What is the difference between central and peripheral stimuli in assessing the motor part of the Glasgow Coma Scale (GCS) and what are the expected responses?

Assessment of Motor Component of GCS: Central vs. Peripheral Stimuli

When assessing the motor component of the Glasgow Coma Scale, you must apply a central stimulus (such as supraorbital pressure or trapezius squeeze) to differentiate true purposeful movement from spinal reflex withdrawal, as peripheral stimuli alone can elicit spinal reflexes that falsely suggest higher cortical function.

Understanding the Critical Distinction

The motor component of the GCS is the most difficult to assess accurately and requires proper understanding of stimulus application 1. The key distinction lies in what each type of stimulus reveals about brain function:

Central Stimulus Application

• Apply pressure to central structures such as the supraorbital ridge, trapezius muscle, or sternal rub to assess cortical motor responses 1
• Central stimuli bypass spinal reflexes and require intact brain function to generate a response, making them essential for accurate GCS motor scoring
• These stimuli test descending motor pathways from the brain rather than isolated spinal cord reflexes

Peripheral Stimulus Application

• Peripheral stimuli (such as nail bed pressure or limb pinching) can elicit spinal reflex withdrawal that occurs at the spinal cord level without any cortical involvement
• This creates a critical pitfall: a patient with severe brain injury may withdraw a limb from peripheral pain through spinal reflexes alone, falsely suggesting better neurological function (M4 score) when they actually have no cortical motor response

Expected Motor Responses and Scoring

M6 - Obeys Commands (Best Response)

• Patient follows verbal instructions such as "squeeze my hand" or "wiggle your toes" 2
• Requires intact cortical function and comprehension
• No painful stimulus needed for this assessment

M5 - Localizes to Pain

• Patient reaches toward and attempts to remove the source of central painful stimulus
• Purposeful, coordinated movement that crosses the midline
• Must use central stimulus to accurately assess this response
• Key distinction: The hand moves to the site of pain application (e.g., reaches up to supraorbital pressure)

M4 - Withdraws from Pain

• Limb pulls away from painful stimulus but does not localize to remove it
• Critical caveat: This response can be a spinal reflex if only peripheral stimulus is used 1
• Always confirm with central stimulus to ensure this represents true cortical function rather than spinal reflex

M3 - Abnormal Flexion (Decorticate Posturing)

• Flexion of arms with adduction and internal rotation at shoulders
• Extension and plantar flexion of legs
• Indicates severe brain injury with damage above the red nucleus 2

M2 - Abnormal Extension (Decerebrate Posturing)

• Extension and adduction of arms with internal rotation
• Extension of legs with plantar flexion
• Indicates brainstem dysfunction and worse prognosis than decorticate posturing 2

M1 - No Motor Response

• No movement to any stimulus including central painful stimuli 2
• Worst motor score indicating severe neurological compromise

Clinical Algorithm for Accurate Assessment

Step 1: Begin with verbal commands (if patient appears conscious)

• Ask patient to follow simple commands
• If compliant → Score M6

Step 2: If no response to commands, apply central painful stimulus

• Use supraorbital pressure or trapezius squeeze
• Observe the motor response carefully

Step 3: Classify the response to central stimulus

• Reaches toward stimulus site and attempts removal → M5 (localizes)
• Pulls limb away but doesn't localize → M4 (withdrawal)
• Abnormal flexion posturing → M3
• Abnormal extension posturing → M2
• No response → M1

Step 4: Avoid the peripheral stimulus trap

• Do NOT rely solely on nail bed pressure or peripheral limb stimulation
• These can trigger spinal reflexes that mimic M4 responses without cortical involvement

Critical Pitfalls to Avoid

• Using only peripheral stimuli is the most common error, leading to overestimation of neurological function 1
• Mistaking spinal reflex withdrawal for purposeful withdrawal occurs when peripheral rather than central stimuli are used
• Inconsistent stimulus application between assessments makes serial monitoring unreliable 1
• Inadequate training leads to 50% error rates even among experienced ICU nurses 1

Prognostic Implications

• The motor score alone preserves virtually all predictive power of the full GCS (ROC 0.87 vs 0.89 for full GCS) and should be prioritized 3
• Motor score >3 on day 4 after cardiac arrest predicts favorable outcome with 84% specificity and 77% sensitivity 4
• Motor score of 4-5 at ICU admission after cardiac arrest predicts favorable outcome with 98% specificity 4
• Serial assessments provide more value than single determinations, with declining scores indicating deterioration 2

Documentation Requirements

• Always record the specific motor score (M1-M6) rather than just contributing to a sum 2
• Document which stimulus was used (central vs peripheral) for clarity
• Note any asymmetry between right and left sides
• Record the best response observed during the assessment period 5