Myotomes
A myotome is the group of muscles innervated by a single spinal nerve. Many muscles are innervated by more than one spinal nerve root, and therefore consist of multiple myotomes, however some muscles commonly have dominant supply by a single nerve root. Understanding the myotomes is integral for performing an accurate motor system examination. In Musculoskeletal Medicine the concept is typically used in localising nerve root and peripheral nerve lesions.
Terminology
Myotome has Greek roots: myo means muscle (or "mouse," as early anatomists thought contracting muscle resembled a mouse moving under the skin) and tome means a section or volume. Therefore, a myotome literally translates to a "section of muscle".
Clinically, a myotome is defined as a group of muscles innervated by a single spinal nerve root. This direct link is what makes myotome testing a powerful diagnostic tool. By testing a specific movement, a practitioner can effectively evaluate the function of the associated spinal cord level and its corresponding spinal nerve.
The concept originates in embryology. During development, the embryo forms segmented blocks of tissue called somites. Each somite differentiates into three parts: Dermatome (forms skin), sclerotome (forms skeleton), and myotome (forms skeletal muscles). As these embryonic myotomes migrate to form the body's muscles, they maintain their connection to their original spinal cord segment, establishing the nerve pathways we test clinically.
Clinical Application
If there is motor loss that occurs within a myotomal distribution then that suggests a radiculopathy (or a rare cervical spondylotic amyotrophy) rather than say a peripheral nerve lesion or a plexopathy. This is the most important clinical utility in MSK medicine.
Knowledge of the myotomes can also be useful for assisting in localising plexopathies and peripheral nerve lesions. One takes account of all affected muscles, then assess which nerve roots are common to all those muscles. This can be combined with other aspects of the neurological examination. For example in a sciatic trunk lesion there may be motor loss affecting both common peroneal and tibial nerve distribution muscles, plus sensory loss involving the foot and lateral leg, but sparing of the medial leg.
Simplified Examination
| Spinal Level | Movement |
|---|---|
| Upper Limb | |
| C5 | Shoulder abduction |
| C6 | Elbow pronation |
| C7 | Elbow extension |
| C8 | Flexor digitorum profundus 4 |
| T1 | Flexor digitorum profundus 1 |
| Lower Limb | |
| L2 | Hip flexion |
| L3 | Hip adduction |
| L4 | Knee extension |
| L5 | Ankle dorsiflexion |
| S1 | Ankle plantarflexion |
Commonly Tested Movements
| Movement | UMN | Root | Reflex | Nerve | Muscle |
|---|---|---|---|---|---|
| Upper Limb | |||||
| Shoulder abduction | ++ | C5 | Axillary | Deltoid | |
| Elbow flexion | C5/6 C5 |
+ + |
Musculocutaneous Radial |
Biceps Brachioradialis | |
| Elbow extension | + | C7 | + | Radial | Triceps |
| Radial wrist extension | + | C6 | Radial | Extensor carpi radialis longus | |
| Finger extension | + | C7 | Posterior interosseous nerve | Extensor digitorum communis | |
| Finger flexion | C8/T1 | + |
Anterior interosseous nerve Ulnar |
Flexor pollicis longus and flexor digitorum profundus (index) Flexor digitorum profundus (ring and little) | |
| Finger abduction | ++ |
T1 T1 |
Ulnar Median |
First dorsal interosseous Abductor pollicis brevis | |
| Lower Limb | |||||
| Hip flexion | ++ | L1/2 | Iliopsoas | ||
| Hip adduction | L2/3 | + | Obturator | Adductors | |
| Hip extension | L5/S1 | Sciatic | Gluteus Maximus | ||
| Knee flexion | + | S1 | Sciatic | Hamstrings | |
| Knee extension | L3/4 | + | Femoral | Quadriceps | |
| Ankle dorsiflexion | ++ | L4 | Deep peroneal | Tibialis anterior | |
| Ankle eversion | L5/S1 | Superficial peroneal | Peronei | ||
| Ankle plantarflexion | S1/S2 | + | Tibial | Gastrocnemius, soleus | |
| Big toe extension | L5 | Deep peroneal | Extensor hallucis longus | ||
Complete Myotome Chart
The below charts are based on multiple sources, however are generally mostly consistent with Sonoo's charts.[2][3][4][5][6][7][8][9] Sonoo notes that there is a lot of discrepancy in the actions assigned to specific myotome levels between textbooks. There are a few important points to bear in mind when applying the below charts to individual patients[2]
- The spinal nerve roots listed for particular muscles are the innervation range across a population, not necessarily the range within a single patient.
- Individuals probably have a much more limited range of innervation, and some muscles may even be innervated by a single spinal nerve root. For example the inferior gluteal nerve arises from L5-S2, but that doesn't necessarily mean the gluteus maximus derives innervation from all three roots.
- Each muscle often has one dominant root that provides the major supply.
- Patients can have pre or post fixed plexus anatomy, and this might be the most important of variation between individuals.
- No existing myotome chart is perfect. The quality of evidence varies substantially, even including vague "clinical experience."
- There is some basic logic in the code to help you localise the lesion within the peripheral nervous system (nerve root vs peripheral nerve) via selecting which muscles are strong and which are weak. Please use your only clinical judgement, this does not replace intimate knowledge of the peripheral nervous system, it is only a basic tool.
Upper Limbs
- Click the left column (❌) or anywhere on a row to mark a muscle as weak.
