• Do not make a simple perpendicular T-extension off the transverse scar if it can be avoided. The corners may have compromised vascularity and add scar tethering.
• If the existing scar is directly over the suspected lesion and must be addressed, consider elliptically excising/reopening the relevant segment, then extend in a curvilinear or zig-zag fashion, rather than merely extending it transversely across the wrist.
• Identify the nerve proximally in normal tissue first, then proceed distally through the scar. At the distal forearm the ulnar nerve lies deep to the lateral edge of FCU; the ulnar artery is lateral to the nerve, and both pass superficial to the flexor retinaculum just lateral to the pisiform. Protect the artery throughout. This relationship is described in Gray’s Anatomy for Students, p. 905.
• If symptoms/localization suggest Guyon canal involvement, the incision needs to permit assessment of the main trunk before division, plus the superficial sensory and deep motor branches. Tingling alone can arise from scar tethering, partial fascicular injury, neuroma-in-continuity, or compression, not necessarily transection.

1. Static and two-point discrimination in little finger and ulnar half of ring finger.
2. Dorsal ulnar cutaneous territory. Preserved dorsal sensation can suggest a lesion distal to the dorsal sensory branch, near the wrist/Guyon region.
3. FDI/ADM strength, Froment sign, Wartenberg sign, clawing, and intrinsic wasting.
4. Tinel’s at the scar, pisiform, and Guyon canal.
5. Ulnar artery perfusion and Doppler if there is any concern.
6. High-resolution ultrasound, ideally by someone experienced in peripheral nerve imaging, to look for continuity, focal enlargement/neuroma, fascicular disruption, and scar adherence. EMG-NCS is useful for baseline and localization, though it should not override a convincing traumatic clinical deficit.

Practical mapping sequence

1. Position
   • Supine, arm on hand table.
   • Forearm supinated, wrist in neutral to slight extension.
   • Mark the palpable pisiform, FCU tendon, distal wrist crease, and hook of hamate if palpable.
2. Map the ulnar artery first
   • Place a high-frequency linear probe transversely at the proximal volar wrist crease, just radial to the FCU tendon/pisiform.
   • Use color Doppler, then spectral Doppler if needed, to identify the pulsatile ulnar artery.
   • Trace it proximally 4-6 cm into normal tissue and distally into Guyon canal/palmar arch.
   • On the skin, draw its course and mark any focal narrowing, pseudoaneurysm, thrombosis, abnormality, or point where it is adherent to scar.
3. Map the ulnar nerve
   • In the same transverse scan, identify the nerve as a noncompressible fascicular, honeycomb structure.
   • At the distal forearm and Guyon canal, it generally lies ulnar/posteromedial to the artery. In Guyon canal, the nerve is more ulnar and the artery more radial. The pair enter just lateral to the pisiform.
   • Trace the nerve from well proximal to the transverse scar, through the scar level, and distally until its division into superficial sensory and deep motor branches.
   • Mark the nerve course, its bifurcation, any focal enlargement, loss of fascicular pattern, hypoechoic edema, neuroma-in-continuity, or tethering to the scar.
4. Dynamic assessment
   • Scan with light transducer pressure only over the scar.
   • Examine in wrist neutral, modest extension, and flexion. Look for a nerve that glides poorly, is angulated, or becomes compressed at the scar.
   • A sonopalpation Tinel-type reproduction of the patient’s exact paresthesia supports clinical correlation, but do not infer nerve discontinuity from symptoms alone.
5. Use the map to plan exposure
   • Choose a longitudinal/curvilinear incision just ulnar to the marked neurovascular bundle, rather than incising directly over an uncertain artery or through the entire transverse scar.
   • Ensure the planned incision provides access to a segment of normal proximal nerve, so the ulnar nerve and artery can be identified before approaching scar.
   • Extend distally only as the ultrasound indicates is necessary to inspect the Guyon canal and branch point.
   • Keep the marked artery visible on the sterile skin and reassess it intraoperatively with sterile Doppler if there is any doubt.

Vascular safety checks

• Document radial and ulnar pulses, capillary refill, and palmar arch perfusion.
• Perform a Doppler-assisted Allen assessment if there is an arterial concern or if arterial control/repair could become necessary. Handheld Doppler assessment is recommended when deciding whether a single forearm vessel provides adequate hand perfusion. Sabiston Textbook of Surgery, Upper Extremity section.
• If the artery is nonvisualized, has absent/abnormal Doppler signal, or there is concern for pseudoaneurysm, obtain formal vascular ultrasound and consider CTA/MRA rather than proceeding based on surface anatomy.

Anatomical landmarks to keep in mind

• The ulnar nerve and artery pass superficial to the flexor retinaculum, immediately lateral to the pisiform.
• In the distal forearm, the ulnar artery is lateral to the nerve; the exact relationship can change as they enter Guyon canal and anatomical variation is common.
• The nerve divides just distal to the pisiform into superficial sensory and deep motor components. A finding confined to the sensory branch may explain tingling despite preserved intrinsic function. Gray’s Anatomy for Students, p. 905.