Epidemiology

Depressed mental status represents an alteration in arousal and is a common presenting complaint in the emergency department (ED). This presentation can be the manifestation of a wide spectrum of diseases, with the degree of impairment ranging along a continuum from sleepiness to decreased alertness to frank coma. The differential diagnosis of stupor and coma is broad and diverse (Table 16-1) but can usually be categorized into metabolic and systemic, structural, or psychogenic causes. The majority of cases are caused by metabolic or systemic derangements, whereas the remainder are usually caused by structural lesions. Psychogenic presentations are much less common. The differential diagnosis for depressed level of consciousness often overlaps with that for confusion (see Chapter 17). Frequently, diagnosis and management occur simultaneously, and a structured systematic approach is used. A thorough grasp of the underlying pathophysiology leading to the acute depressed mental state will lead to timely diagnosis and treatment.

Pathophysiology

Consciousness is defined as the awareness of one’s self or surroundings; it includes the properties of arousal and cognition. Alterations in arousal frequently are described as levels of consciousness, although they actually refer to dynamic points on a continuum ranging from fully alert to stuporous to comatose or complete unconsciousness. Conversely, cognition frequently refers to states of consciousness or awareness and is defined as the combination of orientation, the accurate perception of what is experienced; judgment, the ability to process input data to generate more meaningful information; and memory, the ability to store and retrieve information. There are many examples of medical states that alter cognition, including confusion, inattention, delusions, and dementia. However, these states do not depress the level of arousal and therefore are discussed in other chapters in this book.

The ascending reticular activating system (ARAS) is the neuroanatomic structure primarily responsible for arousal and cortical activation. It is located in the paramedian tegmental zone in the dorsal part of the brainstem (Fig. 16-1). The input of somatic and sensory stimuli to the cerebral cortex is controlled by the ARAS and functions to initiate arousal from sleep. The brain’s cognition centers are located primarily in the cerebral cortex and serve to determine the content of consciousness.

Insults to the cerebral cortex or brainstem can each independently cause depressed consciousness or coma. These structures are vulnerable to metabolic derangements, toxins, or mechanical injury. Typically, both cerebral hemispheres need to be affected to induce coma, and this also depends on the size and speed of progression of the insult. Localized, unilateral lesions in the cerebral cortex usually do not induce depressed consciousness or coma even if other cognitive functions are impaired. In contrast, a completely intact brainstem is necessary for arousal. Small focal lesions in the brainstem can affect the ARAS. If the ARAS is impaired, the cerebral cortex cannot be aroused, and depressed consciousness or coma occurs.

Differential Considerations

Potential causes of depressed consciousness can be divided into a few general categories. Metabolic or systemic causes of coma can include hypoxia, hypoperfusion, infection, toxic drug effects, or electrolyte disturbances. Hypoxia can be the result of congestive heart failure (CHF), pulmonary embolism, carbon monoxide poisoning, or severe pulmonary compromise such as occurs in chronic obstructive pulmonary disease (COPD), cystic fibrosis, and asthma. The various causes of shock can result in global hypoperfusion states leading to depression of consciousness. These include anaphylactic, septic, hypovolemic, cardiogenic, and neurogenic origins of shock. Each type of shock has its own unique characteristics, which are detailed in other chapters. Infection, both systemic (sepsis) and focal, can be another general cause of depressed consciousness. This is particularly true if central nervous system (CNS) structures are involved, as in meningitis, encephalitis, or CNS abscess. Toxic drug effects ranging from recreational drug use and intentional overdoses to therapeutic doses with adverse side effects are common general causes of depressed consciousness seen in the ED. In the elderly, adverse side effects from prescription medications are common. In addition, electrolyte and glucose abnormalities can be caused by multiple conditions, including diabetes, renal dysfunction, malignancy, and medication interactions or dosage errors.

Head trauma, stroke, tumor, and infection are the most common structural causes of coma and depressed consciousness. Traumatic causes can include subdural and epidural hematomas, intraparenchymal or subarachnoid hemorrhage, or simply contusion or concussion. Strokes occur with embolic, thrombotic, or hemorrhagic mechanisms, but it is extremely unusual for ischemic (i.e., nonhemorrhagic) stroke to depress consciousness unless a massive insult to both hemispheres has occurred (e.g., diffuse severe cerebral edema after a massive infarct) or a high-grade basilar artery stenosis or occlusion is present.¹ Depression of consciousness with CNS infections may be caused by mass effect and is common with severe bacterial meningitis, cerebral abscess or empyema, or parasitic mass. Malignancies, whether primary or metastatic, can cause depressed consciousness if the tumor mass elevates intracranial pressure (ICP) or reduces cerebral blood flow, or if surrounding edema develops rapidly.

Special consideration should be given to specific populations of patients. The elderly, in particular, are susceptible to alterations in therapeutic medication dosage and drug-drug interactions. Even seemingly minor infections, such as urinary tract infections, upper respiratory infections, or viral gastroenteritis, may cause altered mental status (see Chapter 13), depressed consciousness, or coma. In addition, immunocompromised patients with acquired immunodeficiency syndrome (AIDS) or those undergoing chemotherapy treatments for transplants, and patients with malignancy or immunologic disease are vulnerable to a multitude of opportunistic infections not commonly seen in the general patient population.

The clinical evaluation and stabilization of patients with depressed consciousness occur simultaneously with the diagnosis in the ED. The differential diagnosis of depression of consciousness is extensive but can be greatly simplified by focusing attention on the distinguishing characteristics of the available patient history and physical examination (Box 16-1).² Approaching the patient’s presentation systematically, beginning with a broad differential diagnosis, usually allows development of a short list of likely diagnoses early in the encounter.

Pivotal Findings