Heel pad sign | Radiology Signs

Answer:

The heel pad sign is thickening of the plantar soft tissue measured on a lateral radiograph as the shortest distance from the inferior surface of the calcaneal tuberosity to the plantar skin surface. The historical threshold is a heel pad thicker than 23 mm in men and about 21.5 mm in women, with some series using 25 mm; values above this were used to suggest growth hormone excess. The measurement is easy to obtain but is a poor discriminator in isolation, because heel pad thickness correlates with body weight, and obesity, dependent oedema, infection, myxoedema and even normal variation shift it upward, so the finding should be interpreted only alongside other features. The supporting radiographic changes of acromegaly are tufting of the terminal phalanges, increased joint spaces from cartilage hypertrophy, thickened sesamoids, enlarged frontal sinuses and mandibular prognathism with an increased mandibular angle, posterior vertebral body scalloping and enlargement of the sella turcica. Modern practice makes the diagnosis biochemically with an age-adjusted IGF-1 level and failure of growth hormone suppression during an oral glucose tolerance test, followed by dedicated pituitary MRI to localise the adenoma, so the heel pad sign is now a legacy observation that can prompt the biochemical work-up when noticed incidentally. Ultrasound can measure the pad more reproducibly than radiography and shows the fibrofatty chambers directly.

Why is it called so?

The sign is named for the plantar fat pad of the heel, whose measured thickness is the observation itself.

Pathophysiology

Excess growth hormone acts through hepatic and local IGF-1 to stimulate fibroblasts and increase deposition of collagen and glycosaminoglycans in the connective tissue septa of the plantar fat pad, while sodium and water retention from growth hormone excess adds interstitial fluid. The result is enlargement of the fibrofatty chambers of the heel pad rather than an increase in fat alone. The same soft tissue and periosteal response accounts for the generalised acral soft tissue enlargement, the phalangeal tufting and the cartilage hypertrophy that widens joint spaces, and it explains why the soft tissue changes partly regress once growth hormone levels are normalised.

Alternative names: Heel pad thickness sign

Other associated named signs: Tufting of the terminal phalanges and posterior vertebral scalloping in the same disease

References

1. Gooding GA, Stress RM, Graf PM, Grunfeld C. Heel pad thickness: determination by high-resolution ultrasonography. J Ultrasound Med. 1985;4(4):173-4. PMID: 3886923.