The Problem With Calling Rare Variants ‘Rare’: A Data-Driven Perspective

It is still common to read that a nerve, vessel or drainage pathway shows a “rare variant,” only to find that the cited meta-analysis reports a prevalence in double digits. Extralaryngeal branching of the recurrent laryngeal nerve (RLN) occurs in roughly 60% of nerves overall, higher in cadaveric than intraoperative series,¹ yet the unbranched RLN is often presented as the unquestioned norm. Similarly, the non-recurrent laryngeal nerve (NRLN) is labelled a curiosity despite pooled prevalence approaching 0.7% on the right side, with even higher rates in selected vascular cohorts.² In the gluteal region, type B–F relationships between the sciatic nerve and piriformis muscle collectively account for around 8–10% of limbs in pooled cadaveric data,^{5, 6} yet are frequently described as exceptional.

At the same time, high-resolution CT and MR angiography have normalised the idea that a single textbook picture cannot represent renal vascular anatomy. Multiple renal arteries, early branching and complex renal venous patterns around the kidney are now routine pre-operative findings rather than surprises,^{7, 8} but much of our vocabulary for these patterns still comes from a pre-imaging era. This editorial argues that the persistent, casual use of the word “rare” has become actively unhelpful. If anatomy has embraced meta-analysis and pooled prevalence, then our language should follow.

“Rare” is a visceral word. To a trainee, it implies something they are unlikely ever to see; to a surgeon, something they may reasonably not anticipate. Yet many so-called rare variants actually inhabit the 5–30% prevalence band in pooled series. In oncological epidemiology or pharmacovigilance, those numbers would never qualify as rare.

Evidence-based anatomy now provides robust statistical tools to synthesise such prevalence across cadaveric and imaging studies.⁴ Random-effects meta-analyses of the RLN, sciatic nerve and renal vasculature routinely include thousands of nerves or vessels^{1, 7} with narrow confidence intervals. Once we know, for example, that type B sciatic nerve–piriformis configurations cluster around 8% in pooled data,⁵ continuing to describe them as “rare” is a choice, not a reflection of the evidence. The problem is not just semantic: it encourages a binary view of anatomy in which one drawing is “normal” and the rest are curiosities.

This binary framing is attractive in early teaching but becomes a liability in clinical translation. If 20–30% of patients have extra renal arteries, and around 5% have significant renal venous variants,^{7, 9} then the real question is not “Does this patient have a variant?” but “Which configuration on the spectrum do they have?” Calling one pattern “normal” and the others “variants” distracts from the underlying distribution.

Language shapes expectation. When an operative atlas describes the NRLN as “very rare,” the unspoken message is that most surgeons will never face it. Yet a 0.7% pooled prevalence in unselected series, and higher rates in individuals with aberrant subclavian arteries, means thyroid surgeons in busy units are almost guaranteed to encounter it across a career.² Injury to a “rare” nerve still produces very real dysphonia.

Similarly, labelling extralaryngeal branching of the RLN as an anomalous pattern subtly legitimises drawings and operative descriptions that show a single trunk.¹ Meta-analytic data make clear that a bifurcated or trifurcated RLN is the dominant configuration, not an exception. Teaching that starts from the most common real-world pattern would force surgeons to search systematically for multiple branches, particularly near Berry’s ligament and the tracheoesophageal groove where branching is frequent.^{3, 12}

In regional anaesthesia and hip surgery, underestimating sciatic nerve variants has parallel consequences. MRI and ultrasound work shows that variant sciatic nerve–piriformis relationships are not curiosities confined to cadaver labs but live anatomy encountered during piriformis injections and posterior approaches to the hip.^{5, 11} If the pre-operative report describes these as “rare anomalies,” the clinician may feel unlucky rather than unprepared.

