What is the diagnostic approach for sciatica?

Diagnostic Approach for Sciatica

The diagnosis of sciatica should be made through a focused history and physical examination that identifies back and leg pain in a typical lumbar nerve root distribution, with positive straight-leg-raise testing and specific neurological findings corresponding to affected nerve roots. 1

Clinical Assessment

History Taking

• Inquire about location of pain, focusing on pain radiating below the knee into the foot and toes, which is the most important symptom of sciatica 2
• Assess frequency of symptoms and duration of pain, as well as any history of previous symptoms, treatment, and response to treatment 1
• Document pain characteristics and distribution, as a typical history for sciatica (back and leg pain in a typical lumbar nerve root distribution) has high sensitivity for herniated disc 1
• Evaluate for risk factors for alternative diagnoses such as cancer (history of cancer, unexplained weight loss, failure to improve after 1 month, age >50 years) or infection (fever, intravenous drug use, recent infection) 1

Physical Examination

• Perform straight-leg-raise testing, which has high sensitivity (91%) but modest specificity (26%) for diagnosing herniated disc 1
• Conduct crossed straight-leg-raise test, which is more specific (88%) but less sensitive (29%) 1
• Complete a focused neurologic examination that includes:
  • Evaluation of knee strength and reflexes (L4 nerve root) 1
  • Assessment of great toe and foot dorsiflexion strength (L5 nerve root) 1
  • Testing of foot plantarflexion and ankle reflexes (S1 nerve root) 1
  • Mapping distribution of sensory symptoms to assess nerve root dysfunction 1

Diagnostic Categorization

After initial assessment, patients with low back pain should be classified into one of three categories 1:

1. Nonspecific low back pain
2. Back pain potentially associated with radiculopathy or spinal stenosis (sciatica)
3. Back pain potentially associated with another specific spinal cause

Red Flags Requiring Urgent Evaluation

• Rapidly progressive or severe neurologic deficits 1
• Motor deficits at more than one level 1
• Fecal incontinence or bladder dysfunction (cauda equina syndrome) 1
• Urinary retention (90% sensitivity for cauda equina syndrome) 1

Imaging Studies

• Initial imaging is generally not required for most patients with sciatica, as the clinical course is favorable with most pain improving within 2-4 weeks with or without treatment 2

• Imaging is warranted in the following circumstances:

  • Evidence of underlying pathology other than disc herniation (infection, malignancy) 2
  • Severe symptoms that do not improve after 6-8 weeks of conservative treatment 2
  • Presence of progressive neurological deficits 1

• When imaging is indicated, MRI is the preferred modality as it:

  • Visualizes soft tissues better than CT 2
  • Does not expose the patient to ionizing radiation 2
  • Can detect nerve root compression and inflammation 1

Differential Diagnosis

It's important to distinguish true radicular pain (sciatica) from referred pain from the lower back, as their management differs 3:

• Non-discogenic causes of sciatica to consider include:

  • Lumbar radicular herpes zoster 4
  • Nerve root schwannoma 4
  • Piriformis syndrome 4
  • Sacroiliitis 4
  • Intrapelvic masses 4

• Inflammatory vs. mechanical causes:

  • Sciatica may involve both nociceptive and neuropathic pain components (mixed pain syndrome) 5
  • Inflammation can sensitize nerve roots even without compression 5
  • Mechanical compression of nerve roots can cause neuropathic pain 5

Common Pitfalls and Caveats

• Relying solely on dermatome patterns can be misleading, as they are often unreliable in clinical practice 6
• Imaging findings may not correlate with symptoms in approximately 28% of cases 6
• Electrophysiological studies may be normal in purely inflamed nerve roots 6
• Assuming all leg pain is sciatica - referred pain from the lower back is common and not related to nerve root compression 3
• Failing to assess for psychosocial factors, which are stronger predictors of low back pain outcomes than physical examination findings or pain severity 1

Confirmation of Diagnosis

• Selective nerve root blocks may be used to confirm the diagnosis in challenging cases, though they carry risks and require specialized settings 6
• The clinical diagnosis based on history and physical examination remains the cornerstone of sciatica diagnosis 1, 2