium. All of these symptoms can be seen in acute psychosis. In mental states where the assessment of sensorium is not possible, the differentiation between an acute delirious and an acute psychotic state may be impossible based on clinical evaluation alone. The DSM-IV further states that there should be evidence from the history, physical examination, or laboratory findings that the disturbance is a direct consequence of a general medical condition.

In their classic 1959 paper, Engel and Romano13 described delirium as a syndrome of cerebral insufficiency. They emphasized that the electroencephalogram (EEG) was by far the most sensitive and reliable indicator of delirium, and the concurrent altered mental state and slowing of background EEG activity provided solid evidence for the reduction of cerebral metabolism underlying delirium. They wrote that a significant EEG finding in delirium was the degree of change in dominant posterior frequency, rather than absolute value alone, which correlated with the altered mental state. They concluded that the presence of a disturbance in the level of awareness and the EEG changes characteristic of that disturbance provide the basis for differentiation of delirium from other psychopathologic entities.

Electroencephalography is a noninvasive, low-cost neurodiagnostic technique widely available in psychiatric hospitals. Clinical applications of the standard (noncomputerized) EEG (SEEG) include evaluation of organic etiologies and symptoms of dementia, delirium, and lithium toxicity; evaluation before electroconvulsive therapy; and routine screening of patients.14 Increasing evidence suggests a more positive role for SEEG in clinical psychiatry.14-16 Subjects with psychiatric disorders exhibit a high proportion of abnormal EEG findings. The presence of diffuse slowing of the background or the superimposition of generalized slow rhythms is strongly associated with delirium and is not commonly associated with the functional disorders (to be differentiated from psychiatric disorders secondary to general medical conditions or specific neurologic disorders). This observation underscores the potential value of the SEEG in situations in which delirium is added to the differential diagnostic list. SEEG can be easily obtained in an acute setting and does not require special expertise and equipment for interpretation. The exact diagnostic value of performing SEEG in the psychiatric emergency room or acute hospital settings is unknown.

In this review, we aim to evaluate the literature to assess whether the indications for a clinical EEG in the emergency room or acute hospital settings are established and whether the degree of usefulness of the findings is known. The role of EEG in psychiatry is ever expanding. We conducted this literature review with 2 goals in mind: (1) To encourage and facilitate the involvement of electrophysiologic laboratories in investigating the diagnosis and prognosis of such cases, and (2) to highlight difficulties and shortcomings in the EEG research of DAMS in order to guide future efforts.


Exhaustive MEDLINE and PsycInfo searches were performed for the period 1950-2007 for all articles cross-referenced for delirium and EEG. Additional searches were conducted using the terms EEG, psychiatry, and psychiatric disorders. Reference citations in these articles that had an electrophysiology component were also reviewed, and review articles were used to identify additional original research articles. For inclusion in our review, papers had to be in English, had to report studies having a group of delirium subjects (with or without other groups), and had to use ≥1 electrophysiological testing technique. Animal investigations were excluded. All articles were independently reviewed in detail by 2 of the authors (K.S.S. and N.N.B.). Papers examining electrophysiological modalities other than the standard clinical EEG (eg, evoked potentials, sleep studies, and quantified EEG) were excluded. The reviewers extracted the following information from each article: the method used for clinical evaluation, particularly the diagnostic criteria applied; the presence or absence of comorbid conditions (both Axis I and Axis II); medications used by subjects at time of study; the number and gender of the study subjects; and the prevalence of electroencephalographic (visually detected) abnormalities as well as the nature of the reported abnormalities.


The initial search identified approximately 300 English-language articles related to the topic. This was further narrowed down to 45 articles based on relevance to this review article, and after making more relevant exclusions (ie, including only SEEG papers and assuring the inclusion of a delirium group), 12 papers remained for inclusion in this review. Although our search was thorough, we cannot claim to be all-inclusive due to the nonuniform, limited nature of the literature on this topic. Therefore, we elected to summarize the findings in chronological order. Our discussion is mainly focused on SEEG, although we also allude to quantitative EEG when described.

This review resulted in a number of significant findings, which are summarized in TABLE 1. Because of both the body of literature and the varied methodology, including inclusion/exclusion criteria, a meta-analysis could not be performed.

