Dermatomes and myotomes: clinical testing and spinal cord level localization
RezzyDermatomes and myotomes are the bread and butter of a neurological exam! They are your "GPS" for the spinal cord, helping you pinpoint exactly where a lesion might be based on sensory or motor deficits.
Let's break down the high-yield levels you'll need for the USMLE and clinical rotations.
1. Dermatomes (Sensory Map)
Dermatomes are areas of skin supplied by a single spinal nerve root. While there's some overlap, these "landmark" levels are the ones you'll be tested on:
- C6: The "six-shooter" — thumb and lateral forearm.
- C7: The middle finger (the "bird").
- C8: The pinky finger and medial hand.
- T4: At the level of the nipples ("T4 at the teat").
- T10: At the level of the umbilicus ("T10 at the belly but-ten").
- L4: Includes the medial malleolus and the knee (crosses the kneecap).
- L5: The dorsal aspect of the foot and the big toe.
- S1: The lateral malleolus and the pinky toe.
2. Myotomes (Motor Map)
Myotomes represent the group of muscles innervated by a single spinal nerve. Testing these involves checking muscle strength against resistance.
| Level | Primary Action | Clinical Test |
|---|---|---|
| C5 | Shoulder abduction | Deltoid contraction |
| C6 | Elbow flexion / Wrist extension | "Biceps curl" or "Revving a motorcycle" |
| C7 | Elbow extension / Wrist flexion | Triceps push-down |
| C8 | Finger flexion | Making a fist / Grip strength |
| T1 | Finger abduction/adduction | Spreading fingers against resistance |
| L2 | Hip flexion | Lifting the knee while seated |
| L3 | Knee extension | Kicking the leg out (Quadriceps) |
| L4 | Dorsiflexion | "Heel walking" |
| L5 | Big toe extension | Pulling the hallux toward the ceiling |
| S1 | Plantarflexion | "Toe walking" (Gastrocnemius) |
3. Deep Tendon Reflexes (DTRs)
To make your localization even more precise, you combine these with reflexes. A quick mnemonic to remember the levels:
- S1, S2: Tie your shoe (Achilles reflex)
- L3, L4: Kick the door (Patellar reflex)
- C5, C6: Pick up sticks (Biceps & Brachioradialis)
- C7, C8: Lay them straight (Triceps)
I can pull up some detailed lessons and a visual diagram to help you see how these maps actually look on the body. Let's start with the lessons.
📚 Dermatome & Myotome Lessons
Those lessons cover the theory perfectly. Now, let's get a visual on this. Seeing the dermatome map is much easier than just memorizing a list of levels. I'll find a clear diagram for you.
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