= Solution
\b[A <hydrocele> contains fluid around the <testis>; a <scrotal hernia> contains abdominal contents that have descended through the <inguinal canal>.] Their distinction rests on a combination of location, continuity with the <inguinal canal>, behaviour and light transmission.
The usual acquired <hydrocele> is a collection between the layers of the <tunica vaginalis>. A <scrotal hernia> is generally an indirect <inguinal hernia>: a sac of <peritoneum>, often containing <intestine> or <omentum>, follows the <spermatic cord> into the <scrotum>. Persistence of the <processus vaginalis> can permit either a communicating <hydrocele> or an indirect <inguinal hernia>, depending on the communication and its contents. Thus the underlying developmental route is related even though the usual clinical appearances differ.<Hydrocele by MedlinePlus>[“Exams and Tests”: examination, light transmission, changes in swelling and imaging]{ref}
* An uncomplicated <hydrocele> is typically smooth, fluctuant or tense, and confined to the <scrotum>. One can usually feel normal <spermatic cord> above it: one can “get above” the swelling. The <testis> may be difficult to feel separately and is usually behind the fluid collection. A <scrotal hernia> extends upwards toward the <inguinal canal>; its upper limit cannot usually be encircled within the <scrotum>.<The Ship Captain's Medical Guide by the Maritime and Coastguard Agency>[Chapter 7, p. 155, distinguishing continuity into the groin from a swelling confined to the scrotum]{ref}
* A typical acquired <hydrocele> has neither an expansile cough impulse nor ready reduction into the abdomen. A reducible <scrotal hernia> enlarges on standing, coughing or straining, and diminishes on lying down or with gentle clinical reduction; <intestine> may give a gurgle. An irreducible <hernia> is an important exception, so failure of reduction does not establish <hydrocele>.
* For <transillumination>, a bright light is applied to one side of the swelling in a darkened setting while the examiner views the other side. Clear fluid in a thin-walled <hydrocele> usually transmits a diffuse glow. Most <scrotal hernias> do not. Blood, thick walls or an underlying <tumour> can prevent a <hydrocele> from transmitting light; apparent transmission alone cannot exclude a <hernia>, especially in a child.
* A longstanding uncomplicated <hydrocele> commonly produces heaviness rather than acute systemic illness. A painful, tender, irreducible <scrotal hernia> with vomiting or abdominal distension raises concern for <intestinal obstruction> or <strangulated hernia>, in which the blood supply of the trapped contents is compromised. That requires urgent surgical assessment.
If examination leaves doubt, <ultrasonography> can establish whether the swelling contains fluid, <intestine> or a solid <tumour>, assess the <testis> and show continuity with the <inguinal canal>. A collection may obscure a diseased <testis>; demonstrating fluid does not finish the diagnostic task. Blind puncture is not an appropriate diagnostic shortcut when a <hernia> remains possible. Even an established <hydrocele> usually reaccumulates after simple aspiration, which also risks <infection>.<Repair of a scrotal hydrocele by the British Association of Urological Surgeons>[September 2024 leaflet, “Key points” and “What are the alternatives?”]{ref}
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