Peripheral Nerves, Myotomes, Dermatomes

Question 1

Axillary Nerve

Answer 1

Motor: Deltoid (shoulder abduction 20-90 deg)

Sensory: Regimental patch

Question 2

Radial Nerve

Answer 2

Motor: Elbow, wrist, finger extension

Sensory:
- Posterior cutaneous nerve of arm - dorsal arm
- Low lesions - only anatomical snuffbox

Question 3

Musculocutaneous Nerve

Answer 3

Motor: Elbow flexion

Sensory: Lateral Forearm

Question 4

Median Nerve

Answer 4

Motor:
- Wrist flexion, finger flexion (apart from ulnar doing medial 2 FDP)
- 1/2LOAF: lateral 2 lumbricals, Oponens pollicis brevis, abductor pollicis brevis, flexor pollicis longus

Sensory:
- lateral 3.5 digits palmar surface

Question 5

Median nerve lesions:

Answer 5

Lesion at the wrist:
- Loss of abductor pollicis brevis (thumb abduction, pen touch test)

Lesion at the elbow:
- Loss of flexon digitorum superficialis
- HAND OF BENEDICT, unable to flex lateral 3 fingers.

Question 6

Ulnar Nerve

Answer 6

Motor: Finger abduction and adduction (PADs and DABs), flexor digitorum profundus digit 4 and 5.

Sensory: medial 1.5 digits

Question 7

Ulnar nerve lesion

Answer 7

Wasting of intrinsic hand muscles apart from 1/2 LOAF (median).
Ulnar claw due to paralysis of lumbricals
- with ulnar paradox (distal lesion results in worsening of ulnar claw as the FDP is still active)

Froments sign:
- failure of adductor pollicis due to ulnar nerve palsy, requires flexion by flexor pollicis longus to compensate

Question 8

How to differentiate between a L5 radiculopathy and common peroneal lesion ?

Answer 8

The differentiator is FOOT INVERSION.

L5 - weakness if dorsiflexion, inversion and eversion.

Peroneal - weakness in dorsiflexion and eversion, but preserved inversion by tibialis posterior innervated by tibial nerve.

Question 9

How to differentiate between a femoral nerve lesion and L3-L4 lesion?

Answer 9

The difference is ADDUCTION.

Femoral nerve lesion will have preserve adduction with is performed by obturator nerve.

L3-4 lesion will have weak adduction.

Question 10

NCS Patterns

Answer 10

Compound Muscle Action Potential (CMAP)
- reduced amplitude could be due to axonal damage, NMJ dysfunction, myopathy.

Sensory Nerve Action Potential (SNAP)

Reduced amplitude - axonal loss

Reduced velocity/increased latency, temporal dispersion - demyelination

Question 11

EMG Patterns

Answer 11

Looks at spontaneous activity, MUPs and recruitment.

Neurogenic - increase spontaneous activity, large/prolonged/polyphasic MUP, reduced recruitment.

Myopathic - maybe increased spontaneous activity, small/short MUPs, increased/early recruitment.

Question 12

Differential Diagnosis of Multiple mononeuropathy

Answer 12

Mononeuritis:
Infection: Leprosy
Inflammatory: Vasculitis (PAN, RA, Cryoglobulinaemia), Sarcoidosis

Mono-Neuropathy:
Extrinsic
- Trauma
- Compression

Intrinsic:
- Vascular
- Genetic
- Diabetes

Question 13

DDx for Peripheral Neuropathy

Answer 13

Neuritis:
- Infection - HIB
- Paraneoplastic - Anti-Hu, Anti-CV2, Paraproteinaemia

Neuropathy:
- Extrinsic - infiltrative
- Intrinsic: Genetic (CMT), Metabolic (Diabetes), Nutritional (B12/B1), Drugs or Toxins (chemo, RTx, HAART, ETOH)

Question 14

HIV neuropathy

Answer 14

Distal, symmetrical, sensory predominant neuropathy.
Usually small fibre predominant.