Usually, a battery of tests including complete blood count (CBC), electrolytes, urine drug screen, urinalysis, etc, are conducted for medical clearance before patients are admitted to a psychiatric ward. One study found that when EEG is included in this battery of tests, the yield of medical illness is significantly higher (90%).17 EEG alone can help to narrow down the differential diagnosis of altered mental status (TABLE 2). A retrospective chart review study reinforced this, despite limitations of having access to only partial clinical information.18 A study of elderly patients with delirium found an increase of slow-wave activity on SEEG,19 a finding in line with Engel and Romano’s suggestion of a significant association between EEG changes and cognitive deterioration in delirium. Another retrospective study found a high percentage of abnormal EEGs in psychiatric patients and suggested that well-designed prospective studies are needed to assess the usefulness of EEG in psychiatry.20

A number of prospective studies have since been conducted on patients diagnosed with delirium. One such study in a long-term care population found a significant relationship between EEG parameters and changes in mental status as assessed by the Mini-Mental State Exam (MMSE).21 Other studies have validated the use of EEG to monitor and determine the severity of delirium.22-27


EEG studies in delirium

Study Sample Control group Assessment used Findings
Hall et al, 198017 100 (55 female); age 18 to 52 None Standard EEG and sleep- deprived EEG; battery of lab and neuropsychiatric tests 9 cases of medical illness were identified by EEGs
Lam et al, 198818 150 (74 female); age 17 to 76 None Standard EEG 11.3% of EEGs were abnormal
Koponen et al, 198922 51 (27 female); age 60 to 88 19 (13 female); age 61 to 83 Standard and quantitative EEG; MMSE; CT (head); CXR; blood tests; CSF analysis EEG slowing in standard EEG and specific changes in spectral parameters
Katz et al, 199019 102 (66 female); age 78 to 90 20 (13 female); age 20 to 40 Standard and quantitative EEG; MMSE EEG background slowing present in patients with delirium
Warner et al, 199020 88 (49 female); age 20 to 86 102 (42 female); age 18 to 73 Standard EEG 36% had an abnormal EEG finding
Katz et al, 199121 102; No well-defined groups; mean age, 85 No well-defined groups Standard and quantitative EEG; MMSE Slowing on standard EEG and specific changes in spectral parameters in patients with delirium
Jacobson et al, 199324 25 (15 female); age 57 to 87 8 (2 female); 8 (2 female); Standard 17-channel EEG and quantitative EEG studies; MMSE EEG slowing present in patients
Jacobson et al, 199323 25 (15 female); age 18 to 75 25 (11 female); 25 (11 female); Standard 17-channel EEG and quantitative EEG studies; MMSE EEG slowing with posterior predominance
Boutros, 199630 25 (11 female); age 18 to 75 25 (11 female); age 18 to 67 Standard EEG Patients with diffuse slowing had increased illness severity compared to controls
Katz et al, 200125 N = 96; 47, hospitalized; 12, diagnosed with delirium; mean age, 84.7; 66% female None Standard and quantitative EEG; CIRS; MMSE; PSMS; Stroop Test; BSRT EEG slowing in delirium group
Reischies et al, 200526 12 (7 female); mean age, 56.7 None Standard and quantitative EEG; DRS; AS; FRT A significant increase of slow brain electrical activity during delirium
Thomas et al, 200827 12, delirium + dementia; age 80 to 100 2 control groups: 23, dementia; 15, cognitively unimpaired Standard and quantitative EEG; MMSE EEG slowing in delirium group
AS: Awareness Scale; BSRT: Buschke Selective Reminding Test; CIRS: Cumulative Illness Rating Scale; CSF: cerebrospinal fluid; CT: computed tomography; CXR: chest x-ray; DRS: Delirium Rating Scale; EEG: electroencephalogram; FRT: Free Recall Test; MMSE: Mini-Mental State Exam; PSMS: Physical Self-Maintenance Scale.


This review highlights a number of important issues. First, the most important finding of this review is that there are a limited number of studies addressing DAMS. More puzzling is the paucity of studies examining patients in acute psychiatric settings presenting with altered mental status due to causes other than delirium. Second, there appears to be a consensus that diffuse slowing has a significant correlation with impairment of functions such as awareness, attention, memory, and comprehension. Third, that this important area of clinical research has been severely neglected, with a limited number of papers identified in the English language literature. Diffuse slowing of the EEG is considered one of the hallmarks of the presence of an encephalopathic process, and is one of the commonly reported EEG findings in psychiatric patients.28 Diffuse slow-wave abnormality is reported to constitute 40% of all EEG abnormalities seen in psychiatric patients.29 EEG findings in cases of delirium may include typical signs of generalized slow activity or slow activity primarily in the posterior leads.21 Thus, the EEG may be helpful in the diagnostic evaluation of patients with acute or subacute delirium, especially as a confirmatory test after clinical evaluation suggests the presence of delirium. Serial EEGs can also detect mental status changes in delirium over time, and can help in monitoring of patients diagnosed with delirium.19,25 The use of EEGs may be of critical importance as potentially reversible states of cognitive impairment may be detected, leading to prevention of disability in elderly and frail individuals. The authors concluded that background slowing is a classic hallmark of delirium.


