Juvenile ankylosing spondylitis (JAS) is more frequent in males, and disease onset usually occurs between the end of childhood and the first years of adolescence. Arthritis and enthesitis (inflammation involving the sites of insertion of tendons, ligaments, and joint capsules) are the more important articular manifestations in JAS. In contradistinction to the adult form of ankylosing spondylitis, in which axial involvement predominates from the start, in JAS, years (or even decades) may elapse before spondylitis and sacroiliitis become evident.

Arthritis is more common in large joints – mainly in the lower extremities – and the peripheral small joints are involved less often. If compared to that found in adult-type ankylosing spondylitis, articular disease in JAS usually follows a more severe course, with unrelenting arthritis, disability, and occasional need for joint replacement (Figs. 4.1 and 4.2). Radiographic findings related to peripheral arthritis include progressive narrowing of joint space and bone erosions of late appearance (Fig. 4.1). MRI is the method of choice for early diagnosis, demonstrating joint effusion, synovial thickening, and inflammation of the soft tissues and of the bones (Fig. 4.1), even in the pre-erosive stage. US is also able to demonstrate joint effusion, synovitis, and soft-tissue swelling as soon as they appear. Enthesitis is frequently found in the insertions of the plantar fascia, of the knee extensor mechanism, and of the Achilles tendon. Enthesitis-related radiographic findings include periosteal reaction, bone spurs, and enthesophytes, as well as sclerosis and lucent areas in the adjacent bone. Periostitis is more common in the hands, while enthesophytes are more frequently seen in the tarsal bones (Fig. 4.1). The affected tendon appears thickened and heterogeneous on US and on MRI, with increased perfusion on Doppler US and post-gadolinium enhancement. Cortical erosions and bone marrow edema pattern are also present, as well as soft-tissue swelling and fluid distention of adjacent bursae.

Sacroiliitis in JAS is prototypical, very similar to that found in adult-type disease. When positive, radiographs show loss of definition of the articular surfaces, bone erosions, and subchondral sclerosis in the sacroiliac joints, which are most often bilateral. As the disease advances, there is gradual narrowing of the joint spaces, and ankylosis may be found in late-stage JAS. Pseudo-widening of the joint space is a peculiar finding, caused by confluence of subchondral erosions (Figs. 4.2 and 4.3). However, radiographs are insensitive for the assessment of the sacroiliac joints, largely because of their complex anatomy; furthermore, the classic radiographic findings are only found in advanced sacroiliitis (Fig. 4.2), and this imaging method is unable to distinguish active from inactive disease. In spite of the excellent bone assessment provided by computed tomography (CT) (Fig. 4.3), this imaging modality is also unable to yield accurate information about inflammation activity. MRI is the most suitable study for sacroiliac evaluation, as it is able to detect subchondral areas of bone marrow edema that show post-gadolinium enhancement and are indicative of active disease (Fig. 4.4); these areas of osteitis are evident since the early stages of sacroiliitis, way before the development of erosions. Increased signal intensity on T2-weighted images (T2-WI) and post-gadolinium enhancement can also be seen inside the articular space in inflamed sacroiliac joints. Bone erosions are usually small, and large subchondral cysts are relatively rare (Fig. 4.5); subchondral sclerosis appears as areas of low signal intensity in all sequences (Fig. 4.5). Fatty replacement of bone marrow is a late finding, indicative of healing and typically found in chronic sacroiliitis. Despite the high sensitivity of bone scintigraphy, its usefulness is limited in the assessment of sacroiliitis in the juvenile spondyloarthropathies, mostly because of technical issues in the evaluation of the immature sacroiliac joints (see Chap. 1) and due to its inherently low specificity and spatial resolution (Fig. 4.6).

Spondylitis usually occurs in late-stage JAS and is rarely found in children, affecting most often the thoracolumbar spine of adults with long-standing disease (Fig. 4.2); sacroiliitis almost always precede vertebral involvement. Involvement of the cervical spine is rare and late, presenting a more benign course if compared to that seen in juvenile idiopathic arthritis. Classic radiographic findings (infrequently seen in skeletally immature patients) include sclerosis of vertebral corners, squaring of the vertebral bodies, syndesmophytosis, and ankylosis of the facet joints.

Juvenile psoriatic arthritis (JPA) is a form of seronegative arthritis associated with psoriasis, much less common and more subtle than the adult form of the disease. The typical cutaneous rash is absent in up to half of the patients. This is the only spondyloarthropathy in which females are more frequently affected (up to 2.5:1), with an incidence peak at 11–12 years of age. Diagnosis is based on clinical features, a compatible family history, laboratory tests, and imaging findings.

Arthritis is usually oligoarticular and asymmetric, affecting most often the knees and the small joints of the hands and feet. Radiographs are insensitive for early-stage JPA; the association of periostitis, bone erosions, and enthesitis is very characteristic of JPA, but these are late findings, as well as sacroiliitis/spondylitis. The hands are affected most often and more severely than the feet, and arthritis of the distal interphalangeal joints is typical (Fig. 4.7). Peripheral erosions may lead to the classic “mouse ear” appearance (Fig. 4.8). Arthritis mutilans is a rare and severe form of JPA, presenting imaging findings similar to those of ­juvenile idiopathic arthritis (Figs. 4.9 and 4.10). Abnormalities commonly found in the adult-type form of the disease, such as acro-osteolysis (resorption of the distal phalanges), ­whiskering (proliferative bone neoformation in the entheses), and periostitis tend to be less aggressive in JPA. If compared to JAS, there is more extensive involvement of the small joints and less ­enthesopathy in JPA, features that help to narrow the differential diagnosis. The so-called “pencil-in-cup” deformity, a classic finding of psoriatic arthritis in adults, is uncommonly seen in children (Figs. 4.8 and 4.11).