Differential Diagnosis: Swelling Over the Right Lower Back (Loin/Lumbar Region)

1. Skin & Subcutaneous Tissue

2. Musculoskeletal / Paraspinal

3. Retroperitoneal / Renal Origin

4. Vascular

5. Referred / Miscellaneous

Key Distinguishing Clinical Points

1. Impulse on coughing - present in lumbar hernia and cold abscess (meningocele also)
2. Ballotability - bimanually palpable, ballotable mass = renal origin
3. Transillumination - positive in cysts and meningocele
4. Temperature - hot abscess = pyogenic; cold, non-tender = TB cold abscess
5. Reducibility - reducible = hernia
6. Haematuria - renal tumour, polycystic kidneys, renal vein thrombosis
7. Age - Wilms' tumour (children); RCC (adults > 50); lipoma/hernia (any age)
8. Associated spinal signs (kyphosis, gibbus deformity, neurological deficit) - Pott's disease with cold abscess

High-Priority Diagnoses Not to Miss

• Renal cell carcinoma (especially if > 40 years, haematuria, weight loss)
• Pott's disease with cold abscess (especially in TB-endemic regions)
• Perinephric abscess (if febrile, unwell)
• Aortic aneurysm (if pulsatile, elderly male)
• Retroperitoneal sarcoma (if large, deep, progressive)

Psoas Muscle - Anatomy Recap

Classification of Psoas Muscle Abnormalities

1. PSOAS ABSCESS

Types

Secondary Causes (by source)

• Spinal / vertebral - Pott's disease (TB spondylitis) - the classic historical cause; vertebral osteomyelitis (S. aureus)
• Gastrointestinal - Crohn's disease (most common today in resource-rich countries), appendicitis, diverticulitis, colorectal cancer, perforated bowel
• Genitourinary - perinephric abscess, pyelonephritis, vaginal delivery complications, post-caesarean section
• Vascular - infected aortic graft, infected retroperitoneal haematoma
• Iatrogenic - post-lumbar puncture, post-spinal surgery

Clinical Features

• Fever (41%), hip/flank pain (43%), abdominal pain (14%), limp (22%)
• Fixed flexion deformity of the hip - patient lies with hip flexed to relieve psoas spasm; passive hip extension causes pain
• Swelling may track distally under the inguinal ligament and appear as a groin mass (fluctuant, non-tender if TB origin = "cold abscess")
• Symptoms may be insidious and non-specific (weight loss, malaise, nausea)

Provocation test: Place hand just proximal to patient's knee and ask the patient to raise the thigh against resistance - this contracts the psoas and reproduces pain. - Tintinalli's Emergency Medicine

The Cold Abscess (Tuberculous Psoas Abscess)

• Arises when TB of the lumbar vertebrae (Pott's disease) causes caseous necrotic material to track into the psoas sheath
• No heat, no redness, non-tender (hence "cold")
• Tracks: lumbar region → psoas sheath → below inguinal ligament → groin swelling (in front of femoral vessels) or into the medial thigh
• Has an impulse on coughing
• Illustrated below - Gray's Anatomy for Students

Investigations

• CT scan with contrast - gold standard; sensitivity >90% (reduced to 33% if symptoms <6 days)
• MRI - better for soft tissue detail and spinal involvement
• USS - can guide drainage
• Blood cultures, CRP, ESR, WBC (leucocytosis in pyogenic; ESR markedly elevated in TB)
• Mantoux/IGRA if TB suspected

Treatment

• Antibiotics covering S. aureus and enteric organisms (e.g. co-amoxiclav + flucloxacillin; or piperacillin-tazobactam)
• Percutaneous CT-guided drainage - first-line
• Open surgical drainage - if percutaneous fails or if bowel resection required (e.g. Crohn's)
• TB psoas abscess: anti-TB therapy (HRZE regimen) for 6-9 months + drainage
• Treat the underlying cause (e.g. spinal stabilisation for Pott's disease)

2. PSOAS HAEMATOMA

3. PSOAS SIGN (Clinical Sign)

• Elicitation: Patient lies on left side; right hip is passively hyperextended - pain = positive sign
• Indicates: Retroperitoneal appendicitis (classic), psoas abscess, pyelonephritis, retroperitoneal mass, Crohn's disease
• Yamada's Textbook of Gastroenterology: "When elicited on the right side, the psoas sign is suggestive of acute appendicitis, although it may also be elicited in other retroperitoneal inflammatory processes including psoas abscess or pyelonephritis."

4. PSOAS OBLITERATION ON X-RAY

• Normally the psoas shadow is clearly visible on plain AXR as a triangular soft-tissue density on each side of the lumbar spine
• Obliteration of the psoas shadow = important radiological sign of:
  • Perinephric haematoma
  • Retroperitoneal abscess (including psoas abscess)
  • Cold abscess (TB)
  • Retroperitoneal haemorrhage (ruptured AAA, trauma)
  • Retroperitoneal tumour or lymphadenopathy

5. ILIOPSOAS BURSITIS

• The iliopsoas bursa lies between the iliopsoas tendon and the hip joint capsule - the largest bursa in the body
• Causes: Osteoarthritis of hip, rheumatoid arthritis, overuse (runners, dancers), trauma
• Clinical: Anterior groin/hip pain, worsened by hip extension and resisted flexion; may cause a palpable mass in the groin
• Diagnosis: USS or MRI
• Treatment: Rest, NSAIDs, USS-guided corticosteroid injection, surgical bursectomy (rare)