The implication of these findings is that diffuse slowing of the EEG of an individual presenting with DAMS or altered mental status should prompt the clinician to include encephalopathy (including medication toxicity) and other structural pathologies in the differential diagnosis.30 The possible etiologies of patients presenting with a difficult-to-assess mental status appear in TABLE 2. We are introducing the term DAMS and defining it as an altered or a difficult-to-assess mental status (DAMS) ranging from delirium and nonconvulsive status epilepticus to functional disorders like catatonic schizophrenia or elective mutism. The degree of the EEG changes correlates with the severity of the encephalopathy; and thus the EEG may be used to help monitor therapy. In some delirious patients, the EEG may indicate whether the patient is suffering from focal, rather than global, impairment.16 The EEG can be an important addition to the traditional clinical evaluation, particularly when it could rule out a potential organic contributor to a psychiatric phenotype. The psychiatrists considering recommending an EEG should look for acute changes in the history or examination suggestive of an organic cause.31 A study that did not specifically target delirious patients,30 found that psychiatric inpatients with diffuse EEG slowing had significantly longer lengths of hospital stay and needed increased numbers and dosages of medications. These findings suggest that diffuse slowing of the EEG is an important variable in need of further exploration.

There are other important issues to consider, including detailed cost analysis, sensitivity, specificity, and difficulties in categorization of EEGs.32 While these issues are of great importance, they need to be addressed with more detail and clarity in future papers. It may, however, be fair to say that the EEG is a low-cost procedure and is likely to be cost-effective. However, use of EEGs for routine screening purposes has been questioned in the literature, and an approach directed at specific problems, including monitoring changes in mental status, may be of a higher value.33

Finally, the data presented above underscores the serious need for additional research addressing the role of EEG in occasions when the mental status cannot be fully assessed. EEG abnormalities that may be predictive of clinical changes in patients presenting with acute psychiatric symptoms, the nature of the abnormalities likely to be encountered, the impact of the findings on treatment choices, as well as prognosis are all areas that remain significantly underinvestigated.


Differential diagnosis and evaluation of difficult-to-assess mental status (DAMS)

Condition Commonly used initial diagnostic tests/procedures
Stroke CT scan, neuropsychological testing
COPD exacerbation Pulse oximetry, ABG, CXR
Myocardial infarction Troponins, CK-MB, ECG, CPK
CO poisoning ABG, carboxyhemoglobin
Hypoglycemia Serum glucose
Thyroid dysfunction TSH, thyroid panel
Hepatic encephalopathy AST, ALT, GGT, LDH, ALP, bilirubin
Renal failure Serum and urine electrolytes, BUN, Cr
B12 deficiency CBC, B1212 levels
Epileptiform disorders EEG, prolactin level
Intracranial mass CT scan, MRI
Intracranial bleed CT scan, MRI
Normal pressure hydrocephalus Lumbar puncture
Syphilis RPR, VDRL
Urinary tract infection Urinalysis
All substance intoxication/withdrawal syndromes Urine drug screen
Wernicke-Korsakoff syndrome History of alcohol use, MSE; ataxia, confusion, ophthalmoplegia
Neuroleptic malignant syndrome Vital signs, CBC, CPK, AST, ALT, BUN, Cr
Serotonin syndrome History of medication use, vital signs; no specific labs
Psychotic and thought disorders History and MSE
Catatonia History and MSE
Mood disorders History and MSE
Pseudodementia History and MSE
Conversion disorder History and MSE
Malingering History and MSE
Elective mutism History and MSE
Insomnia/sleep deprivation History and MSE
Dementia History and MSE
Delirium History and MSE
ABG: arterial blood gases; ALP: alkaline phosphatase; ALT: alanine aminotransferase; AST: aspartate aminotransferase; BUN: blood urea nitrogen; CBC: complete blood count; CK-MB: creatine kinase MB fraction; CO: carbon monoxide; COPD: chronic obstructive pulmonary disease; CPK: creatine phosphokinase; Cr: creatinine; CT: computed tomography; CXR: chest x-ray; ECG: electrocardiogram; EEG: electroencephalogram; GGT: gamma-glutamyl-transferase; LDH: lactate dehydrogenase; MRI: magnetic resonance imaging; MSE: mental status examination; RPR: rapid plasma reagin; TSH: thyroid-stimulating hormone; VDRL: venereal disease research laboratory.

DISCLOSURE: The authors report no financial relationship with any company whose products are mentioned in this article or with manufacturers of competing products.


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CORRESPONDENCE: Kanwaldeep S. Sidhu, MD, Wayne State University School of Medicine, 2751 East Jefferson Avenue, Suite 400, Detroit, MI 48207-4166 USA. E-MAIL: