Anatomy chapters
Dermatomes of the upper limb
Learn the upper-limb sensory landmarks and use them alongside strength and reflexes to distinguish root, plexus and nerve lesions.
- A dermatome belongs to a spinal segment, not one named peripheral nerve.
- C6 thumb, C7 middle finger, C8 little finger are practical anchors.
- Combine sensory findings with movements and reflexes.
On this page
Overview
A dermatome is an area of skin whose sensory fibres mainly reach one spinal cord segment through a spinal nerve. A peripheral cutaneous nerve territory is the area supplied by one named nerve after fibres have mixed in the plexus.
These maps answer different questions.
Upper-limb dermatomes run roughly along the limb rather than forming the horizontal bands seen on the trunk. C5 to T1 are central to upper-limb examination, while C4 and T2 help define the shoulder and axillary borders.
How the map is formed
Sensory fibres pass from skin into peripheral nerves, through the brachial plexus and into spinal nerves. Their cell bodies are in dorsal root ganglia. Their central processes enter the spinal cord through dorsal roots.
A named nerve normally contains fibres from several roots. Conversely, one root contributes to several named nerves.
For example, C6 sensation can be affected in more than one peripheral nerve territory when the lesion is proximal to the plexus.
Growth and rotation of the developing limb account for its longitudinal sensory pattern. The thumb is on the preaxial, radial side and the little finger on the postaxial, ulnar side.
This provides orientation, but it does not replace learning the actual landmarks.
Use thumb C6, middle finger C7 and little finger C8 as examination anchors.
The main sensory landmarks
Snell teaches segmental sensory territories with overlap between adjacent levels. The following landmarks are practical examination anchors, rather than precise borders around an exclusive strip of skin.
| Segment | Useful landmark | Broader region | Interpretation |
|---|---|---|---|
| C4 | Top of shoulder near the acromioclavicular region | Shoulder cape | Helps distinguish the shoulder border from C5 |
| C5 | Lateral upper arm | Deltoid region and lateral arm | Assess with shoulder abduction and elbow flexion |
| C6 | Thumb | Radial forearm and radial side of hand | Assess with wrist extension and biceps reflex |
| C7 | Middle finger | Central hand and adjoining forearm region | Assess with elbow extension and triceps reflex |
| C8 | Little finger | Ulnar hand and adjoining distal forearm | Assess with finger flexion |
| T1 | Medial forearm | Medial forearm towards elbow | Assess intrinsic hand function as well |
| T2 | Axilla and proximal medial arm | Upper medial arm adjoining chest | Includes the intercostobrachial region |
The sequence thumb C6, middle finger C7 and little finger C8 is especially useful. T1 should not be reduced to a finger landmark; the medial forearm is the more useful sensory anchor.
Dermatome boundaries overlap and vary; a diagram is not an exact sensory border.
Overlap and differences between maps
Adjacent roots share skin territories. A single root lesion may cause pain, tingling or reduced sensation without producing a sharply bounded area of complete anaesthesia.
A normal light-touch examination therefore does not rule out a root lesion.
Published maps also differ. Snell's regional account gives the conventional segmental pattern; modern clinical maps similarly recognise substantial individual variation and overlap.
Use one consistent map for revision and describe the actual sensory findings in a patient.
A line drawn on a diagram is a teaching boundary. It is not a prediction that every patient will lose sensation on one side of that exact line.
Medial forearm sensation is not an ulnar nerve territory.
Dermatomes versus peripheral nerves
A little-finger deficit could fit C8 disease or ulnar neuropathy. The decision depends on the rest of the examination.
Medial forearm sensation travels mainly through the medial cutaneous nerve of the forearm, rather than the ulnar nerve.
Similarly, the lateral forearm is supplied by the lateral cutaneous nerve of the forearm, the continuation of the musculocutaneous nerve.
The dorsal first web space belongs to the superficial radial nerve, while the palmar index fingertip belongs to the median nerve.
| Name | Origin/roots | Supply/action | Clinical note |
|---|---|---|---|
| Upper lateral cutaneous nerve of arm | Axillary nerve, C5–C6 | Skin over lower deltoid | A peripheral test site, not a whole dermatome |
| Lateral cutaneous nerve of forearm | Musculocutaneous, C5–C7 | Lateral forearm skin | Useful when comparing a root with a terminal nerve lesion |
| Superficial radial nerve | Radial nerve, C5–T1 parent nerve | Dorsolateral hand skin | Test dorsal first web space |
| Median digital nerves | Median nerve, C5–T1 parent nerve | Lateral palmar digits and distal dorsal surfaces | Test palmar index fingertip |
| Ulnar digital nerves | Ulnar nerve, mainly C8–T1 | Medial one and a half digits | Test palmar little fingertip |
| Medial cutaneous nerve of forearm | Medial cord, C8–T1 | Medial forearm skin | Usually spared in isolated ulnar neuropathy |
| Intercostobrachial nerve | Usually T2 | Axillary floor and proximal medial arm | May be affected during axillary surgery |
The parent-nerve roots in this table should not be read as the exact root contribution to every individual skin point.
