Medicine – Carpal Tunnel Syndrome

Carpal tunnel syndrome is a compression neuropathy of the median nerve at the wrist, where the nerve passes beneath the flexor retinaculum through the carpal tunnel.

It is the most common entrapment neuropathy and typically causes pain, numbness, and paraesthesiae in the median nerve distribution, often worse at night.

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1. Basic Mechanism

The carpal tunnel is a narrow space at the wrist containing:

The median nerve.

Flexor tendons.

The roof of the tunnel is formed by the flexor retinaculum, also called the transverse carpal ligament.

Compression of the median nerve within this tunnel produces the characteristic symptoms.

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2. Median Nerve Involvement

Carpal tunnel syndrome is therefore a median nerve palsy or entrapment neuropathy at the wrist.

Because the lesion is distal, forearm muscles supplied by the median nerve are spared.

The main deficits involve:

Median-innervated sensation in the hand.

and

Median-innervated intrinsic muscles of the thenar region.

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3. Pain and Paraesthesiae

The most common symptoms are:

Pain.

Tingling.

Pins and needles.

Numbness.

Symptoms commonly affect the hand but may radiate proximally into the forearm.

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4. Worse at Night

Symptoms are classically:

Worse at night.

Patients may wake from sleep because of tingling or pain.

They may report that shaking or moving the hand temporarily relieves the symptoms.

This is a common and useful clinical clue.

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5. Sensory Distribution

Paraesthesiae typically involve the:

Thumb.

Index finger.

Middle finger.

Radial half of the ring finger.

This corresponds to the:

Lateral three and a half fingers.

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6. Important Sensory Sparing

An important localisation clue is that sensation over the thenar eminence is usually spared.

This is because the palmar cutaneous branch of the median nerve arises proximal to the carpal tunnel and passes superficial to the flexor retinaculum.

Therefore:

Carpal tunnel syndrome → numb lateral 3½ digits, but thenar skin sensation is usually preserved.

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7. Motor Weakness

More advanced carpal tunnel syndrome may cause weakness of the median-innervated intrinsic hand muscles.

The classic muscles are remembered using:

LOAF.

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8. LOAF Muscles

L = Lateral two lumbricals.

O = Opponens pollicis.

A = Abductor pollicis brevis.

F = Flexor pollicis brevis, particularly its superficial head.

These are the main median-innervated intrinsic hand muscles.

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9. Lateral Two Lumbricals

The first and second lumbricals are supplied by the median nerve.

They help:

Flex the metacarpophalangeal joints.

and

Extend the interphalangeal joints.

Weakness may contribute to impaired fine finger movement.

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10. Opponens Pollicis

Opponens pollicis is essential for thumb opposition.

Weakness may cause difficulty:

Touching the thumb to the little finger.

Picking up small objects.

Buttoning clothes.

Using keys or coins.

Thumb opposition is therefore an important bedside test.

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11. Abductor Pollicis Brevis

Abductor pollicis brevis is commonly tested in suspected carpal tunnel syndrome.

The patient is asked to lift the thumb perpendicular to the palm against resistance.

Weakness suggests median nerve dysfunction at the wrist.

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12. Flexor Pollicis Brevis

The superficial head of flexor pollicis brevis is typically supplied by the median nerve.

Weakness may contribute to impaired thumb movement.

The deep head can receive ulnar nerve innervation, so the muscle has variable dual innervation.

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13. Thenar Wasting

Severe or longstanding median nerve compression may cause:

Thenar muscle wasting.

This is seen as flattening of the thenar eminence.

It suggests significant chronic motor involvement.

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14. Functional Consequences

Patients may have difficulty with:

Fine finger movements.

Thumb opposition.

Pinching objects.

Holding small objects.

Buttoning clothes.

They may also report dropping objects because of sensory loss and weakness.

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15. Pregnancy

Pregnancy is a recognised association with carpal tunnel syndrome.

Fluid retention may increase pressure within the carpal tunnel.

Symptoms often appear during later pregnancy and may improve after delivery.

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16. Hypothyroidism

Hypothyroidism is associated with carpal tunnel syndrome.

Tissue swelling and mucopolysaccharide deposition may contribute to median nerve compression.

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17. Acromegaly

In acromegaly, enlargement of soft tissues and connective tissues around the wrist may narrow the carpal tunnel.

Therefore, bilateral carpal tunnel syndrome can occasionally be a clue to underlying acromegaly.

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18. Amyloidosis

Amyloid deposition can contribute to median nerve compression within the carpal tunnel.

Bilateral carpal tunnel syndrome may sometimes precede other manifestations of systemic or transthyretin amyloidosis.

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19. Rheumatoid Arthritis

Rheumatoid arthritis may produce inflammation and swelling of flexor tendon sheaths within the carpal tunnel.

This increases pressure on the median nerve.

Carpal tunnel syndrome is therefore a recognised complication of inflammatory arthritis.

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20. Obesity

Obesity is associated with increased risk of carpal tunnel syndrome.

The mechanism is likely multifactorial and may include increased tissue pressure within the carpal tunnel.

