Ulnar Nerve Lesions

Ulnar nerve lesions are characterized predominantly by:

High lesions:

• Affects flexor carpi ulnaris, flexor digitorum profundus (digits IV–V), and intrinsic hand muscles
• Functional loss includes:
  • Opening of the hand
  • Loss of grip strength
  • Reduced pincer strength

Low lesions:

• At or near wrist/palm
• Paralysis of:
  • Hypothenar muscles
  • All interossei
  • Adductor pollicis
  • ½ of flexor pollicis brevis
  • Palmaris brevis

Sensory deficits may involve:

• Medial forearm
• Medial palm and dorsal hand
• Medial 1½ fingers

Anatomy

Course of the ulnar nerve

• Origin: C7–T1, from the medial cord of the brachial plexus
• Travels medial to the brachial artery, pierces the medial fascial septum
• Behind medial epicondyle, enters forearm between flexor carpi ulnaris heads
• Descends with ulnar artery, superficial at the wrist
• Divides into deep and superficial branches at the palm

Ulnar Nerve Innervations

Proximal forearm:

• Flexor carpi ulnaris
• Flexor digitorum profundus (digits III & IV)

Distal forearm:

• Dorsal cutaneous branch: medial hand and 1½ fingers
• Palmar cutaneous branch

Hand (deep branch):

• Hypothenar muscles
• Adductor pollicis
• ½ of flexor pollicis brevis
• All interossei
• 3rd & 4th lumbricals

Hand (superficial branch):

• Palmaris brevis
• Sensory to medial 1½ fingers

Actions of Ulnar-Innervated Muscles

Muscle	Action
Flexor carpi ulnaris	Flexes/adducts hand
FDP (digits III & IV)	Flexes distal phalanges
Abductor digiti minimi	Abducts little finger
Opponens digiti minimi	Rotates 5th MC, cupping
Flexor digiti minimi	Flexes little finger
Adductor pollicis	Thumb adduction
½ Flexor pollicis brevis	Flexes thumb (MCP joint)
Interossei	Finger adduction/abduction; MCP flexion, IP extension
Lumbricals (III & IV)	Flex MCP, extend IP (digits 3 & 4)

Pathology

Causes of Ulnar Nerve Lesions

1. Trauma:
   • Penetrating or blunt trauma
2. Neuropraxia:
   • Compression at medial epicondyle
   • Prolonged elbow pressure
   • Compartment syndrome
3. Mass lesions:
   • Tumours
   • Abscesses
4. Neuropathies (mononeuritis):
   • Diabetes mellitus
   • Malignancy
   • Infection: leprosy, HIV
   • CTDs: RA, SLE, PAN, scleroderma, sarcoidosis
   • Other: amyloidosis, Lyme disease, heavy metals

Clinical Assessment

Ulnar Nerve Deficits

At the elbow:

• Motor:
  • Paralysis of FCU and medial FDP
  • Weak wrist flexion and deviation
• Sensory:
  • Loss over medial third of hand (both surfaces), medial 1½ fingers

At the wrist — Claw Hand:

• Intrinsic hand muscles paralyzed
• Thenar muscles and lumbricals I–II (median nerve) spared

Motor deficits:

1. Loss of finger ab/adduction
2. Impaired pincer grip
3. MCP hyperextension (esp. digits IV–V)
4. IP flexion (digits IV–V)
5. Hypothenar wasting
6. Dorsal interosseous wasting

Sensory loss:

• Palmar and dorsal surfaces of medial 1½ fingers
• Palmar cutaneous and dorsal cutaneous branches often involved

Investigations

When clear-cut, investigations may not be necessary.

Consider:

• Nerve conduction studies
  • Diagnostic clarification
  • Rule out generalised disease
  • Prognostic information
• Blood tests:
  • FBC, U&Es/glucose, CRP, ESR, others (e.g. lead levels)
• CT/MRI:
  • Mass lesion delineation
  • Nerve visualisation
• Biopsy:
  • Rarely for mononeuritis or suspected leprosy

Management

1. Treat the cause
2. Compartment syndrome:
   • Fasciotomy if appropriate
3. Occupational therapy:
   • Splinting, support devices
4. Physiotherapy:
   • Rehab, prevention of secondary complications

Disposition

Dependent on:

• Cause
• Severity
• Onset

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Appendix 1

Ulnar Nerve Anatomy

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Appendix 2

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Appendix 3

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What is the Ulnar paradox?

An injury to the ulnar nerve will result in the classical ulnar claw. However this is mainly seen in more distal injuries. The ulnar nerve also innervates the ulnar half of the flexor digitorum profundus (FDP) muscle. If the ulnar nerve lesion occurs more proximally (closer to the elbow), the flexor digitorum profundus muscle may also be denervated. As a result, flexion of the IP joints is weakened, which reduces the claw-like appearance of the hand.

Simply put, as reinnervation occurs along the ulnar nerve after a high lesion, the deformity will get worse (FDP reinnervated) as the patient recovers – hence the use of the term “paradox”.

A simple way to remember this is: ‘the closer to the Paw, the worse the Claw’.

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Coni R. Neuro 101: Peripheral Nervous System. LITFL
• Nickson C. Myotomes and Differentiating Nerve Lesions. LITFL