Nonspecific back pain.
Mechanical low back pain (>85% causes)
Lumbar strain or sprain  — most common cause.
Osteoarthritis of the facet joints of the spine
Spondylosis (discs)
Herniated disc
Vertebral compression fracture (2/2 to osteoporosis)
Fractures
Spinal stenosis
Spondylolisthesis
Spondylosis
Congenital disease: Severe kyphosis, Severe scoliosis
Possible spondylolysis
Possible facet joint asymmetry
Non-mechanical spine disease
Infection: Osteomyelitis; Septic discitis; Paraspinous abscess; Epidural abscess
Neoplasia: Multiple myelomas; Metastatic carcinoma; Lymphoma and leukemia; Spinal cord tumors; Retroperitoneal tumors
Inflammatory arthritis: Ankylosing spondylitis (sacroiliitis): Psoriatic spondylitis: Reactive arthritis: Inflammatory bowel disease; Paget disease; Osteochondrosis; Polymyalgia Rheumatica
Causes outside the spine
Pelvic organ: Prostatitis, Endometriosis, Pelvic inflammatory disease (chronic)
Renal disease: Nephrolithiasis; Pyelonephritis; Perinephric abscess; Perinephric hematoma
GI disease: Pancreatitis; Pancreatic carcinoma; Abdominal tumor; Cholecystitis / Gallbladder disease; Penetrating ulcer/ perforate bowel
Pulmonary disease: Pulmonary embolus; Pneumonia
Shingles (Herpes Zoster)
Psychologic distress
Abdominal or thoracic aortic aneurysm
Retroperitoneal fibrosis
Others
Spinal cord or cauda equina compression; Cauda equina syndrome
Radiculopathy
Iliocostal impingement syndrome
Meralgia paresthetica
Poor posture; Pregnancy
Piriformis syndrome
Fat herniation of lumbar space
Back mouse
Ehlers Danlos (Hypermobility syndrome)
Vitamin D deficiency
Bertolotti’s syndrome (Back pain in the setting of a transitional vertebra)

Thinking through the differential diagnosis
Acute back pain is back pain ≤ 4 weeks duration. Back pain that lasts 4 to 12 weeks is called subacute back pain. Chronic back pain (persists for ≥12 weeks).
Note: “Even when back pain is associated with specific imaging findings such as degenerative disk disease, spondylolysis, spondylolisthesis, or osteoporosis, it may be impossible to determine whether the finding is the cause of the patient’s symptom.” J Gen Intern Med. 2001

Nonspecific back pain.
The majority of back pain is nonspecific. Nonspecific low back pain is pain that is not attributed to a specific recognizable pathology (e.g., infection, tumor, osteoporosis, rheumatoid arthritis, lumbar spine fracture, inflammatory disorder, radicular syndrome, or cauda equina syndrome.


Further Reading

J Gen Intern Med. 2001 Feb; 16(2): 120–131. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1495170/

http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/neurology/low-back-pain/ (Last Accessed 12/2017)

Am Fam Physician. 2007 Apr 15;75(8):1181-1188. https://www.aafp.org/afp/2007/0415/p1181.html

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