lac mac and pac

Upper limb proximal Sensorystudies

D M W DharmakeerthiSPR clinical Neurophysiology

NHNN Queen squareLondon

LAC MAC PAC

Lateral Antebrachial cutaneous nerve

C5,6 – upper trunk

anterior division

lateral cord

musculo cutaneous nerve

Lateral Antebrachial cutaneous nerve

Clinical applications

• Direct LACN injuries1. Venepuncture at lateral ACF

2. Compression by the deep fascia at ACF3. Rupture of the long head of the biceps

4. Rarely an anomalous portion of the biceps brachii muscle may injure LACN

• Secondary to musculo cutaneous damage1. Anterior dislocation of the shoulder

2. weight lifting

3. malpositioning during anesthesia

4. traumatic arm extension

5. neuralgic amyotrophy

• Lateral cord lesions

• Upper trunk lesions

• Root lesions (C5,6)

• Median neuropathy vs lateral cord lesion

• Side to side comparison

• Inter personal and intra personal variability of amplitude

Filter- 10Hz – 5kHz

Analysis time- 20 ms

gain – 10 - 20 uV per division

Pitfalls of LACN study

• spread of the stimulus to the radial nerve, in an antidromic sensory nerve conduction study of the (LAC) nerve

• Subjects - 80 healthy - 39men and 41 women (aged 24–82 years)

• stimulus current was increased gradually until the SNAP became unidentifiable due to contamination of (CMAPs)

• 3 possible patterns according to the type of spread

1. Only LAC (true SNAP)

2. LAC Radial (true + false SNAPS) Apparent SNAPS

3. Radial + LAC (true + false SNAPS)

• The stimulus intensity that achieved maximum stimulation of the LAC nerve was usually low, 4.7 mA (1.6–11.0 mA).

• Spread to the radial nerve started at 10 +/- 5.5 mA (0.8–26 mA), and at 10 mA for 63% of pattern B or C arms.

Conclusion

• The maximum intensity for a genuine LAC SNAP was usually very low, 10 mA in 99% of arms (all but 1) and 5 mA in 75% of arms using a 0.2-ms duration

• Therefore, if the LAC amplitude continued to increase over 10 mA, this was probably due to spread

• However, this pitfall cannot be avoided by this strategy, because even spread to the radial nerve often started at 10 mA (63% of arms where the spread occurred).

• Amplitude of a false SNAP may be almost 7 times the genuine LAC SNAP

• Intersubject variation

• Interside difference for genuine SNAPs exceeded 50% in 11% of subjects for whom the difference was measurable. In this regard, the “50% rule” is not always applicable to the LAC nerve

Medial antebrachial cutaneous nerve

C8/T1

Lower trunk

Anterior divisions

Medial cord

MACN

• Direct MACN lesionsSteroid injection due to medial epicondylitisRoutine VenepunctureCubital tunnel surgeryElbow arthroscopyOpen fractures fixationRepeated minor trauma (from tennis)Rarely by tuberculoid leprosy neuritis Subcutaneous lipoma

• Medial cord lesions

• Lower trunk lesions

• C8/T1 roots

• Ulnar neuropathy vs lower trunk lesions

• MACN neuropathy should be taken into account for the differential diagnosis of the patients with complaints of pain and dysestesia in medial forearm and anteromedial aspect of the elbow.

Posterior antebrachial cutaneous nerve of forearm (C5-8)

• Branches from the radial nerve just distal to the posterior brachial cutaneous nerve (PBC) in the axilla and they course through the arm in close proximity to each other.

• In the proximal forearm, the PABCN is found exactly on the lateral border of the brachioradialis muscle.

• Below elbow – superficial radial

• Above elbow – SR + PACN