Acromegaly

an endocrine disorder caused by hypersecretion of growth hormone and characterized by enlargement of many skeletal parts. Endochondral bone formation is reactivated and periosteal bone formation is stimulated, with concurrent proliferation of connective tissue.

Characteristic radiographic findings are produced, including increased soft tissue thickness and bone overgrowth. Joint abnormalities result from proliferation of chondrocytes in articular cartilage. The radiographic features of joint involvement are joint space narrowing, bone sclerosis, cyst formation and osteophytosis, which resemble the changes of primary degenerative joint disease. Continued hypersecretion of growth hormone (somatotropin) affects both the mature and immature skeleton. In the immature skeleton, in which the growth plates are still open, the result is excessive growth of bone in both length and width, a condition known as hyperpituitary gigantism. This disorder is characterized by extreme height. In the mature skeleton, in which the growth plates have closed, growth hormone hypersecretion may reactivate endochondral bone formation at various existing cartilage - bone junctions, such as the costochondral junctions. Acromegaly can result in periosteal bone formation, leading to widening of osseous structures and enlargement of soft tissue. Such excessive growth is prominent particularly in the hands, feet and lower jaw and is termed acromegaly. Articular cartilage may be affected by a continuous cycle of fragmentation, disordered joint mechanics and attempted regenerative tissue repair with remodelling.

Radiographic features of skeletal involvement include changes in skin thickness and abnormalities of the skull, hand, foot, vertebral column, thoracic cage, pelvis and other sites. Hypertrophy of cartilage and degeneration of both cartilage and bone also are seen (Table 1) (Fig.1).

Certain bone and soft tissue measurements are of value in the diagnosis of acromegaly. Collagen tissue demonstrates a marked response to excessive amounts of growth hormone, manifested as thickening of the skin. Measurement of heel pad thickness (greater than 21.5 mm in women or 23 mm in men) can be suggestive or diagnostic of acromegaly. The sesamoid index (obtained by measuring the medial sesamoid at the first metacarpophalangeal joint and then multiplying this value by the greatest diameter of the same sesamoid image that is perpendicular to the first measurement), is another helpful measurement. A sesamoid index of greater than 40 in men and greater than 32 in women is suggestive of acromegaly. Other measurements that may indicate presence of the disease are a tuftal width of the third finger greater than 10 mm in women or 12 mm in men; joint space thickness of the second metacarpophalangeal joint of 2.5 mm in either men or women; and thickness of the phalangeal soft tissues of greater than 26 mm in women or 27 mm in men.

Although accurate diagnosis of acromegaly is not difficult, particularly in advanced cases, some of the individual radiographic signs may be noted in other disorders. An acromegaly-like syndrome has been associated with pachydermoperiostosis. Radiographic findings are similar to those of acromegaly, with enlarged sinuses, prominent supraorbital ridges, and thickening of the phalanges. However, the sella turcica is not enlarged, and severe prominence of the phalangeal tufts and enlargement of articular space are not observed.

Acromegaly, Table 1. Radiographic features in acromegaly.

DR/RB