= Solution
\b[A <symmetrical disease> produces corresponding lesions in matching parts on the two sides of the body.] The correspondence may involve site, form, extent and timing; it need not be mathematically exact in every patient or at every stage.
The idea concerned the distribution of pathological change. For example, similar lesions at both elbows have a stronger anatomical correspondence than two unrelated lesions anywhere on the right and left. Bilateral involvement is therefore not always the same as close mirror symmetry. Conversely, a disorder may characteristically show a symmetrical pattern while an individual case is uneven because lesions began at different times or received different local irritation.
The concept was already a subject of organized study before 1866. <William Budd>'s paper on diseases of corresponding parts described paired articular and periarticular lesions and developed the argument that a systemic influence acts selectively on tissues with corresponding susceptibilities. The inference was that neither the circulating influence alone nor the local tissue alone explains the complete pattern; their interaction determines where disease appears. The historical account used the language of disordered nutrition and morbid material in the <blood>, which should not be mistaken for identification of a modern molecular cause.<On diseases which affect corresponding parts of the body in a symmetrical manner by William Budd>[Opening clinical observations and discussion of constitutional disease and tissue susceptibility]{ref}
Useful examples include:
* <Psoriasis>, whose plaques often occupy matching extensor sites such as the elbows and knees. This is a distributional tendency, not a requirement that every plaque have an identical partner.<Chronic plaque psoriasis by DermNet>[Definition and typical symmetrical distribution]{ref}
* Chronic inflammatory polyarthritis, exemplified today by <rheumatoid arthritis>, with corresponding small <anatomical joints> of the hands or feet affected. The modern diagnosis is more specific than the historical phrase chronic rheumatism.<Rheumatoid Arthritis by Merck Manual Professional Edition>[Symptoms and Signs, characteristic symmetric peripheral arthritis]{ref}
* A <distal symmetric polyneuropathy>, for example in <diabetes>, with similar sensory deficits beginning in both feet and later involving the hands. The matching distribution reflects a systemic process acting on similarly vulnerable long <nerves>, rather than one local injury at each spot.<Polyneuropathy by Merck Manual Professional Edition>[Clinical features, distal symmetric patterns and length-dependent axonal neuropathies]{ref}
The first two also illustrate a distinction between surface lesions and deeper structural disease; the third illustrates functional deficits following anatomical correspondence of a nerve population. The symmetry is the shared descriptive property, not evidence that these disorders share one agent.
To document it, compare matching sites, the same tissue compartments and the stage of each lesion; record exceptions rather than filling in an absent partner. \b[Symmetry is a clue to systemic influence and selective tissue susceptibility, not a diagnosis or proof of a single mechanism.]
Back to article page