6. SNAPPING HIP / PSOAS TENDINOPATHY (Coxa Saltans Interna)

• Movement of the iliopsoas tendon snapping over the iliopectineal eminence or femoral head
• Produces an audible/palpable click/snap at the anterior hip with flexion/extension
• Common in dancers, gymnasts, runners
• May coexist with iliopsoas tendinitis or bursitis
• Treat with physiotherapy, stretching, US-guided injection; rarely surgery

7. PSOAS MUSCLE AS A ROUTE OF SPREAD FOR TUMOURS

• Direct invasion: Renal cell carcinoma, retroperitoneal sarcoma, colorectal cancer, lymphoma
• Metastatic deposits: Rare; lung, breast, prostate
• Lymphoma: Retroperitoneal lymphadenopathy may displace or invade the psoas
• Liposarcoma: Can arise in the retroperitoneum adjacent to or within the psoas

8. PSOAS MINOR ABNORMALITIES

• Psoas minor (absent in ~40% of people) can rarely develop calcification or enthesopathy
• Of little clinical significance

Summary Table

The Myenteric (Auerbach's) Plexus - Background

• Peristaltic coordination (propulsive contractions)
• Inhibitory relaxation (via NO and VIP-releasing inhibitory neurons)
• Interstitial cells of Cajal (ICC) act as the pacemaker and relay signals

Classification of Myenteric Plexus Degeneration Diseases

1. HIRSCHSPRUNG DISEASE (Congenital Aganglionosis)

• Newborns: failure to pass meconium within 48 hours, abdominal distension, bilious vomiting
• Older children/adults (missed cases): chronic constipation, abdominal distension
• Complications: Hirschsprung-associated enterocolitis (HAEC) - most common cause of death; presents with alternating diarrhoea/obstipation, fever, haematochezia, toxic megacolon, shock

• Absent ganglion cells in myenteric and submucosal plexus
• Hypertrophied nerve trunks (acetylcholinesterase-positive)
• Calretinin immunostaining - now standard adjunct (negative in aganglionic segment)

• Contrast enema: narrow calibre distal segment, transition zone, dilated proximal bowel; failure to evacuate barium after 24 hours
• Anorectal manometry: failure of internal sphincter to relax on balloon distension
• Rectal biopsy - gold standard (suction biopsy in neonates, full-thickness in older children; must be >1 cm above dentate line)

2. CHAGAS DISEASE (Acquired Aganglionosis)

• Oesophagus, duodenum, and colon most affected
• Progressive ganglion cell destruction → loss of peristaltic coordination
  • Abnormal peristalsis begins after 50% ganglion cell loss
  • Dilatation occurs after 90% loss
• Results in: megaoesophagus, megaduodenum, megacolon, megarectum, megaureter

• Acute phase: parasites visible on blood smear
• Chronic phase: serological tests (complement fixation, PCR)

• Sleisenger and Fordtran's GI & Liver Disease

3. CHRONIC INTESTINAL PSEUDO-OBSTRUCTION (CIPO) / Enteric Neuropathy

• Neuropathic CIPO = degenerative or inflammatory changes in the myenteric plexus (neuronal inflammation = myenteric ganglionitis, neuronal loss, neuronal inclusion bodies)
• Myopathic CIPO = degeneration of intestinal smooth muscle (separate entity)

• Familial visceral neuropathy (autosomal dominant or recessive)
• Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE - TYMP gene mutation; neuropathic degeneration of the myenteric plexus + leukoencephalopathy)
• Intestinal neuronal dysplasia type B (IND-B) - colonic submucosal hyperganglionosis

4. PARANEOPLASTIC ENTERIC NEUROPATHY

• Most commonly associated with small cell lung carcinoma, also thymoma, carcinoid
• Anti-Hu antibodies (ANNA-1) target RNA-binding proteins in ENS neurons
• Leads to degeneration of myenteric plexus neurons throughout the GI tract, including the colon
• Presents with severe constipation, intestinal pseudo-obstruction, autonomic neuropathy
• May precede the cancer diagnosis

5. DIABETIC AUTONOMIC NEUROPATHY

• Long-standing diabetes damages autonomic nerves supplying the gut, including the vagal input and the myenteric plexus itself
• Loss of interstitial cells of Cajal (ICC) - the pacemaker cells - contributes
• Colonic involvement: diabetic colonic dysmotility → slow transit constipation, megacolon in severe cases
• Associated with diabetic gastroparesis (same mechanism in the stomach)

6. ACHALASIA (Oesophageal Analogue - Same Plexus, Different Level)

7. INTESTINAL NEURONAL DYSPLASIA TYPE B (IND-B)

• Colonic submucosal hyperganglionosis (giant ganglia, >8 neurons per ganglion)
• May mimic Hirschsprung or cause chronic constipation
• Diagnosis: rectal/colonic biopsy
• Often resolves with age; treatment conservative

8. SLOW TRANSIT CONSTIPATION (Colonic Inertia)

• A subset of patients with intractable constipation show reduced myenteric plexus neurons and ICC loss on full-thickness colonic biopsy (particularly in the right and transverse colon)
• Not a distinct neuropathy but a spectrum of degenerative change
• Simultaneous degeneration of myenteric plexus + pelvic parasympathetic colonic nerve fibres has been documented in slow transit constipation

Summary Table