What could be causing nighttime pain when laying down?

Nighttime Pain When Laying Down: Differential Diagnosis and Evaluation

Nighttime pain that occurs specifically when laying down requires immediate evaluation for serious underlying pathology, particularly inflammatory spondyloarthropathy, infection, or malignancy, as these conditions characteristically worsen at night and represent potentially life-threatening causes that demand urgent intervention.

Red Flag Conditions Requiring Immediate Workup

Inflammatory Spondyloarthropathy (Ankylosing Spondylitis)

• Awakening during the second part of the night specifically due to back pain is a classic presentation of ankylosing spondylitis 1
• Additional features include younger age, morning stiffness that improves with exercise, and alternating buttock pain 1
• This pattern distinguishes inflammatory from mechanical causes, which typically worsen with activity 1

Malignancy

• Night pain or pain at rest is characteristic of infection or tumor and must be excluded urgently 1
• History of cancer increases likelihood from 0.7% to 9% in patients with back pain 1
• Unexplained weight loss, failure to improve after 1 month, and age >50 years each increase cancer probability 1
• In pediatric populations, constant night pain lasting ≥4 weeks constitutes a clinical red flag requiring immediate imaging 1

Infection

• Night pain at rest is a hallmark feature of vertebral infection 1
• Risk factors include fever, intravenous drug use, or recent infection 1
• Periprosthetic infection (if joint replacement present) characteristically causes night pain or pain at rest, distinguishing it from mechanical loosening which causes pain on weight bearing 1

Sleep-Related Disorders Presenting as Nighttime Pain

Restless Legs Syndrome (RLS)

• Uncomfortable leg sensations with urge to move that are worse in evening/night and relieved by movement 1
• Symptoms brought on specifically by rest and lying down 1
• Prevalence increases with age, affecting ~10% of population 1
• Circadian pattern with worsening at night and improvement toward morning 1

Positional/Mechanical Causes

• Pelvic congestion syndrome can cause deep pelvic pain that may worsen with recumbency 1
• Cervical stenosis and chronic inflammatory pelvic disease may present with positional pain 2

Diagnostic Approach Algorithm

Initial Clinical Assessment

1. Determine pain location and character:

   • Spinal/back pain → evaluate for inflammatory vs. mechanical vs. malignancy 1
   • Leg discomfort with urge to move → screen for RLS using diagnostic criteria 1
   • Deep pelvic pain → consider gynecologic/vascular causes 2

2. Assess for red flags requiring urgent imaging:

   • Age >50 years with new onset pain 1
   • History of cancer 1
   • Unexplained weight loss 1
   • Fever or recent infection 1
   • Awakening specifically in second half of night 1
   • Pain lasting >4 weeks in children 1
   • Neurologic deficits 1

3. Laboratory evaluation when red flags present:

   • ESR and CRP for inflammatory/infectious causes 1
   • Complete blood count 1
   • Iron studies if RLS suspected 1

Imaging Strategy Based on Suspected Etiology

For spinal pathology with red flags:

• Plain radiographs are initial study but contribute only limited diagnostic information 1
• MRI without contrast is the screening tool of choice when red flags present 1
• Do not delay imaging if infection or malignancy suspected 1

For pelvic pain:

• Combined transabdominal and transvaginal ultrasound is initial imaging of choice 2
• Plain radiography has no role 2

For RLS:

• Diagnosis is clinical; polysomnography not needed in majority of cases 1

Common Pitfalls to Avoid

• Do not dismiss nighttime-specific pain as "just insomnia" - this temporal pattern is a critical diagnostic clue for serious pathology 1
• Do not attribute all nighttime leg discomfort to neuropathy - RLS has specific diagnostic criteria requiring urge to move and relief with movement 1
• Do not delay imaging in presence of red flags - serious causes like infection and malignancy require urgent evaluation 1
• In children, do not assume pain is benign - persistent night pain ≥4 weeks mandates imaging regardless of normal physical exam 1
• Recognize that mechanical causes typically improve with rest, whereas inflammatory, infectious, and neoplastic causes characteristically worsen at night 1

Medication Review

• Tricyclic antidepressants, SSRIs, lithium, and dopamine antagonists can exacerbate RLS symptoms 1
• Benzodiazepines should be avoided in older patients with sleep complaints due to cognitive impairment risk 1
• Review all medications that may contribute to nighttime symptoms before attributing pain to primary pathology 1