Ankylosing spondylitis (AS) is the prototype of spondyloarthritis, characterized by spinal inflammation and new bone formation that can affect entheses, sacroiliac joints, zygapophyseal joints, vertebrae, costovertebral joints, sternocostal joints, and peripheral joints (Jacobson et al. 2008a; Braun and van der Heijde 2002). Its prevalence may vary from 0.03 to 0.9 % according to the population and is higher in men (Helmick et al. 2008; Jacobson et al. 2008a).

Onset of AS after 55 years is uncommon, with an incidence of 2.2/100,000 per year, and the frequency of HLA B27 negative patients is higher (Olivieri et al. 2009; Feldtkeller et al. 2003). These patients may have axial symptoms with or without peripheral arthritis, peripheral enthesitis, or ­dactylitis (Olivieri et al. 2009). It is believed that there are more similarities than differences between the clinical presentations of late-onset and of classical-onset AS (Olivieri et al. 2009). As in younger patients, MR imaging is particularly useful for the depiction of inflammatory changes of the axial skeleton (Figs. 6.5 and 6.6). Ultrasound is also helpful for the assessment of peripheral enthesitis.

However, the elderly may present with long-standing AS, and the physician must be aware of some special complications related to this situation. The first is the risk of horizontal fractures of the rigid osteoporotic spine, even after minor trauma (Fig. 6.7) (Barozzi et al. 1998; Campagna et al. 2009; El Maghraoui 2004). Most of these fractures occur in the cervical spine and present a significant risk of initial or secondary neurologic complications (Fig. 6.8) (Westerveld et al. 2009). Due to multilevel bony fusion, long lever arms develop in the spine, on which forces act during trauma (Westerveld et al. 2009). Hyperextension is the most common cause of fracture, often involving the vertebral body, ossified ligaments, and surrounding tissue and leading to instability (Westerveld et al. 2009). In fractures complicating AS, when paravertebral hemorrhages and injuries to the ligamentum flavum or posterior spine are found, a horizontal fracture should be sought (Koivikko and Koskinen 2008). The fracture line may be transdiscal and/or transvertebral, horizontal or oblique (Michel et al. 2007). In MR imaging, narrow-gap fractures present a low signal in both T1- and T2-weighted images, and with increasing width the gaps present a higher signal on T2-weighted images (Koivikko and Koskinen 2008). The bone marrow edema varies greatly, and in most instances, it is relatively subtle and best depicted in STIR sequence (Koivikko and Koskinen 2008). Such fractures can be missed in conventional radiographies, especially in the aged, but are well evidenced by CT and MR imaging (Michel et al. 2007).

Another late but uncommon complication of AS is the cauda equina syndrome (Kotil and Yavasca 2007). This disorder has several possible mechanisms, including fibrosis following arachnoiditis, root damage by dural ectasia and diverticula, and reduced compliance of the caudal sac (Fig. 6.9) (Kotil and Yavasca 2007; Van Hoydonck et al. 2010).

On imaging, the main differential diagnosis of SA in the elderly is represented by diffuse idiopathic skeletal hyperostosis (DISH) (Olivieri et al. 2007a, 2009). Even though both diseases affect the axial skeleton, extraspinal entheses, and occasionally involve similar kyphotic posture, DISH is a noninflammatory disease in which the spinal longitudinal ligaments and entheses slowly ossify, leading to decreased mobility (Westerveld et al. 2009; Olivieri et al. 2007a). The diagnosis of DISH can be made when ossification of the anterior longitudinal ligament is present on at least four consecutive vertebral bodies, with relative preservation of the intervertebral disk height (Westerveld et al. 2009; Taljanovic et al. 2009). Besides, widespread enthesopathy and arthritis in AS result in intra- and extra-articular fusion of the facet joints, costovertebral joints, and sacroiliac joints, which is not a common feature in DISH (Taljanovic et al. 2009).

Psoriatic arthritis has been defined as a unique form of inflammatory arthritis, usually seronegative, associated with psoriasis (Gladman et al. 2005). This disease affects about 0.15 % of the general population (Helmick et al. 2008; Olivieri et al. 2009; Denis et al. 2010). A majority of patients are adults between 30 and 50 years of age, but in one population study, the disease began after the age of 55 in about a quarter of the PA patients (Helmick et al. 2008; Olivieri et al. 2009; Denis et al. 2010; Kaipiainen-Seppänen 1996). The adult sex ratio is close to 1, but in elderly patients, the male to female ratio is 3:1 (López-Montilla et al. 2002; Denis et al. 2010).

The features that distinguish PA from other forms of spondyloarthritis are association with cutaneous psoriasis (66 % of cases), asymmetric involvement of the skeleton (although symmetric involvement is possible), frequent damage to the distal interphalangeal joints, the inconstant presence of sacroiliitis and the presence of bony erosions with adjacent proliferation (Figs. 6.10 and 6.11), the pencil-and-cup appearance of PIP, DIP, MCP, or MTP joint (s), and the frequent development of intra-articular bony ankylosis, mainly of the interphalangeal joints, rarely of the carpal bones or larger joints (Denis et al. 2010). Destruction of the phalangeal tufts and periostitis can also be seen.

Elderly people have a significantly higher number of active joints and foot erosions and higher levels of serum C-reactive protein and synovial IL1β and IL6 than younger patients (Olivieri et al. 2009). Besides, late-onset PA is characterized by a more severe acute onset and more severe outcome (Punzi et al. 1999; Olivieri et al. 2009).

The progression of joint damage in PA is highly variable between subjects, and factors such as the presence of tender and/or swollen joints, raised erythrocyte sedimentation rate, and preexisting joint damage at the time of referral to the clinic predict future progression of joint damage (Coates et al. 2010). About 20 % of the patients develop a very destructive disabling arthritis (Gladman et al. 2005).

The term “undifferentiated spondyloarthritis” refers to a condition involving inflammatory backache, oligoarticular asymmetric arthritis, and enthesitis, but not fulfilling the diagnostic criteria for any of the currently established forms of spondyloarthritis (Olivieri et al. 2007b, 2009; Aggarwal et al. 1997). Patients may also test positive for B27 and present pitting edema, marked constitutional symptoms, and elevated ESR (Olivieri et al. 2009). Some authors believe that late-onset undifferentiated spondyloarthritis is more common than late-onset ankylosing spondylitis (Olivieri et al. 2009).