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 DDU (Emergency) CCPU. Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner
Educator, magister, munus exemplar, dicata in agro subitis medicina et discrimine cura | FFS |