Myotomes
A myotome refers to the spinal nerve root that innervates a particular skeletal muscle.
It should be noted that:
- Each spinal segmental nerve root innervates more than one muscle.
- Most muscles are innervated by more than one spinal root (commonly by two).
For the purpose of clinical assessment, particular muscles or muscle groups are correlated with a single spinal nerve root segment to simplify examination.
Key Muscle Myotomes
The key muscle myotomes for rapid clinical assessment include the following:
| Spinal Segmental Root | Predominant Muscle Group |
|---|---|
| C5 | Shoulder abduction (deltoid) |
| C6 | Wrist extensors (biceps, extensor carpi radialis longus and brevis) |
| C7 | Elbow extensors (triceps) |
| C8 | Finger flexors to the middle finger (flexor digitorum profundus) |
| T1 | Small finger abductors (abductor digiti minimi) |
| L2 | Hip flexors (iliopsoas) |
| L3, L4 | Knee extensors (quadriceps) |
| L4, L5, S1 | Knee flexors (hamstrings) |
| L5 | Ankle and great toe dorsiflexors (tibialis anterior, extensor hallucis longus) |
| S1 | Ankle plantar flexors (gastrocnemius, soleus) |
The Grading of Muscle Power
Muscle power is clinically graded according to a scale out of 5.
Documentation helps to objectively define disability and track improvement or deterioration.
| Score | Examination Findings |
|---|---|
| 0 | Total paralysis |
| 1 | Palpable or visible contraction only |
| 2 | Full range of motion provided gravity is eliminated |
| 3 | Full range of motion against gravity |
| 4 | Full range of motion, but less than normal strength |
| 5 | Normal strength |
| NT | Not testable |
Reflexes
The table below shows the spinal segmental level of the common muscle reflex arcs:
| Spinal Segment | Muscle Reflex |
|---|---|
| C5, C6 | Biceps jerk |
| C7, C8 | Triceps jerk |
| C5, C6 | Brachioradialis (supinator) jerk |
| C8 | Finger jerks |
| L3, L4 | Knee jerk |
| S1, S2 | Ankle jerk |
| L5, S1, S2 | Plantar reflex |
Reflex Activity Grading
Reflex activity is recorded as follows:
| Score | Activity |
|---|---|
| 0 | Absent reflexes |
| + | Reduced reflexes |
| ++ | Normal reflexes |
| +++ | Exaggerated reflexes |
| ++++ | Exaggerated reflexes with clonus |
References
FOAMed
- Nickson C. Myotomes and Differentiating Nerve Lesions. CCC
- Coni R. Neuro 101: Spinal Cord. LITFL
- Nickson C. Spinal Injury Patient Hot Case. CCC
- Nickson C. Acute Traumatic Spinal Cord Injury. CCC
Fellowship Notes
MBBS FACEM DDU (Emergency) CCPU. Emergency Physician in Melbourne, Australia. Co-Ultrasound Lead for Emergency Medicine at The Alfred Hospital. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor of the LITFL ECG Library.
Educator, magister, munus exemplar, dicata in agro subitis medicina et discrimine cura | FFS |