Mononeuropathy

Diseases, ,

Synonyms:

Mononeuropathy, including Compressive neuropathy a.k.a. nerve compression

Diagnosis:

This is a clinical diagnosis supported by electrophysiology (NCS/EMG)

Clinical features:

  • Dysfunction limited to one isolated peripheral nerve for example the median nerve, the ulnar nerve, the radial nerve, the femoral nerve, the peroneal nerve etc.
  • Only one nerve involved. This distinguishes the syndrome from mononeuropathy multiplex
  • Each nerve may have multiple clinical syndromes depending on how proximal or distal the lesion is
  • No involvement outside the isolated nerve. This distinguishes the syndrome from radiculopathy or polyneuropathy
  • Some conditions like diabetes mellitus predispose to compression mononeuropathies

Findings on Investigations:

+NCS/EMG:
  • Confirms the pattern of isolated peripheral nerve involvement and helps distinguish between mononeuropathy and radiculopathy, polyneuropathy or mononeuropathy multiplex
  • Conduction slowing occurs
  • Temporal dispersion occurs
  • EMG findings of denervation and renervation in affected muscles e.g. fibrillations and positive sharp waves PSW)

Investigations to consider:

Blood tests:

  • Diabetes mellitus screening: Fasting blood glucose, HbA1c
  • Thyroid function tests

Imaging:

  • X-ray: fractures or bone abnormalities e.g. humeral fractures in radial neuropathy
  • MRI: for structural lesions
  • CT: for structural lesions e.g. retroperitoneal hematoma in femoral neuropathy

Types of mononeuropathy (see details below):

  • Median nerve compression a.k.a. median neuropathy:
    • Carpal tunnel syndrome (median neuropathy at the wrist)
    • Entrapment at ligament of Struthers, very rare
    • Pronator syndromeAnterior interossesous nerve AIN syndrome (a.k.a. Kiloh-Nevin syndrome)
  • Ulnar nerve and ulnar neuropathy:
    • Ulnar nerve compression at the wrist
    • Ulnar nerve compression at the Elbow
  • Radial nerve and radial neuropathy:
    • Radial nerve compression at the axilla
    • Radial nerve compression at the spiral groove
    • Posterior interosseous nerve neuropathy (PIN)
    • Superficial radial nerve neuropathy
  • Long thoracic nerve a.k.a. Thoracic nerve of Bell
  • Obturator nerve palsy a.k.a. obturator neuropathyFemoral nerve palsy a.k.a. femoral neuropathy
  • Sciatic nerve a.k.a. sciatic neuropathy
  • Peroneal nerve a.k.a. peroneal neuropathy
  • Tibial nerve neuropathy, rare
  • Tarsal tunnel syndrome (posterior tibial nerve compression in tarsal tunnel)
  • Miscellaneous syndromes:
    • Suprascapular nerve: Spinoglenoid notch
    • Lateral fermoral cutaneous nerve (Meralgia paresthetica; at the Inguinal ligament)
    • Obturator nerve at the Obturator canal
    • Plantar branches of the posterior tibial nerve (Morton metatarsalgia, plantar fasica and heads of 3rd and 4th metatarsals)

 

Median nerve compression a.k.a. median neuropathy:

Carpal tunnel syndrome (median neuropathy at the wrist):

Clinical features:

  • Pain up to the shoulder, worse at night
  • Weakness of thenar muscles: LOAF muscles especially adductor pollicis brevis APB
  • Tinels sign and Phalen’s sign
  • Sensory: palm (radial), palmer aspect of 3.5 fingers and over the tips, NO supply to forearm.

