Anatomy chapters
Dermatomes of the lower limb
Use the groin, knee, ankle and foot landmarks to localise root disease and distinguish it from peripheral neuropathy.
- L4 medial ankle, L5 dorsal foot and S1 lateral foot are useful anchors.
- Dermatomes describe spinal segments; cutaneous nerves describe peripheral pathways.
- Combine sensation with strength and reflexes, because adjacent roots overlap.
On this page
Overview
A dermatome is a skin territory linked mainly to one spinal segment through its sensory root. It is not the same as the territory of a named cutaneous nerve, which may contain fibres from several roots.
The lower limb shows a roughly longitudinal arrangement, modified by limb growth and rotation. Lumbar territories are prominent anteriorly and medially; sacral territories become prominent posteriorly and along the lateral foot.
Sensory landmarks
Use the following clinical anchor points alongside Snell’s regional nerve distributions. They are practical testing sites, not boundaries enclosing exclusive one-root territories.
| Segment | Useful test region | Broader association | Clinical use |
|---|---|---|---|
| L1 | Groin/inguinal region | Proximal anterior thigh border | Separates groin from more distal lumbar territories |
| L2 | Anterior thigh | Upper and middle thigh | Pair with hip flexion |
| L3 | Medial knee region | Distal anterior thigh | Pair with knee extension |
| L4 | Medial malleolus | Medial leg and ankle | Pair with knee reflex and ankle movements |
| L5 | Dorsum of foot and great-toe region | Anterolateral leg/dorsal foot | Pair with great-toe extension |
| S1 | Lateral foot and heel | Posterolateral calf | Pair with plantarflexion and ankle reflex |
| S2 | Posterior thigh/popliteal region | Posterior proximal limb | Adds information beyond the distal foot |
| S3 | Lower buttock and saddle region | Perineal sensory territory | Relevant to sacral root assessment |
| S4–S5 | Perianal region | Most caudal sensory territory | Relevant to sacral sparing and cauda equina assessment |
A common formal L5 test point lies over the dorsum near the third metatarsophalangeal joint. Great-toe-region sensation is also widely used clinically. State the exact site tested rather than assuming every map chooses the same point.
L4 is medial ankle, L5 dorsal foot, and S1 lateral foot/heel.
Overlap, variation and sensory modalities
Neighbouring dermatomes overlap. A partial lesion may cause tingling or altered pinprick rather than complete loss of light touch. The affected area may be smaller or less sharply defined than a textbook diagram suggests.
Different published maps use different evidence and testing conventions. Use a consistent map for revision, but record the patient's actual distribution. Do not force an irregular clinical finding into an exact coloured strip.
Test light touch and, when appropriate, pinprick independently. Compare corresponding sites on the two sides with the patient's eyes closed. A change in one sensory modality may be more evident than a change in another.
One normal sensory point does not exclude a root lesion; overlap and incomplete damage can preserve sensation.
Dermatomes versus peripheral territories
| Name | Origin/roots | Supply/action | Clinical note |
|---|---|---|---|
| Lateral femoral cutaneous | L2–L3 | Anterolateral thigh skin | Sensory symptoms without quadriceps palsy suggest a cutaneous lesion |
| Saphenous | Femoral, mainly L3–L4 | Medial leg and medial foot skin | Not a branch of sciatic |
| Superficial fibular | Common fibular, mainly L5–S2 | Most dorsum of foot | Usually excludes first web space and lateral foot border |
| Deep fibular | Common fibular, mainly L4–S1 | First dorsal web space | A peripheral nerve test point, not an entire L5 dermatome |
| Sural | Mainly S1–S2 | Lateral foot and posterolateral distal leg | Runs with small saphenous vein |
| Plantar branches | Tibial nerve | Most sole sensation | Different from dorsal fibular territories |
| Posterior femoral cutaneous | S1–S3 | Posterior thigh skin | Separate from sciatic motor supply |
Medial malleolar sensation can therefore be described both as an L4 landmark and as a saphenous territory. The first statement identifies a segment; the second identifies the peripheral route.
