Muscle:External Oblique
External oblique is the most superficial of the three lateral abdominal wall muscles. Its trigger points (TrPs) produce a wide range of referred pain patterns — from "heartburn" in the epigastric region to groin and testicular pain — and can initiate viscerosomatic disturbances capable of closely mimicking appendicitis, cholecystitis, and other acute visceral pathology. Because its fibres interdigitate with the serratus anterior and latissimus dorsi, dysfunction in those muscles and in the thoracolumbar region can produce satellite TrPs in the external oblique and vice versa.
Anatomy
The external oblique is the largest and most superficial of the lateral abdominal wall muscles. Its fibres run diagonally downward and forward from the external surfaces and inferior borders of the lower eight ribs. The lower three rib attachments interdigitate with the latissimus dorsi; the upper five interdigitate with the serratus anterior. Anteriorly the muscle joins the abdominal aponeurosis, attaching to the linea alba in the midline and to the anterior half of the iliac crest.
Primary action: Increases intra-abdominal pressure (bilaterally); flexes and rotates the vertebral column — the external oblique rotates the vertebral column toward the contralateral side. Functions eccentrically to control and brake trunk rotation in the opposite direction.
Innervation: Branches of the eighth through twelfth intercostal nerves; segmental innervation T8–T12.
Main synergists: Internal oblique (contralateral), serratus anterior, external intercostals, vertical costal fibres of latissimus dorsi.
Memory aid for fibre direction: Place the right hand flat on the lower left abdomen with fingers pointing downward toward the opposite hip — the fingers represent the external oblique fibre direction on that side (same as sliding hands into the front trouser pockets).
Referred Pain Patterns
The external oblique TrPs have multiple referred pain patterns that may reach into the chest, travel straight or diagonally across the abdomen, and extend downward. Variability in patterns likely represents the successively deeper layers of this muscle and the diagonal crisscross arrangement of its fibres, analogous to the plies of a tyre.
Upper Attachment TrPs — "Heartburn"
Active TrPs in the upper external oblique, in the part of the muscle overlying the anterior rib cage, are likely to produce deep epigastric pain described by the patient as "heartburn." This pain pattern may occasionally extend to other parts of the abdomen. These are sometimes called costal or subcostal TrPs. The same patterns have been observed from TrPs in the external oblique at its rib cage attachments and from TrPs in the pectoralis major, which overlies this region.
Lower Lateral Wall TrPs — Groin and Testicular Pain
Active TrPs in the lower lateral abdominal wall — possibly in any one of the three muscle layers — refer pain to:
- The groin and testicle (or labium majus in females)
- Other parts of the lower abdomen
Experimental injection of hypertonic saline into the external obliques near the anterior superior iliac spine induced referred pain over the lower portion of that quadrant of the abdomen, along the inguinal ligament and into the testicle. A left external abdominal oblique TrP in a 10-year-old child referred severe pain from the left upper quadrant to the left inguinal region.
TrPs along the upper rim of the pubis and the lateral half of the inguinal ligament may lie in the lower internal oblique or in the lower rectus abdominis; when needled, such TrPs often refer pain to the urinary bladder region.
The "Belch Button" TrP
The belch button is an uncommon but clinically important TrP. It has not been consistently localised to a specific muscle — it may lie in the posterior fringe of the external oblique, or it may be a fascial TrP in the lumbodorsal fascia. It is found at, or just below, the angle of the twelfth rib. When located by palpation, a rib is palpable beneath the examining finger.
When sufficiently active, this TrP causes spontaneous belching and, in severe cases, projectile vomiting — a serious postoperative complication risk. The patient is likely to complain of a "stomach problem" with much belching of gas. See Muscle:Abdominal Wall/Belch Button for full details.
Somatovisceral and Viscerosomatic Effects
TrPs in the external oblique participate in strong reciprocal somatovisceral and viscerosomatic interactions:
- Somatovisceral: Active TrPs can initiate or worsen diarrhoea, nausea, vomiting, urinary bladder irritability, and sphincter spasm — without any underlying visceral pathology
- Viscerosomatic: Visceral disease (peptic ulcer, intestinal parasites, dysentery, ulcerative colitis, diverticulitis, cholelithiasis) can activate and perpetuate TrPs in the external oblique, which may then persist long after the initiating visceral disease has resolved, continuing to refer pain that closely mimics the original visceral symptom
A direct linear correlation has been demonstrated between the severity of visceral pain episodes and hyperalgesia of the ipsilateral external oblique muscle.
Activation and Perpetuating Factors
Visceral Disease
Peptic ulcer, intestinal parasites (Entamoeba histolytica, fish or beef tapeworm), dysentery, ulcerative colitis, diverticulosis, diverticulitis, and cholelithiasis are important activating factors.
Trauma and Surgery
Acute trauma, direct blow, or abdominal scar (appendicectomy, hysterectomy) — the initiating stresses during surgery include excessive stretch by retractors and associated ischaemia.
Occupational and Postural Strain
- Sustained twisted posture — sitting sideways at a desk due to monitor placement
- Activities requiring vigorous twisting body motion (throwing the discus)
- Forward-head posture or slumped sitting posture
Other Factors
- Paradoxical respiration — asynchronous chest-diaphragm breathing patterns
- Over-enthusiastic or poorly conditioned abdominal exercise
- Emotional stress, cold exposure, constipation (straining at stool)
- Satellite TrP activation — paraspinal TrPs at T7–T12 levels may activate external oblique TrPs as satellites; dorsal TrPs at the belch button location may be key TrPs activating the external oblique
Clinical Examination
Abdominal Tension Test
The Abdominal Tension Test distinguishes abdominal wall TrP pain from pain originating inside the abdomen, and is essential whenever abdominal TrPs are suspected:
- With the supine patient at rest, compress the sensitive area with sufficient pressure to cause steady pain
- Ask the patient to raise both heels several inches off the table — this tenses the abdominal muscles and lifts the palpating finger away from the viscera
- If the pain increases: the pain originates in the abdominal wall
- If the pain decreases: the pain more likely originates inside the abdomen
Modified techniques to increase abdominal tension include a partial sit-up (Llewellyn and Jones), the Carnett technique (patient crosses arms and sits half-way forward), raising only the head and shoulders (Wilson / Kelsey), or elevating both feet and head simultaneously (de Valera and Raftery).