NCS and EMG, carpal tunnel syndrome:

  • SNAP, if motor is negative: median nerve latency 0.4 ms > ulnar latency, Median nerve conduction velocity <50 ms. SNAP amplitude is reduced.
  • CMAP: prolonged median nerve latency > 4.4 ms, or median nerve latency more than 1.4 ms greater than ulnar latency. CMAP reduced amplitude
  • EMG: fibrillation potentials and positive sharp waves PSW in Abductor pollicis brevis and not in median innervated muscles proximal to the wrist
  • Motor involvement indicates more severe CTS

Investigations to consider in carpal tunnel syndrome:

  • Fasting blood glucose, Thyroid function tests
  • Testing for acromegaly

Treatment of carpal tunnel syndrome:

  • Avoidance of movements that can exacerbate the condition
  • Splints at night
  • Local steroid injection
  • Consider Surgery:
    • Section of the carpal ligament

Entrapment at ligament of Struthers, very rare:

Clinical features:

  • Forearm pain
  • Paresthesia in median innervated fingers
  • Worsened by hand/forearm supination and extension of elbow
  • Weakness may occur, including pronator teres

NCS/EMG:

  • Stimulate at the axilla as well
  • CMAP: conduction block, temporal dispersion between axilla and antecubital fossa.
  • Prolonged motor latency may occur

X-ray humerus and elbow: boney spur (supracondylar process)

Pronator syndrome:

Clinical features:

  • Weakness: mild median innervated muscles
  • Pain: Nonspecific, worsening by supination and pronation
  • Paresthesia: worsened by forearm pronation with elbow in extension and by elbow flexion with forearm supinated.

Anterior interossesous nerve AIN syndrome (a.k.a. Kiloh-Nevin syndrome):

Clinical features:

  • Weak pincer grip, can’t make the “OK” sign/make a circle with index finger and thumb (Dip extension occurs). Test pronation with arm flexed (pronator quadratus weakness). Weak FDP in 1 and 2.
  • No sensory deficit
  • Pain can occur

NCS/EMG:

  • flexor digitorum profundus FDP 1 and 2, Flexor pollicis longus FPL, pronator quadratus PQ (difficult to study)

Ulnar nerve and ulnar neuropathy:

Clinical features:

  • Weakness of the intrinsic muscles of the hand (esp. Digiti minimi muscles and 1st interossei)
  • Sensation of medial 1 and a half fingers +palm

Points of compression:

  • Bicipital groove
  • Cubital tunnel syndrome: at the elbow
  • Guyon canal (at the wrist)

Guyon canal (at the wrist):

  • The triangular canal at the base medial part of the palm
  • Borders: laterally= hook of the hamate and transverse carpal ligament, medially= the pisiform.
  • Spares sensation
  • Palmar fascia-pisiform bone

Findings on Investigations:

Ulnar nerve compression at the wrist:

NCS/EMG:

  • SNAP:
    • Reduced SNAP amplitude in the 5th digit (may be normal in purely motor lesions)
    • SNAP in dorsal ulnar cutaneous nerve is normal.
  • CMAP:
    • Slowing of distal motor nerve conduction velocity with normal conduction velocity in the rest of the nerve.
    • Normal conduction studies across the elbow
  • EMG:
    • Fibrillation potentials and positive sharp waves PSW in ulnar innervated muscles in the hand. Normal flexor carpi ulnaris and flexor digitorum profundus.

Ulnar nerve compression at the Elbow:

NCS (test with elbow flexed 70-90%):

  • SNAP: decreased amplitude in dorsal ulnar cutaneous nerve. Note that SNAP should be normal in C8 radiculopathy.
  • CMAP:
    • Slower motor nerve conduction velocity across the elbow (>10m/s slowing is significant)
    • Conduction block across the elbow may be present (false positive in Martin-Gruber anastomosis, in this case check median nerve CMAP it may show positive initial deflection, increased conduction velocity and increased CMAP amplitude at the elbow compared to the wrist)
    • Inching may reveal the site where amplitude drops.
  • EMG:
    • Fibrillation potentials and positive sharp waves PSW in ulnar innervated muscles in the hand. Flexor carpi ulnaris and flexor digitorum profundus may also show these signs, however normal EMG in these muscles doesn’t exclude a lesion at the elbow.