Add movements and reflexes
A myotome is the muscle or movement contribution of a spinal segment. Most movements receive more than one root, so the following are dominant clinical associations.
| Root | Movement to assess | Useful associated reflex |
|---|---|---|
| C5 | Shoulder abduction and elbow flexion | Biceps, C5–C6 |
| C6 | Wrist extension and elbow flexion | Brachioradialis, C5–C6, mainly C6 |
| C7 | Elbow extension | Triceps, mainly C7 |
| C8 | Distal finger flexion | No routinely used isolated tendon reflex |
| T1 | Finger abduction and adduction | No routinely used isolated tendon reflex |
A root lesion may weaken muscles served by different peripheral nerves. C7 involvement, for example, can affect radial-supplied triceps and median-supplied forearm muscles.
An isolated radial neuropathy should not weaken median-supplied muscles.
A practical sensory examination
Explain the test and demonstrate the stimulus on an unaffected area. Ask the patient to close their eyes, then compare corresponding sites on both limbs. Start with the main landmarks and expand around any abnormal finding.
Test light touch and, when appropriate, pinprick. Record whether sensation is reduced, absent, unpleasant or simply different.
Avoid documenting only “C6 reduced” when the finding can be described more precisely as reduced thumb and radial forearm sensation.
Next test relevant movements and reflexes. Finally, compare the distribution with both a root map and a peripheral nerve map. Pain, effort and pre-existing disease can complicate interpretation.
Clinical relevance
Cervical radiculopathy
How it happens: A cervical root is compressed or irritated; a C5–C6 disc lesion commonly affects C6.
What you see: Radiating pain with thumb symptoms, weak wrist extension and reduced brachioradialis reflex supports C6 involvement. Neck pain may accompany the limb findings.
Ulnar neuropathy versus C8–T1 disease
How it happens: A lesion may affect the ulnar nerve alone or the roots/lower plexus proximal to several terminal nerves.
What you see: Little-finger sensory loss and interosseous weakness can occur in either. Additional medial forearm numbness or median-supplied intrinsic weakness suggests a more proximal lesion.
Herpes zoster
How it happens: Virus reactivates in a sensory ganglion.
What you see: Pain followed by segmental vesicles follows a dermatome rather than the entire distribution of one terminal nerve. Atypical or widespread eruptions need broader assessment.
Link thumb, middle finger and little finger to C6, C7 and C8 respectively. Use medial forearm sensation and muscles supplied by different nerves to distinguish root disease from ulnar neuropathy. Combine the map with reflexes; do not treat dermatome borders as absolute.
Quick revision
- A dermatome refers to a spinal segment; a nerve territory refers to a named nerve.
- C5 is lateral arm; C6 thumb; C7 middle finger; C8 little finger.
- T1 is tested over the medial forearm and T2 near the axilla.
- Adjacent dermatomes overlap and published maps vary.
- Median sensation includes the palmar index fingertip.
- Radial sensation is tested in the dorsal first web space; root lesions can involve muscles from several peripheral nerves.
Sources: Snell's Clinical Anatomy by Regions, supplied Upper Limb chapter; UHS PGET Paper I MBBS Table of Specifications. Supplementary clinical testing and developmental context follow standard examination conventions and Duke Embryology where relevant. Picture links are visual study resources. Text and tables written independently for The House Job.
FAQ
Is the entire thumb supplied by the median nerve?
No. Palmar and dorsal regions have different peripheral nerve supplies, although the thumb is a useful C6 dermatome landmark.
Is medial forearm numbness typical of isolated ulnar nerve injury?
No. This skin is mainly supplied by the medial cutaneous nerve of the forearm. Its involvement prompts assessment for a more proximal lesion.
Can a root lesion have normal skin sensation?
Yes. Overlap and incomplete involvement may leave bedside sensation normal despite pain, weakness or reflex changes.
Which root is associated with the triceps reflex?
Mainly C7. The reflex travels through the radial nerve and also receives a C8 contribution.