Why is the left side of the body clinically significant?

Why the Left Side is Clinically Important

The left side of the body is clinically significant primarily because left-sided pathology is more easily recognized and diagnosed than right-sided pathology, leading to systematic biases in clinical detection, and because certain anatomical structures on the left (particularly the spleen and left hemidiaphragm) lack the protective effects present on the right side.

Differential Recognition and Diagnosis

• Left-sided strokes are recognized significantly more often than right-sided strokes (57.7% vs 42.3%), despite MRI studies showing equal distribution of actual infarcts (51.9% left vs 48.1% right), indicating a systematic clinical detection bias favoring left-sided neurological events 1
• This recognition bias extends to transient ischemic attacks, with 57.8% of clinically diagnosed TIAs being left-sided, suggesting that right-sided symptoms are frequently missed or underappreciated 1
• The clinical implication is that providers must maintain heightened vigilance for right-sided pathology, as it is systematically under-recognized despite occurring with equal frequency 1

Anatomical Vulnerability in Trauma

• Left-sided diaphragmatic injuries occur in 50-80% of blunt trauma cases that reach the hospital, compared to only 12-40% on the right side, because the liver provides a protective cushioning effect on the right hemidiaphragm that is absent on the left 2
• The spleen, being significantly smaller than the liver, provides a much more limited sonographic window during ultrasound examination, requiring more posterior probe placement and making left upper quadrant evaluation technically more challenging 2
• Left-sided traumatic injuries have better survival rates precisely because they are more easily detected—the protective liver effect on the right means that right-sided injuries severe enough to be detected often indicate more catastrophic trauma with higher mortality 2

Ultrasound and Imaging Considerations

• During FAST examinations, the left flank (perisplenic) view requires evaluation of four potential spaces: pleural space, subphrenic space, splenorenal space, and left paracolic gutter, but the small size of the spleen necessitates extensive use of posterior intercostal approaches 2
• The liver's superior size and acoustic properties make right upper quadrant evaluation substantially easier than left-sided evaluation, as the liver serves as an excellent sonographic window while the spleen does not 2, 3
• Probe placement for left-sided views typically requires positioning on the posterior axillary line or more posteriorly to avoid gas-filled splenic flexure and descending colon, adding technical complexity 2

Cardiovascular Monitoring

• In patients with suspected aortic dissection, arterial line placement should be in the right radial artery by default, with left-sided placement only if brachiocephalic trunk involvement is suspected, because pressure measurement on both arms is essential to rule out pseudo-hypotension from aortic arch branch obstruction 2
• Left-sided cardiac pathology (left ventricular dysfunction, mitral and aortic valve disease) generally produces more overt clinical symptoms than right-sided disease, contributing to earlier recognition 2

Gastrointestinal Pathology

• Left-sided complicated diaphragmatic hernias more commonly present with gastrointestinal symptoms including intestinal obstruction, making them clinically apparent earlier than right-sided hernias 2
• There is an observed trend toward proximal (right-sided) migration of colorectal polyps in Asian populations with aging, meaning that reliance on left-sided symptoms alone will miss an increasing proportion of pathology 2

Clinical Pitfalls to Avoid

• Never assume that absence of left-sided findings rules out bilateral pathology—actively search for right-sided disease, as it is systematically under-recognized 1
• In trauma patients, do not be falsely reassured by negative left upper quadrant findings; right-sided injuries may be present but harder to detect and associated with worse outcomes 2
• When performing ultrasound examinations, recognize that technically adequate left-sided views require more skill and posterior probe positioning than right-sided views 2
• In patients with laterality markers on imaging, be aware that AI algorithms can inappropriately use these markers as shortcuts rather than evaluating actual pathology, potentially introducing systematic bias 2