Radial nerve and radial neuropathy:

Clinical features:

  • Motor:
    • Triceps (Elbow extension), brachiradialis (elbow flexion in mid pronation), supinator
    • Extensor carpi radialis longus (abduct and extend wrist), extensor carpi ulnaris (adduct and extend wrist), Extensor digitorum,
    • Extensor pollicis (brevis and longus), abductor pollicis longus
    • Wrist drop
  • Sensory: posterior cutaneous nerve of arm, posterior cutaneous nerve of forearm, dorsum of first web-space
  • Posterior interosseus nerve PIN lesions:
    • Cause weakness of wrist extensors and index and thumb extensors. Spares triceps

Findings on investigations:

Radial nerve compression at the axilla:

NCS/EMG:

  • SNAP: decreased amplitude, normal if done early
  • CMAP: decreased amplitude
  • EMG:
    • Abnormal (fibrillations and positive sharp waves PSW) triceps, anconeus, brachioradialis, extensor carpi radialis as well as distal muscles
Radial nerve compression at the spiral groove:
  • SNAP: decreased amplitude, normal if done early
  • CMAP: temporal dispersion and conduction block may occur.
  • EMG:
    • Normal triceps,
    • Abnormal (fibrillations and positive sharp waves PSW) anconeus, brachioradialis, extensor carpi radialis and distal muscles
Posterior interosseous nerve neuropathy (PIN):

NCS/EMG:

  • SNAP: normal
  • CMAP: decreased amplitude
  • EMG:
    • Normal triceps, anconeus, brachioradialis, extensor carpi radialis
    • Abnormal (fibrillations and positive sharp waves PSW) distal muscles
Superficial radial nerve neuropathy:

NCS/EMG:

  • SNAP: abnormal,
  • CMAP: normal amplitude, latency and conduction velocity
  • EMG: normal

Long thoracic nerve a.k.a. Thoracic nerve of Bell:

Clinical Features:

  • Winging of the scapula
  • Pain around the shoulder
  • Exclude other causes such as myopathies (such as Facioscapulohumeral Muscular Dystrophy and others)
  • Typical causes are trauma (blunt or sports), surgery (thoracic, radical mastectomy and 1st rib resection) and occasionally systemic causes such as SLE

Obturator nerve palsy a.k.a. obturator neuropathy:

Clinical features:

  • Weakness of hip adduction
  • Medial thigh pain during exercise
  • Loss of obturator reflex L3
  • Sensory loss and pain around medial side of thigh
  • Unusual to occur in isolation

Causes:

  • Surgery, hemorrhage, tumor, sports injuries

Findings on investigations:

EMG/NCS:

  • Allows localization of the dysfunction to the obturator nerve muscles
  • Adductor muscles: Fibrillation potentials or high-amplitude, long-duration complex motor unit potentials
  • Sparing of quadraceps, sparing of iliospoas, sparing of L-2, L-3 and L-4 paraspinal muscles

CT abdomen, pelvis:

  • Pelvic tumours

Femoral nerve palsy a.k.a. femoral neuropathy:

Clinical features:

  • Weakness of knee extension
  • Loss of knee reflex
  • Sensory loss and pain around knee and medial side of leg

Findings on investigations:

EMG/NCS:

  • Helps distinguish between femoral neuropathy and other conditions

Fasting blood glucose, HbA1c: diabetes mellitus
CT abdomen, pelvis: retroperitoneal hematoma, psoas hematoma, psoas abscess, pelvic tumours
MRA: femoral artery aneurysm

Sciatic nerve a.k.a. sciatic neuropathy:

Clinical features:

  • Peroneal part is more likely to be damaged than tibial part
  • Biceps femoris reflex (Lateral hamstring reflex) is absent in high sciatic/peroneal lesions (above the knee) and spared in peroneal nerve lesions below the knee
  • Absent ankle reflexes
  • Weakness in tibial and peroneal nerve distribution
  • Rarely sensory loss on lateral aspect of foot, sole and foot dorsum

Findings on investigations:

NCS/EMG:

  • Localizes the lesion (above or below the fibula)

CT abdomen, pelvis: pelvic tumours,

Peroneal nerve a.k.a. peroneal neuropathy:

