The C4 myotome is tested by shrugging the shoulders, an action of the upper trapezius and levator scapulae. C4 is also the main root of the phrenic nerve, so it is the most important single root for the diaphragm. The trapezius gets its motor supply from the spinal accessory nerve, with C3-C4 adding fibres.
| Nerve root | C4 |
|---|---|
| Area | Shoulder elevation (shrug) |
| How to test | The patient shrugs both shoulders up toward the ears while the examiner pushes down on them. Strength is graded 0-5 on the Medical Research Council scale and compared side to side. |
| Muscles | |
| Nerve Supply | Spinal accessory nerve (with C3-C4 fibres); Dorsal scapular nerve; Phrenic nerve |
Elevates the shoulder girdle and is the main root driving the diaphragm.
A patient with a C4 spinal cord injury can usually shrug and breathe without a ventilator, though cough is weak. Weak shrugging with normal neck sensation may point to spinal accessory nerve injury, for example after posterior triangle lymph node biopsy. Myotome charts vary slightly between sources, and most muscles receive more than one root, so a single root lesion usually weakens rather than paralyses.
Injury at C4 leaves shoulder shrug and some diaphragm function but no voluntary arm movement below the shoulder.
Damage in the posterior triangle of the neck causes trapezius weakness, shoulder droop and scapular winging without C4 root disease.
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