There are 6 common clinical scenarios which can be related to sacroiliac (SI) joint region pain: 1) acute onset of SI joint region pain characterised by a reluctance to go forwards when ridden; 2) sudden onset of bucking and kicking out, usually only when ridden; 3) insidious onset of reduced hindlimb impulsion associated with SI joint region pain; 4) thoracolumbar and sacroiliac region pain; 5) unilateral or bilateral hindlimb lameness and SI joint region pain, especially associated with proximal suspensory desmopathy; 6) as a sequel to a previous ilial stress fracture (Dyson and Murray 2003; Dyson 2008).
Typical features of the history may include loss of performance, poor hindlimb action, stiffness, reluctance to work on the bit, poor quality canter: 4 time, changing legs behind in canter; difficulty in flying changes, difficulty in lateral work, resistance to work, loss of jump; stopping. Bucking when ridden is usually sudden in onset. The horse often kicks out repeatedly to one side. The horse may refuse to go forwards. This bucking behaviour is occasionally seen on the lunge, but is usually only evident when the horse is ridden and is not typical of 'cold backed' behaviour. Other relevant history may include that the horse has become difficult to shoe. The horse may move over in the stable awkwardly. The clinical signs may be worse after travelling. Some horses adopt an odd posture at rest with the hindlimbs camped under, or shifting weight.
The hindquarters may be poorly muscled, but most commonly with chronic SI joint region pain there is atrophy of the longissimus dorsi muscles cranial to the tubera sacrale, resulting in prominence of the lumbar spinous processes. The tubera sacrale may be symmetrical or asymmetrical. There may be an exaggerated response to pressure applied over the tubera sacrale, with the hindquarters dropping. Rocking the pelvis may elicit pain.
In some horses there are no detectable gait abnormalities either in hand or on the lunge. Other horses exhibit poor hindlimb impulsion, some times with a mild toe drag. The horse may track straight, tend to plait or have a base-wide hindlimb gait. The horse may step awkwardly when turned in small circles or when backed.
Gait abnormalities are invariably worst ridden and in some horses are only apparent when ridden. The horse moves with a stiff back, sometimes throwing the rider forwards out of the saddle. There is poor hindlimb impulsion and lack of hindlimb engagement. The canter is often the worst gait, with the horse being crooked, with a poor quality canter and a tendency to break or change legs behind. The horse may show nappy or resistant behaviour. There is often unwillingness to work on the bit and the problems may be worse in lateral work. The horse may break to canter rather than increase hindlimb impulsion in canter. The horse may skip behind in transitions from walk to trot. There may be a unilateral or bilateral low grade hindlimb lameness, which may appear to switch from limb to limb. Affected horses often feel much worse to a rider than they appear to an experienced observer.
In contrast horses with primary thoracolumbar pain may show a 'disconnection' of the hindlimbs; the rider thrown upwards rather than forwards. The rider may also get back pain! A saddle pad may ride backwards. A horse may buck when ridden, but does not kick out.
The diagnosis requires elimination of other potential sources of pain. Ideally diagnosis is confirmed by the response to periarticular analgesia, using 15 - 20 ml of local anaesthetic solution per side. The horse is walked for 15 min after injection and then reassessed ridden. If local anaesthetic solution is placed too far caudally there is the potential to induce hindlimb ataxia or sciatic nerve paralysis. The nerve block technique is not specific and it is not possible to differentiate between pain due to SI joint disease, lumbosacral and ventral sacroiliac desmitis.
Nuclear scintigraphy may be helpful for verifying the diagnosis, but apparent radiopharmaceutical uptake (RU) is influenced by both conformation of the pelvis and the symmetry of overlying musculature (Erichsen and Berger 2001; Erichsen et al. 2002, 2003; Gorgas et al. 2009). Therefore it may be difficult to interpret scintigraphic images in isolation. There is also an overlap between the patterns of RU between horses with and without SI joint region pain (Dyson et al. 2003a,b). Moreover normal RU does not preclude significant SI joint region pain. Left right asymmetry of RU is abnormal and intense increased RU is usually significant.
Ultrasonography should be performed to evaluate not only the caudal aspect of the SI joint per rectum but also the lumbosacral joints. It is not uncommon for lumbosacral pathology and SI joint region pain to coexist. Fusion of the lumbosacral joint has potential biomechanical implications for the thoracolumbar and pelvic regions and may predispose to the development of SI joint region pain. Radiological assessment of the thoracolumbar spine is also useful to identify any concurrent abnormalities which also may be a predisposing factor for the development of SI joint region pain.