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21. PHARO Association Mnemonic

The traditional mnemonic is:

P – Pregnancy

H – Hypothyroidism

A – Acromegaly

R – Rheumatoid arthritis

O – Obesity

The original note also includes amyloidosis, which is an important association even though it does not fit neatly into PHARO.

A practical expanded memory pattern is:

Pregnancy + Hypothyroidism + Acromegaly + Amyloidosis + RA + Obesity.

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22. Other Associations

Other recognised associations include:

Diabetes mellitus.

Chronic kidney disease.

Previous wrist fracture or trauma.

Repetitive wrist activity.

Tenosynovitis.

However, many cases occur without a clearly identifiable underlying disorder.

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23. Tinel Sign

Tinel sign is tested by tapping over the median nerve at the carpal tunnel.

A positive test reproduces:

Tingling or electric sensations in the median nerve distribution.

This may support the diagnosis.

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24. Phalen Sign

Phalen test is performed by holding the wrists in maximal flexion, usually with the backs of the hands pressed together.

A positive test reproduces:

Numbness or tingling in the lateral three and a half fingers.

Symptoms often appear within about a minute.

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25. Limitations of Tinel and Phalen Tests

Tinel and Phalen signs can support the diagnosis, but neither is completely sensitive or specific.

Therefore:

A negative test does not exclude carpal tunnel syndrome.

The overall clinical pattern remains important.

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26. Nerve Conduction Studies

Nerve conduction studies are useful when:

The diagnosis is uncertain.

Symptoms are severe.

There is motor weakness or thenar wasting.

Surgery is being considered.

Typical findings include slowed median nerve conduction across the wrist.

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27. Electromyography

EMG may be added when severe nerve damage is suspected.

It can show evidence of denervation in median-innervated thenar muscles.

It is particularly useful in advanced cases or when another neuropathy needs to be excluded.

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28. Wrist Splinting

Initial treatment often includes a neutral-position wrist splint.

This is especially useful at night.

Keeping the wrist neutral reduces pressure within the carpal tunnel and may improve nocturnal symptoms.

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29. Activity Modification

Patients may also benefit from reducing activities that involve:

Repeated wrist flexion.

Repeated wrist extension.

Prolonged pressure over the wrist.

Ergonomic modification can be useful when occupational factors contribute.

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30. Corticosteroid Injection

A local corticosteroid injection into the carpal tunnel can provide symptom relief.

The injection is placed into the carpal tunnel region, taking care to avoid direct injury to the median nerve.

It is more accurate to say carpal tunnel steroid injection rather than steroid injection “to the flexor retinaculum.”

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31. Surgical Decompression

Surgical treatment involves division of the:

Flexor retinaculum / transverse carpal ligament.

This releases pressure on the median nerve.

The procedure is known as:

Carpal tunnel release.

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32. Indications for Surgery

Surgery is more strongly considered when there is:

Persistent symptoms despite conservative treatment.

Severe nerve conduction abnormalities.

Thenar weakness or wasting.

Persistent sensory loss.

Progressive neurological deficit.

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33. Carpal Tunnel Syndrome – Note Form

Nerve: median nerve.

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Site: wrist, beneath the flexor retinaculum.

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Sensory symptoms: pain, numbness and paraesthesiae.

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Timing: often worse at night.

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Sensory distribution: lateral three and a half fingers.

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Thenar skin: usually spared because palmar cutaneous branch does not pass through the tunnel.

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Motor weakness: LOAF muscles.

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L: lateral two lumbricals.

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O: opponens pollicis.

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A: abductor pollicis brevis.

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F: flexor pollicis brevis.

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Advanced feature: thenar wasting.

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Associations: pregnancy, hypothyroidism, acromegaly, amyloidosis, rheumatoid arthritis and obesity.

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Tinel sign: tapping over median nerve produces tingling.

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Phalen sign: wrist flexion reproduces paraesthesiae.

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Investigation: nerve conduction studies when indicated.

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Initial treatment: neutral wrist splint.

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Further treatment: local corticosteroid injection.

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Definitive treatment in persistent/severe disease: surgical carpal tunnel decompression.

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34. Median Nerve Lesion at the Wrist

Carpal tunnel syndrome should be distinguished from a more proximal median nerve lesion.

In carpal tunnel syndrome:

Forearm median-innervated muscles are spared.

Thenar muscles may be weak.

Lateral 3½ digits develop sensory symptoms.

Thenar eminence sensation is usually preserved.

This helps localise the lesion specifically to the carpal tunnel.

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Key Clinical Pattern

Think of carpal tunnel syndrome as:

MEDIAN NERVE COMPRESSION AT WRIST → NOCTURNAL PAIN/PARAESTHESIA + LATERAL 3½ FINGERS + LOAF WEAKNESS.

Remember:

LOAF = Lateral 2 lumbricals + Opponens pollicis + Abductor pollicis brevis + Flexor pollicis brevis.

And the useful localisation clue is:

Lateral 3½ fingers numb, but thenar eminence sensation usually spared.