I first heard of the “slip­ping slip­per sign” from Pro­fes­sor Su­ru­j­pal Teelucks­ingh. The name is like a play­ful tongue-twister. The ques­tion is sim­ple: “Have you ever lost a slip­per or flip-flop from your foot while walk­ing and not re­alised it had come off?”

The im­por­tant de­tail is the ab­sence of aware­ness. The footwear has gone but the pa­tient is none the wis­er. That may be a warn­ing of se­vere di­a­bet­ic dam­age to the nerves (neu­ropa­thy), usu­al­ly be­gin­ning in the feet, that can dull pain, pres­sure, tem­per­a­ture and po­si­tion sense. A per­son may stand on a sharp ob­ject, de­vel­op a blis­ter from a tight shoe or burn a foot with hot wa­ter and feel sur­pris­ing­ly lit­tle. Pain is an alarm, and a foot that has fall­en silent is not nec­es­sar­i­ly safe.

Pro­fes­sor Teelucks­ingh and col­leagues for­mal­ly de­scribed the sign and com­pared it with tra­di­tion­al clin­i­cal tests for neu­ropa­thy. Among peo­ple with di­a­betes, it showed an 83 per cent sen­si­tiv­i­ty and 91 per cent speci­fici­ty for se­vere neu­ropa­thy.

An in­de­pen­dent Ja­maican team lat­er test­ed the sign in 69 pa­tients. The sen­si­tiv­i­ty was low, mean­ing that many peo­ple with neu­ropa­thy did not re­port los­ing a slip­per. But its speci­fici­ty and pos­i­tive pre­dic­tive val­ue for neu­ropa­thy were both 100 per cent in that study. In plain Eng­lish, the sign could miss dis­ease and there­fore could not re­place prop­er screen­ing; but when it was present, it de­served at­ten­tion. A neg­a­tive an­swer did not grant ab­so­lu­tion. A pos­i­tive one rang a bell.

Years af­ter I first heard of the sign he chal­lenged me at a med­ical con­fer­ence to go fur­ther for my Mas­ters re­search. I had the priv­i­lege of ex­plor­ing an­oth­er ques­tion with a bril­liant team in­clud­ing oph­thal­mol­o­gist Dr Neela Ram­dass: “if the slip­per iden­ti­fies se­vere nerve dam­age, could oth­er or­gans be in­volved?”

In our 2016 study of 100 peo­ple with di­a­betes, 33 had the slip­ping slip­per sign and 67 did not. Every sin­gle one of the 33 with the sign had di­a­bet­ic eye dis­ease (retinopa­thy), com­pared with 12 of the 67 with­out it. More ad­vanced, pro­lif­er­a­tive retinopa­thy was al­so much more com­mon in the pos­i­tive group. Pro­tein in the urine was found in 46 per cent of those with the sign, com­pared with on­ly six per cent of those with­out it.

The num­bers were strik­ing, but this was an ob­ser­va­tion­al study. A wan­der­ing slip­per does not cause blind­ness or kid­ney dis­ease, and it can­not di­ag­nose ei­ther. The same long ex­po­sure to high glu­cose, high blood pres­sure and oth­er vas­cu­lar risks can in­jure nerves, reti­nal ves­sels and kid­ney fil­ters.

Com­pli­ca­tions hunt in packs.

That is the larg­er les­son. Find­ing neu­ropa­thy should prompt a search for retinopa­thy, kid­ney dis­ease, vas­cu­lar dis­ease and the oth­er haz­ards that may be trav­el­ling with it. An eye find­ing should re­mind us to ex­am­ine the feet and as­sess the kid­neys. Pro­tein in the urine should lead us be­yond the urine bot­tle.

Mod­ern med­i­cine is ca­pa­ble of as­ton­ish­ing things. We can pho­to­graph the reti­na with­out di­lat­ing the pupil, mon­i­tor glu­cose minute by minute and in­spect nerves with ul­tra­sound. We stare at screens while clues sit at our pa­tients’ feet.

Peo­ple with di­a­betes re­quire reg­u­lar, com­pre­hen­sive foot ex­am­i­na­tions, in­clud­ing as­sess­ment of skin, cir­cu­la­tion and pro­tec­tive sen­sa­tion. They need ap­pro­pri­ate reti­nal screen­ing and kid­ney checks. Feet should be in­spect­ed dai­ly, in­clud­ing be­tween the toes and be­neath the sole; a mir­ror or an­oth­er per­son can help. New blis­ters, cuts, colour changes, swelling, dis­charge or ar­eas of warmth de­serve ear­ly at­ten­tion. Walk­ing bare­foot is haz­ardous and footwear should pro­tect with­out rub­bing or squeez­ing.

Con­trol of glu­cose mat­ters, but blood pres­sure, cho­les­terol, smok­ing, kid­ney health, nu­tri­tion and phys­i­cal ac­tiv­i­ty al­so in­flu­ence the fu­ture. A per­son with numb feet may need more fre­quent pro­fes­sion­al re­view from a po­di­a­trist and prop­er­ly fit­ted footwear. Any­one who has lost a slip­per with­out notic­ing should men­tion it to a doc­tor and re­quest a thor­ough as­sess­ment.

In busy clin­ics, es­pe­cial­ly where spe­cial­ist equip­ment and time are scarce, sim­ple val­i­dat­ed ques­tions can help iden­ti­fy those need­ing ur­gent at­ten­tion. A com­mu­ni­ty nurse, rel­a­tive or trained health work­er can ask about the slip­per. That does not re­place ex­am­i­na­tion; it opens the door to it. Low-cost med­i­cine is not in­fe­ri­or med­i­cine when it is ev­i­dence-based, prop­er­ly un­der­stood and con­nect­ed to ap­pro­pri­ate care.

The hum­ble slip­per teach­es three things. Lis­ten to the pa­tient’s or­di­nary sto­ries. Look for the un­pleas­ant com­pan­ions of any com­pli­ca­tion you find. Nev­er con­fuse the ab­sence of pain with the ab­sence of dan­ger.

Med­i­cine of­ten looks up­ward but, every so of­ten, wis­dom asks us to look down. If a slip­per slips away un­no­ticed, do not mere­ly re­place the pair. Ask what else may be dis­ap­pear­ing silent­ly: sen­sa­tion from the feet, clar­i­ty from the eyes, pro­tec­tion from the kid­neys.

The first rus­tle in the bush may be no loud­er than a slip­per falling to the floor.