Renal vascular anatomy illustrates how quickly dogma can be overturned once large, imaging-based datasets accumulate. Multi-detector CT angiography and MR angiography now routinely map hundreds or thousands of renal arteries and veins in single studies.^{7, 8} Multiple renal arteries appear in roughly one quarter of patients, early branching in a further 6–10%, and complex venous patterns in around 5–10%, depending on definitions and thresholds.^{7, 9}

These figures have quietly reshaped transplant and partial nephrectomy planning. No transplant team now considers an accessory renal artery to be a freak occurrence; instead, it is a routine technical variable that must be anticipated. Yet in everyday language, students still hear that a kidney “has a rare accessory artery.” The data say otherwise, and so should we.

Evidence-based anatomy has provided a practical roadmap for performing these pooled analyses, including methods for handling heterogeneous definitions and multi-categorical prevalence, especially when combining cadaveric dissection with modern radiological anatomy.⁴ Once such analyses exist, failing to update our vocabulary is not a minor editorial oversight – it is a barrier to translating quantitative anatomy into safer clinical practice.

We cannot abolish the word “variant,” but we can use it more honestly. A pragmatic, data-informed framework might look like this:

• Dominant pattern: the single most frequent configuration, typically >50% prevalence in pooled series (for example, a bifurcating RLN; type A sciatic nerve–piriformis relationship; a single renal artery without early branching in some populations).^{1, 5}
• Common variant: patterns in roughly the 5–30% band, such as extralaryngeal trifurcation of the RLN in certain series, type B sciatic configurations, or multiple renal arteries in CT angiography cohorts.^{1, 5}
• Uncommon variant: generally 1–5%, including many renal venous patterns and some specific RLN–landmark relationships.^{2, 9}
• Rare variant: <1% in pooled data or genuinely case-report–level occurrences, such as bilateral NRLN without arch anomalies or extreme venous duplications.

These bands are deliberately approximate, but they force us to state what we mean. Once an RLN configuration crosses into the 10–20% range, calling it “rare” becomes indefensible. Conversely, reserving “rare variant” for truly sub-percent patterns preserves the term for situations where it actually conveys useful information about risk and expectation.

Crucially, a prevalence-based vocabulary is portable. An atlas can still draw a single “index” configuration, but the legend should state its pooled prevalence and name the next two most frequent patterns with their approximate percentages, rather than hiding them in a caption or appendix. Radiology and operative reports could follow the same logic, describing the observed configuration, its usual label and its approximate prevalence band.

For educators, the challenge is to move beyond a single static diagram of “normal” anatomy. If a first-year student sees the RLN only as an unbranched nerve or the sciatic nerve only as a single trunk below piriformis, we have already misled them.^{1, 5} A better approach is to teach the dominant pattern alongside one or two common variants, explicitly labelled with rough prevalence bands.

For researchers, the next step is consistency. Meta-analyses increasingly provide high-quality pooled prevalence estimates for key structures – RLN, sciatic nerve, renal arteries and veins and many others.^{1, 8} If authors standardise thresholds for describing variants as common, uncommon or rare, future systematic reviews will be easier to synthesise and compare. Adopting established evidence-based anatomy checklists for study design and reporting will help.⁴

For clinicians, especially surgeons and interventionalists, the message is straightforward: if a configuration occurs in 5–30% of pooled series, it is not a surprise finding – it is part of the expected spectrum. Operative planning, consent and intraoperative checklists should reflect that reality. Describing a 20% pattern as “rare” might sound dramatic, but it is ultimately a disservice to patients.

Anatomy has entered the meta-analytic era, but much of our vocabulary remains stuck in the age of case reports and heroic single drawings. As long as we casually call 10–30% patterns “rare variants,” we will continue to miscalibrate expectations in the theatre, the radiology suite and the seminar room. Reframing our language around pooled prevalence – distinguishing dominant patterns, common variants, uncommon variants and truly rare variants – is a simple, achievable step that aligns teaching and practice with the data we already have.

Words matter. If we want clinicians to respect anatomical variability as a predictable, quantifiable part of human biology rather than an afterthought, then “rare” must once again mean what it says.