Spinal Nerves, Dermatomes & Myotomes: Understanding Nerve Root Signs

 When a patient has back pain with leg symptoms — tingling, numbness, or weakness — your job is to figure out which nerve root is involved. 

That means understanding dermatomes (sensory territories) and myotomes (motor control), and linking them to the underlying pathophysiology 👇

📦 What are spinal nerves?

Spinal nerves emerge from the spinal cord as dorsal (sensory) and ventral (motor) roots. These roots unite to form a mixed spinal nerve, which exits through the intervertebral foramen. Each segmental nerve root contributes to specific dermatomes and myotomes.

➡️ Radiculopathy occurs when one of these nerve roots is compressed or inflamed.

🧠 Pathophysiology: What causes nerve root compression?

• The most common cause is intervertebral disc herniation — the nucleus pulposus bulges or extrudes through a weakened annulus fibrosus.
• If the herniation is posterolateral, it presses on the traversing nerve root — e.g. an L4/5 disc herniation compresses the L5 nerve root.
• This compression leads to:
• Sensory disturbance (e.g. numbness or tingling) in the corresponding dermatome
• Motor weakness in the associated myotome
• Possibly reduced or absent reflexes, depending on the level

Inflammation from disc material and local chemical mediators (e.g. TNF-α, interleukins) also contribute to neuropathic pain, even in the absence of significant mechanical compression.

🦵 Clinical Case:
A 40-year-old woman presents with low back pain radiating down the lateral thigh and across the dorsum of the foot. She has trouble dorsiflexing her ankle and says her foot sometimes “slaps” the ground when she walks.
🤔 Diagnosis? This is consistent with L5 radiculopathy, likely due to an L4/5 disc herniation.

🔍 Key Dermatomes & Myotomes – Lumbar and Sacral Roots

📌 L4

• Dermatome: medial shin to medial ankle
• Myotome: knee extension (quadriceps)
• Reflex: patellar reflex
• Deficits: weak quadriceps, trouble with stairs or rising from seated

📌 L5

• Dermatome: lateral thigh to dorsum of foot and big toe
• Myotome: ankle dorsiflexion, toe extension
• Reflex: none reliably tested
• Deficits: foot drop, tripping on toes

📌 S1

• Dermatome: lateral foot and sole
• Myotome: ankle plantarflexion
• Reflex: Achilles reflex
• Deficits: can't toe-walk, loss of ankle jerk

🔎 Disc herniation vs peripheral nerve injury?

Nerve root (radicular) lesions cause segmental deficits — following dermatomes and myotomes.
Peripheral nerve injuries (e.g. peroneal nerve injury) affect broader motor/sensory areas not confined to a single root. Understanding this distinction helps localise lesions clinically.

🖐️ Sensory testing tip:
Use light touch, pinprick, or temperature sensation tests to map dermatomes. Compare both sides — often the patient feels “different” before you can measure it.

🦵 Motor testing tip:
– Heel-walking = L5
– Toe-walking = S1
– Squatting and rising = L4
Watch for subtle asymmetry or fatigue after repeated movements.

📚 Clinical Pearl:
Not all patients will have a textbook pattern — but combining history, sensory testing, motor exam, and reflexes helps you localise the lesion and understand what structure is affected.
A strong grip on dermatomes and myotomes turns back pain into a diagnostic opportunity, not just a vague complaint.