What is the expected limb response to a central stimulus on the supraorbital ridge in an M5 (localizing to pain) patient?

Expected Limb Response to Central Supraorbital Stimulus in M5 Patient

In an M5 patient (localizing to pain on the Glasgow Coma Scale motor response), applying a central painful stimulus to the supraorbital ridge will produce purposeful movement where the patient's hand crosses the midline to attempt to remove or push away the painful stimulus source.

Understanding M5 Motor Response

• M5 represents "localizing to pain" - the highest motor response below normal (M6), indicating the patient can purposefully move toward and attempt to remove a painful stimulus 1
• The key distinguishing feature is that the limb crosses the body's midline to reach the source of pain, demonstrating cortical processing and purposeful motor planning 1
• This differs from M4 (withdrawal) where the limb simply pulls away without crossing midline, or M3 (abnormal flexion/decorticate posturing) which shows stereotyped responses 1

Expected Specific Limb Movements

Upper extremities:

• The ipsilateral or contralateral arm will reach up and across the midline toward the supraorbital ridge where stimulus is applied 1
• The hand will attempt to grasp, push away, or remove the examiner's hand or the source of pain 1
• This represents purposeful, coordinated movement requiring intact cortical function and motor planning 1

Lower extremities:

• Should remain relatively still or show minimal non-purposeful movement unless separately stimulated 2
• Any lower limb movement would be non-localizing and not directed toward the supraorbital stimulus site 2

Neurophysiological Basis

• Central stimuli (like supraorbital pressure) activate nociceptors that transmit pain signals through C and Aδ fibers to the dorsal horn of the spinal cord, then ascend to higher cortical centers 1
• The supraorbital ridge is innervated by the trigeminal nerve (V1 branch), and painful stimulation here produces robust activation of pain-processing networks including the thalamus and cortex 3
• M5 response requires intact cortical function to process the noxious stimulus location and generate appropriate motor commands for purposeful localization 1

Clinical Pitfalls to Avoid

• Do not confuse M5 with M4 (withdrawal): M4 shows limb retraction away from stimulus but does NOT cross midline; M5 specifically involves crossing midline to reach the pain source 1
• Ensure adequate stimulus intensity: Insufficient pressure may not elicit the full localizing response, leading to underestimation of neurological function 1
• Avoid peripheral stimuli when assessing M5: Nail bed pressure or peripheral stimuli can produce withdrawal (M4) that may be mistaken for localization; central stimuli like supraorbital pressure are more reliable for distinguishing M5 from M4 1
• Observe both upper extremities: Some patients may localize with only one arm due to focal weakness; test both sides to accurately assess best motor response 1

Why Supraorbital Stimulus is Preferred for M5 Assessment

• Central location ensures the response requires crossing midline, making M5 versus M4 distinction clearer 1
• Trigeminal nerve stimulation produces reliable, reproducible pain responses that activate core pain-processing areas in the brain 3
• Less likely to produce reflexive withdrawal compared to peripheral stimuli, allowing better assessment of purposeful localization 1