Myotomes and reflexes
A movement is supplied by several roots through several muscles. The following are useful dominant associations, not exclusive one-root assignments.
| Main root association | Movement to test | Reflex or companion finding |
|---|---|---|
| L2–L3 | Hip flexion | Compare anterior thigh sensation |
| L3–L4 | Knee extension | Patellar reflex, L2–L4, mainly L4 |
| L4–L5 | Ankle dorsiflexion/inversion | Assess tibialis anterior and tibialis posterior separately |
| L5 | Great-toe extension and hip abduction | No single dependable routine tendon reflex |
| S1–S2 | Plantarflexion | Achilles reflex, mainly S1 |
| S2 | Knee flexion contribution | Compare posterior thigh sensation |
Patellar reflex mainly tests L4; Achilles reflex mainly tests S1.
A root lesion may affect muscles supplied by different peripheral nerves. L5 disease can weaken tibialis posterior through the tibial nerve and gluteus medius through the superior gluteal nerve, as well as fibular-supplied dorsiflexors.
A practical localisation sequence
First map the sensory complaint without suggesting an answer. Compare groin or thigh sites if symptoms are proximal, then the medial ankle, dorsal foot and lateral foot distally.
Next test hip flexion, knee extension, dorsiflexion, great-toe extension and plantarflexion. Add inversion, eversion and hip abduction when foot drop is present. Complete the pattern with knee and ankle reflexes.
Finally ask whether the abnormalities share a root, plexus or terminal nerve. A tidy peripheral nerve territory with matching motor loss points differently from weakness distributed across several nerves but one root.
Foot drop is a sign, not a complete localisation. Weak inversion or hip abduction argues against an isolated common fibular lesion.
Clinical relevance
Lumbar disc-related radiculopathy
How it happens: a posterolateral disc protrusion usually compresses the traversing root; L4–L5 commonly affects L5, and L5–S1 commonly affects S1. Foraminal lesions can affect the exiting root instead.
What you see: L5 involvement may produce dorsal foot symptoms and weak great-toe extension; S1 involvement may reduce lateral-foot sensation, plantarflexion and the Achilles reflex.
Common fibular neuropathy
How it happens: local compression or trauma injures the nerve around the fibular neck.
What you see: foot drop and eversion weakness with dorsal sensory change. Preserved tibialis posterior inversion and preserved hip abduction favour this over an L5 root lesion.
Cauda equina involvement
How it happens: several descending lumbosacral roots are compressed within the spinal canal.
What you see: saddle sensory alteration may accompany bladder, bowel or sexual dysfunction and bilateral or asymmetric limb findings. New sacral sensory or sphincter symptoms need urgent assessment, even without a complete textbook pattern.
Segmental zoster
How it happens: viral reactivation affects a sensory ganglion.
What you see: pain and vesicles follow a segmental distribution rather than all the skin supplied by a major peripheral nerve.
Localise using sensory anchor, movement and reflex together rather than a dermatome diagram alone. Distinguish L5 root disease from common fibular palsy by inversion and hip abduction. Separate the traversing-root pattern of a posterolateral disc from the exiting root at the same level.
Quick revision
- L2 anterior thigh; L3 medial knee; L4 medial ankle.
- L5 dorsal foot; S1 lateral foot/heel; S2 posterior thigh.
- S3–S5 contribute to saddle and perianal sensation.
- Adjacent root territories overlap and maps vary.
- Knee reflex is mainly L4; ankle reflex mainly S1.
- A root lesion can cross several peripheral nerve territories.
Sources: Snell's Clinical Anatomy by Regions, supplied Lower Limb chapter, with clinical dermatome landmarks supplementing its referenced dermatomal charts; Sensory Thresholds at Different Sites of the Foot and standard neurological testing conventions. UHS PGET Paper I MBBS TOS determines the topic. Picture links are visual study resources. Original wording and tables.
FAQ
Is the first dorsal web space identical to the L5 dermatome?
No. It is a useful deep fibular sensory site. A dermatome covers a broader segmental territory and is not confined to one terminal nerve.
Which root is most associated with great-toe extension?
L5, assessed through extensor hallucis longus. The muscle receives its peripheral supply from the deep fibular nerve.
Which root is tested over the medial malleolus?
L4 is the usual clinical anchor. The peripheral cutaneous pathway is the saphenous nerve.
Can sensory overlap hide a single-root deficit?
Yes. Pain, weakness or altered reflexes may be more informative than light-touch loss in an incomplete lesion.