- Click the right column (✔) to mark a muscle as strong.
- Clicking a selected marker again will deselect it.
- The analysis below will update automatically, suggesting possible nerve root, plexus, and peripheral nerve involvement based on your selections.
Lower Limbs
- Click the left column (❌) or anywhere on a row to mark a muscle as weak.
- Click the right column (✔) to mark a muscle as strong.
- Clicking a selected marker again will deselect it.
- The analysis below will update automatically, suggesting possible nerve root, plexus, and peripheral nerve involvement based on your selections.
Myotome Dance
| Spinal Levels | Memory Aid | Action |
|---|---|---|
| Upper Limb | ||
| C1, 2 | I nod to you | Neck flexion/extension |
| C3 | Don’t tickle me | Neck lateral flexion |
| C4 | I’m not sure | Shoulder elevation |
| C5 | Feel alive | Shoulder flexion, abduction, & lateral rotation |
| C6, 7, 8 | Close the gate | Shoulder extension, adduction & medial rotation |
| C5, 6 | Pick up sticks | Elbow flexion |
| C7, 8 | Lay them straight | Elbow extension |
| C5, 6 | Flick my wrists | Forearm supination |
| C7, 8 | The time is late | Forearm pronation |
| C6, 7 | Fly up to heaven | Wrist flexion & extension |
| C7 | Paper | Finger extension |
| C8 | Rock | Finger flexion (though some sources say C7, 8 does both finger extension and flexion) |
| T1 | Scissors | Finger abduction & adduction |
| Lower Limb | ||
| L2, 3 | Lift my knee | Hip flexion |
| L3, 4 | Kick the door | Knee extension (& knee-jerk reaction) |
| L4, 5 | Foot points to the sky | Ankle dorsiflexion |
| L4, 5 | Extend my thigh | Hip extension |
| L5, S1, (S2) | Kick my bum (Run to poo) | Knee flexion |
| S1, 2 | Stand on my shoes | Ankle plantarflexion (& ankle jerk) |
| L2, 3, 4 | Modestly close the door | Hip adduction & internal/medial rotation |
| L4 - S2 | The opposite is true | Hip abduction & external/lateral rotation |
| C3, 4, 5 | Keeps the diaphragm alive | Innervates the diaphragm |
| S2, 3, 4 | Keeps poo off the floor | Innervates bowel, bladder, sex organs, anal sphincter, pelvic muscles. (& anal wink reflex) |
Note
- T1-12 Supplies chest wall and abdominal muscles
- L1 Contributes to hip flexion & adduction
- L5, S1, S2 Babinski plantar reflex/extensor response in UMN lesion
Resources
The article by Sonoo is highly recommended reading.
References
- ↑ The Noted Anatomist. Myotomes. Feb 29, 2020. Available on youtube
- ↑ 2.0 2.1 2.2 2.3 Sonoo, Masahiro (2023-08). "Recent advances in neuroanatomy: the myotome update". Journal of Neurology, Neurosurgery & Psychiatry (in English). 94 (8): 643–648. doi:10.1136/jnnp-2022-329696. ISSN 0022-3050. Check date values in:
|date=(help) - ↑ 3.0 3.1 Brain. Aids to the Examination of the Peripheral Nervous System, 4th edition. 2000
- ↑ Bell et al.. Refinement of myotome values in the upper limb: Evidence from brachial plexus injuries. The surgeon : journal of the Royal Colleges of Surgeons of Edinburgh and Ireland 2017. 15:1-6. PMID: 26409623. DOI.
- ↑ McGee, Steven R. Evidence-based physical diagnosis. Philadelphia, PA: Elsevier, 2018.
- ↑ Wilbourn, Asa J; Cherington, Michael (2000-02). "The lower plexus innervates the opponens pollicis and abductor pollicis brevis". The Annals of Thoracic Surgery (in English). 69 (2): 664–665. doi:10.1016/S0003-4975(99)01277-1. Check date values in:
|date=(help) - ↑ Chiba, Takashi; Konoeda, Fumie; Higashihara, Mana; Kamiya, Hisao; Oishi, Chizuko; Hatanaka, Yuki; Sonoo, Masahiro (2015-04). "C8 and T1 innervation of forearm muscles". Clinical Neurophysiology (in English). 126 (4): 837–842. doi:10.1016/j.clinph.2014.07.031. Check date values in:
|date=(help) - ↑ Stoker, Geoffrey E.; Kim, Han Jo; Riew, K. Daniel (2014-02). "Differentiating C8–T1 Radiculopathy from Ulnar Neuropathy: A Survey of 24 Spine Surgeons". Global Spine Journal (in English). 4 (1): 1–5. doi:10.1055/s-0033-1354254. ISSN 2192-5682. Check date values in:
|date=(help) - ↑ Caetano, Edie Benedito; Nakamichi, Yuri da Cunha; Alves de Andrade, Renato; Sawada, Maico Minoru; Nakasone, Mauricio Tadeu; Vieira, Luiz Angelo; Sabongi, Rodrigo Guerra (2017-11-23). "Flexor Pollicis Brevis Muscle. Anatomical Study and Clinical Implications". The Open Orthopaedics Journal (in English). 11 (1): 1321–1329. doi:10.2174/1874325001711011321. ISSN 1874-3250. PMC 5721304.CS1 maint: PMC format (link)
