How is an electroencephalogram (EEG) interpreted in patients with locked-in syndrome?

EEG Interpretation in Locked-In Syndrome

In locked-in syndrome, the EEG is typically normal or shows only minimal slowing with preserved reactivity to stimuli, which is the key feature distinguishing these conscious patients from those in coma or vegetative states. 1

Characteristic EEG Patterns

Normal or Near-Normal Background Activity

• The EEG in locked-in syndrome usually demonstrates either normal background activity or minimal slowing, distinguishing it from extensive brainstem lesions causing coma 1
• The basic EEG activity resembles that of the normal waking state, reflecting the preserved consciousness despite complete motor paralysis 1

Preserved Reactivity is the Critical Diagnostic Feature

• All locked-in syndrome patients demonstrate EEG reactivity to various stimuli, which is the most important distinguishing characteristic 1
• Photic stimulation typically elicits a normal photic driving response in these patients 1
• This preservation of alertness and EEG reactivity is what differentiates locked-in patients from comatose patients with extensive brainstem lesions 1

Important Diagnostic Pitfall: The "Alpha Coma" Pattern

When EEG Reactivity May Be Absent

• A critical caveat exists: some locked-in syndrome patients can exhibit an unreactive alpha rhythm pattern (so-called "alpha coma"), which can be mistakenly interpreted as indicating a vegetative state 2
• Three documented cases of acute brainstem strokes with locked-in syndrome showed repeated EEG recordings with unreactive alpha rhythm to multimodal stimuli 2
• This finding is critically important because failure to recognize locked-in syndrome due to unreactive EEG leads to abandonment of communication efforts with these conscious, aware patients 2

The Fundamental Principle

• Alpha reactivity cannot define consciousness—the absence of EEG reactivity does not exclude locked-in syndrome 2, 3
• EEG reactivity was present in only two of five locked-in syndrome patients in one series, while absent in three cases, yet all were conscious 3

Quantitative EEG Findings in Chronic Locked-In Syndrome

Abnormal Cortical Rhythms

• Resting state eyes-closed EEG in locked-in syndrome patients shows reduced power of alpha 2 and alpha 3 sources across all cortical regions compared to controls 4
• Delta source power is increased in central, parietal, occipital, and temporal regions 4
• These findings suggest functional impairment of cortical neuronal synchronization mechanisms, possibly due to disrupted neural synchronization between brainstem and cerebral cortex 4

Clinical Implications for Diagnosis

The Diagnostic Challenge

• The diagnosis of locked-in syndrome is frequently delayed, taking on average over 2.5 months, with some cases taking 4-6 years before conscious patients are recognized as aware 5
• More than half the time, it is the family—not the physician—who first realizes the patient is conscious 5

Practical Approach to EEG Interpretation

• In an unresponsive patient with a brainstem lesion, an EEG resembling the waking state with preserved reactivity is highly suggestive of locked-in syndrome rather than coma 1
• However, be aware that a similar EEG pattern can occasionally occur with diffuse cortical damage secondary to prolonged cardiopulmonary arrest 1
• Never rely solely on the presence or absence of EEG reactivity to determine consciousness in patients with brainstem lesions 2, 3

Somatosensory Evoked Potentials

• There is no specific pattern of somatosensory evoked potential (SEP) abnormality characteristic of locked-in syndrome—findings vary from unilaterally normal to bilaterally absent 3
• SEPs cannot be used as a reliable diagnostic marker for locked-in syndrome 3

Critical Clinical Caveat

The most dangerous error is assuming that an unreactive EEG indicates lack of consciousness in a patient with a brainstem lesion. This misinterpretation leads to tragic abandonment of communication attempts with fully conscious, aware individuals who are trapped in paralyzed bodies 2. Always attempt eye-coded communication regardless of EEG reactivity patterns when locked-in syndrome is suspected based on the clinical presentation and neuroimaging showing ventral pontine lesions 5, 1.