Question 15

Paraprotein associated neuropathy

Answer 15

IgM most common
Distal Large Fibre Sensory Predominant
50% of IgM MGUS have anti-MAG Ab
Can mimic CIDP

Question 16

AL amyloidosis associated neuropathy

Answer 16

painful, length dependent peripheral neuropathy with prominant autonomic failure

Question 17

Charcot Marie Tooth Disease

Answer 17

Group of genetic peripheral neuropathies. Caused by genetic defect is important proteins within the neuron.
CMT1 - demyelinating, motor, autosomal dominant
CMT2 - Axonal, sensory, autosomal dominant
X linked CMTX
Autosomal recessive CMT4 (Rare)

Suspicion for genetic neuropathy if:
- early onset
- long and slowly progressive
- foot deformities
- lack of positive symptoms with clear sensory involvement.

Prevalence 1:2500

Question 18

Management of diabetic neuropathy

Answer 18

1 - Amitriptyline (NNT 3.5 for reduction in pain)
2 - Duloxetine and venlafaxine
3 - Gabapentin and Pregabalin

Question 19

Chemotherapy induced neuropathy

Answer 19

Platinum based (cisplatin, carboplatin, oxaliplatin)

Taxanes (paclitaxel, docetaxel)

Vinca alkaloids (Vincristine, vinblastine)

Bortezomib (proteasome inhibitor)

Question 20

AIDP vs CIDP

Answer 20

AIDP nadir <4 weeks
- IVIG = PLEX
- NO STEROIDS, worse outcomes

CIDP nadir > 4 weeks
- Steroids used
- IVIg, PLEX, Steroid sparing agents, Rituximab in Ab positive disease

Question 21

Antibodies in AIDP

Answer 21

GM1 GD1a in Axonal Subtype

GQ1b in Miller Fisher Syndrome

Question 22

Pathophysiology of AIDP

Answer 22

Infection (C Jejuni) –> molecular mimicry –> auto Ab production by B cells –> IgG and complement depoition on Schwann cells –> Myelin stripping affecting predominantly the NERVE ROOTS and PROXIMAL NERVE (Blood nerve barrier lacking)

Question 23

Presentation AIDP

Answer 23

Prodrome within 4 weeks prior - URTI/Diarrhoea

Acroparesthesia followed 7-10 days later by symmetric ascending weakness (legs, arms, face)

Severe radicular lumbar and neuropathic pain in 2/3 cases.

Nadir 2-4 weeks.

30% need ventilation.

Question 24

Investigations in AIDP

Answer 24

Bloods:
- LFT and CK may be elevated
- Serology for C jejuni, HIV, CMV, EBV
- GQ1b Ab

CSF:
- Albuminocytologic dissociation

PFTs:
- Measure FVC, if <1L need to go to ICU

Question 25

NCS in AIDP

Answer 25

Prolonged F wave latency

Prolonged distal latency / reduced velocity (motor before sensory)

Temporal Dispersion or Conduction block at 2 weeks

Question 26

Tx of AIDP:

Answer 26

PLEX = IVIG for efficacy
- usually IVIG is used as is easier to deliver.
- Give within 2-4 weeks of onset

NO role for steroids, worse outcome.

Question 27

Multifocal Motor Neuropathy

Answer 27

Immune mediated condition affecting only motor neurons. Asymmetric.

Serum Anti-GM1 Ab

NCS showing demyelination with conduction block.

Question 28

Dorsal Root Ganglionopathy

Answer 28

THink of when pure sensory neuropathy with hands and feet onset at same time.

Think:
- Sjogrens
- Paraneiplastic
- B6 toxicity

Question 29

HINTS Exam

Answer 29

Head Impulse - Abnormal in peripheral cause, normal in stroke.

Nystagmus:
- Unidirectional horizontal nystagmus, Follows Alexanders Law (nystagmus increased with gaze towards the fast phase - away from lesion) - peripheral.
- Vertical, bidirectional - central

Test of Skew - Abnormal in Central cause.