Trigger Point Examination
The patient lies supine and takes a deep diaphragmatic breath, holding it to passively stretch and relax the abdominal muscles and increase sensitivity to palpation.
Attachment TrPs: Palpate along the lower border of the rib cage and along the line of attachment to the iliac crest using flat palpation.
Central TrPs: The patient lies on the contralateral side and takes a similar deep lateral breath. In thin patients, flex the hips to slacken the abdominal wall; the lateral wall can then be grasped between the fingers and thumb in a pincer grip. Roll the muscle between digits to identify tender nodules in palpable bands.
Umbilical deviation test: At rest, the umbilicus deviates away from a weaker (inhibited) muscle and toward a stronger (hyperactive) one. Deviation may also become apparent during activities such as laughing, coughing, or raising one leg from the bed.
Differential Diagnosis
| Condition | Distinguishing features |
|---|---|
| Appendicitis | Right lower quadrant pain from lower external oblique TrPs mimics appendicitis; abdominal wall shows palpable taut band and nodule rather than board-like diffuse rigidity; Abdominal Tension Test positive; ESR and WBC normal; Rovsing's sign and rebound tenderness absent |
| Cholecystitis | Right upper quadrant TrPs in the costal portion of the external oblique confused with gallbladder pain; ultrasound and liver function tests distinguish; subcutaneous infiltration of the painful area with procaine relieves TrP-generated pain |
| Inguinal hernia | Lower lateral TrP pain radiating into the groin along the inguinal ligament; no palpable hernia sac; Valsalva does not reproduce pain |
| Testicular or ovarian pathology | TrPs in lower lateral abdominal wall refer pain to testicle or labium — ultrasound required to exclude primary pathology |
| Peptic ulcer | Upper external oblique TrPs produce epigastric "heartburn"; upper GI series and endoscopy differentiate; TrP injection or pressure release reproduces and relieves the pain |
| Fibromyalgia | Widespread pain present for ≥3 months; fibromyalgia and TrPs are different diseases that cause pain for different reasons and respond to different treatments; more than half of fibromyalgia patients also have TrPs |
Treatment
Trigger Point Release — Spray and Stretch
- Patient supine with the hip joint at the edge of the treatment table; lower limbs extend over the end; one limb supported on a stool to avoid lumbosacral overextension; arms raised overhead
- Vapocoolant spray applied in sweeps in a caudal direction (down-sweep pattern) over the abdomen and extending to the iliopsoas attachment
- Patient takes a very deep breath, allowing the diaphragm to strongly protrude the relaxed abdominal musculature — this is the critical stretch step
- The contralateral muscles must also be treated
- After release, patient assumes bilateral knee-to-chest position; moist heat applied promptly
Postisometric relaxation and contract-relax techniques are also effective for central TrPs. Pressure release (ischaemic compression) is most successful for TrPs close to the pubic arch; less successful in patients with excess adipose tissue.
Trigger Point Injection
Injection of lateral wall oblique TrPs employs the pincer technique:
- Flex the patient's hips to slacken the abdominal wall
- Grasp the abdominal wall between the fingers and thumb so that no abdominal contents remain within the grasp
- Locate the TrP by rolling the musculature between the digits to identify a tender nodule in a palpable band
- Direct the needle precisely into the TrP fixed within the operator's grasp
- Avoid penetrating the peritoneal cavity
Active full range of motion with repetition of vapocoolant spray is performed slowly after injection, then followed by moist heat.
Corrective Actions
- Self-administration of TrP pressure release
- Abdominal (diaphragmatic) breathing — the most effective active stretch exercise for lateral abdominal muscles, especially when performed prone
- Pelvic-tilt exercise (see Muscle:Rectus Abdominis — Corrective Actions)
- Sit-back/Abdominal-curl/Sit-up exercise sequence (see Muscle:Rectus Abdominis — Corrective Actions)
- Laughter — a vigorous isometric exercise for all abdominal muscles; "pleasant medicine"
Satellite Trigger Points
- Internal oblique — primary functional partner; commonly co-active
- Transversus abdominis — deep synergist; commonly co-active
- Rectus abdominis — medial synergist
- Latissimus dorsi — interdigitates at lower rib attachments; bilateral satellite relationship
- Serratus anterior — interdigitates at upper rib attachments
- Paraspinal muscles T7–T12 — key TrPs that activate external oblique as satellite
- Iliopsoas — lower external oblique stretch also stretches iliopsoas; commonly co-active TrPs
Related Pages
- Pain:Right Upper Quadrant — External oblique as source of pseudo-cholecystitis
- Pain:Groin and Testicle — Lower lateral wall referral pattern
- Pain:Abdominal — Diagnostic algorithm
- Muscle:Internal Oblique — Primary functional partner
- Muscle:Transversus Abdominis — Deep synergist
- Muscle:Rectus Abdominis — Medial synergist; corrective exercises shared
- Muscle:Abdominal Wall/Belch Button — Belch button TrP full description
References
- Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 2: The Lower Extremities. Baltimore: Williams & Wilkins; 1992. Chapter 49.Bold text