Clinical features:

  • Foot drop, weakness of ankle dorsiflexion and eversion
  • Tinel’s sign at the fibular head
  • Sensation loss and paraesthesia in anterior and lateral shin, dorsum of the foot (superficial peroneal) and 1st web space (deep peroneal nerve a.k.a. anterior tibial nerve).
  • Biceps femoris reflex (Lateral hamstring reflex) is absent in high sciatic/peroneal lesions (above the knee) and spared in peroneal nerve lesions below the knee
  • Present knee and ankle reflexes

Findings on investigations:

NCS:

  • SNAP:
    • Of superficial peroneal nerve normal in purely demyelinating lesions. Reduced amplitude in axonal or mixed axonal/demyelinating lesions.
    • Normal sural nerve sensory.
  • CMAP:
    • If Extensor digitorum brevis is atrophied, place pick-up on tibialis anterior
    • Lesions at the fibular head: slowing across the fibular head may occur
    • Conduction block at the fibular head
    • Reduced amplitudes if axonal
    • Accessory deep peroneal nerve (branch of superficial peroneal nerve): CMAP amplitude at extensor digitorum brevis is larger with stimulation at fibular head than at the ankle.
  • F-waves: may be reduced in lesions at the fibular head. Nonspecific.
  • H-reflex: normal

EMG:

  • Normal in demyelinating neuropathies. Abnormal in axonal neuropathies.
  • Fibrillation and positive sharp waves PSW in affected muscles.
  • Short belly of biceps femoris is affected in lesions proximal to the fibula and helps distinguish them from distal lesions.
  • Superficial peroneal nerve muscles (peroneus longus, peroneus brevis).
  • Deep peroneal nerve muscles (Extensor digitorum brevis-see anomaly below, tibialis anterior)
  • Extensor digitorum brevis may be preserved in deep peroneal nerve muscles when it is supplied by accessory deep peroneal nerve (branch of superficial peroneal nerve).
  • Tibialis anterior is abnormal in fibular lesions
  • Test tibial nerve muscles below the knee (rule out tibial neuropathy/sciatic neuropathy)
  • Test paraspinal muscles (rule out radiculopathy)
  • MUAP: decreased recruitment (in axonal and demyelinating lesions), long duration, increased amplitude and polyphasia in axonal lesions.

Tibial nerve neuropathy, rare:

Clinical features:

  • Weakness of plantar flexion
  • Absent ankle reflex
  • Rarely sensory loss on lateral aspect of foot and sole

Findings on investigations:

NCS/EMG

  • To exclude S1 and sciatic nerve lesions

MRA: popliteal aneurysm

Tarsal tunnel syndrome (posterior tibial nerve compression in tarsal tunnel):

Clinical features:

  • Medial ankle pain. paresthesia and sensory loss in Plantar aspect of the foot, usually unilateral, no weakness. Tinel’s sign positive at tarsal tunnel flexor retinaculum.

Findings on investigations:

  • SNAP: reduced or absent amplitude. False positives occur
  • CMAP of medial and lateral plantar nerves:
    • Demyelinating: reduced distal latency,
    • Axonal: reduced amplitude
  • H-reflex: normal
  • F-waves: abnormal, nonspecific
  • EMG: Painful in this location
    • Fibrillations and positive sharp waves PSWs in involved muscles distal to the tunnel e.g. (lateral plantar nerve) abductor digiti minimi, dorsal and plantar interossei, and (medial plantar nerve) abductor hallucis and flexor digitorum brevis
    • Spared muscles proximal to the tunnel e.g. Gastrocnemius, soleus, popliteaus,

 

Miscellaneous syndromes:

Clinical syndromes:

Suprascapular nerve: Spinoglenoid notch
Lateral fermoral cutaneous nerve:
  • Meralgia paresthetica; at the Inguinal ligament
Obturator nerve at the Obturator canal
Plantar branches of the posterior tibial nerve (Morton metatarsalgia, plantar fasica and heads of 3rd and 4th metatarsals)

 

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