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Rheumatology Advance Access originally published online on March 3, 2008
Rheumatology 2008 47(5):561-562; doi:10.1093/rheumatology/ken032
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© The Author 2008. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

EDITORIALS

What is the primary lesion in SpA dactylitis?

I. Olivieri1, S. D'Angelo1, E. Scarano2 and A. Padula1

1Rheumatology Department of Lucania, San Carlo Hospital of Potenza and Madonna delle Grazie Hospital of Matera and 2Radiology Department, San Carlo Hospital of Potenza, Potenza, Italy.

Correspondence to: I. Olivieri, Rheumatology Department of Lucania, San Carlo Hospital, Contrada Macchia Romana 85100 Potenza, Italy. E-mail: ignazioolivieri{at}tiscalinet.it; i.olivieri{at}ospedalesancarlo.it

Dactylitis or ‘sausage-like’ digit is a characteristic and highly specific manifestation of SpA (Figs 1 and 2) [1]. It totally differs from other digit diseases called dactylitis [2]. Even though more frequent in PsA [3, 4], SpA dactylitis has been observed in all types of SpA, including the undifferentiated ones [5]. In the latter cases, dactylitis usually occurs together with one or more manifestations of the HLA-B27-associated disease, i.e. peripheral arthritis, peripheral enthesitis, inflammatory spinal pain, chest wall pain, alternate buttock pain, acute anterior uveitis and aortic insufficiency with conduction disturbances [6]. Like these, dactylitis may sometimes occur for a long time alone as the only clinically manifest feature of the HLA-B27-associated disease [7–10]. This has been observed in children [7, 8], in young and middle-aged adults [9] and in the elderly [10]. In addition, there are also patients with psoriasis who only exhibit dactylitis and/or enthesitis for months or years [11].


Figure 1
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  		FIG. 1. Dactylitis involving the second finger of the right hand.

 

Figure 2
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  		FIG. 2. Dactylitis of the second toe of the right foot.

 
Dactylitis was ignored by clinical research till the eighties. When we developed our interest in dactylitis we wondered first what the lesion responsible for the ‘sausage-shaped’ appearance was. The simultaneous occurrence of arthritis of the three-digit joints (MCP or MTP, proximal and distal interphalangeal) did not appear to be a sufficient explanation since in this case the digit should have presented three bulges and not a diffuse swelling. On the other hand, physical examination of the dactylitic finger or toe always shows a painful swelling along the flexor tendons. In finger dactylitis, the enlargement of the flexor surface is often so marked that the patient cannot flex his finger. In 1996 and 1997, we investigated the role of tenosynovitis and arthritis in producing the ‘sausage-like’ feature by using ultrasonography (US) and MRI [12, 13]. Twelve dactylitic fingers and 12 dactylitic toes and their corresponding normal contralateral digits were studied. The diameter of the flexor tendons and their sheaths was measured at the midpoint of the proximal phalanx. The MCP, MTP and interphalangeal joints were examined for capsule distension and synovial proliferation. In all ‘sausage-like’ digits, both MRI and US showed fluid collection in the flexor synovial sheaths. Of the 72 joints of the 24 dactylitic digits, only 3 (one MCP and two MTP) showed capsule distension. We concluded that dactylitis is due to flexor tenosynovitis and that the distension of the joint capsule is not an indispensable condition for the ‘sausage-like’ feature. Another important conclusion was that clinical examination is a sufficient method for the diagnosis of tenosynovitis since it showed 100% sensitivity and specificity compared with MRI.

The results of our studies were confirmed by two other US studies on dactylitis [14, 15]. Both found flexor tenosynovitis in all dactylitic digits. The only difference from our studies was on the frequency of joint synovitis. Kane et al. [14] found a frequency of 52% and Wakefield et al. [15] a value of 62.5% higher than 12.5% in our studies [12, 13]. In both studies, a subcutaneous soft tissue that was difficult to assess quantitatively, was observed [14, 15]. A diffuse inflammation of the digital soft tissue, termed ‘pseudosynovitis’, has been observed in a more recent US study [16]. In our most recent MRI study on foot dactylitis, we found that a mild joint effusion was present in 8 (66.6%) out of the 12 dactylitic toes and a mild-to-severe subcutaneous oedema in 11 (91.6%) [17]. Flexor tenosynovitis was present in all 12 dactylitic toes.

The importance of the involvement of the flexor synovial sheaths has been confirmed by some clinical observations. The first is the possible extension of the dactylitic swelling of the second, third and fourth fingers in the palm of the hand [14, 18, 19]. The most common configuration of digital and palmocarpal synovial sheaths is as follows. The index, middle and ring fingers have synovial sheaths unconnected to those of the radial and ulnar bursae that are in communication with the synovial sheaths of the thumb and the little finger, respectively. However, variants showing communication of the sheaths of the index, middle or ring fingers with the ulnar bursa occur frequently. When dactylitis involves a finger with synovial sheaths communicating with the ulnar bursa, the painful swelling also extends into the palm of the hand. The second clinical observation is the spread of the tenosynovial inflammation of dactylitis of the thumb and fifth finger to the radial and ulnar bursa, respectively [20, 21].

In conclusion, flexor tenosynovitis together with the peritendinous soft tissue oedema seems to be the conditio sine qua non for the development of the ‘sausage-like’ appearance. Joint synovitis is often present but cannot give the ‘sausage-shaped’ digit without the simultaneous presence of tenosynovitis and soft tissue oedema.

McGonagle and co-workers [22–24] have approached dactylitis in a different way. They wondered as to where the inflammation starts in the ‘sausage-like’ digit and, of consequence, what is the primary lesion of dactylitis. They hypothesized that enthesitis is the primary lesion in SpA dactylitis and that synovitis of the various structures (joint, tendon, and bursa) represents a secondary phenomenon due to the release of pro-inflammatory cytokines from the inflamed entheses [22, 23]. In their opinion, the flexor tenosynovitis of dactylitis is due to enthesitis as a consequence of the diffusion of cytokines along the tenosynovial sheaths [24]. In a recent study, we demonstrated, by using fast spin echo (FSE)–T2-weighted sequences with fat saturation, that in SpA dactylitis there is no evidence of enthesitis of the flexor digitorum tendons and joint capsule [25]. The same seems to occur in toe dactylitis [17]. However, McGonagle and co-workers [26, 27] argue that some facts and considerations are in favour of the involvement of the entheses in dactylitis. The first is the more frequent US [28] and MRI [29] evidence of extracapsular and peri-entheseal changes in PsA compared with RA. Second, the digits are the sites of numerous entheses and functional entheses that are frequently associated with the presence of fibrocartilage [26, 27]. The flexor tendons themselves form functional entheses with their retinacula or pulleys. Some other entheses are intra-articular such as the extensor tendons at the interphalangeal joints [30]. All these insertions could be involved in dactylitis. Third, bone marrow oedema adjacent to the involved entheses might have been overlooked in our studies because of limitations in spatial resolution of conventional 1.5 T MRI of small joints. Lastly, in a recent study of the distal interphalangeal joint using high-resolution microscopy MRI coil, bone marrow oedema was located near the insertions of the collateral ligaments emanating from the enthesis [31]. In addition, a recent review reported a preliminary study on dactylitis, which showed enthesitis by using high-resolution MRI [27].

In the January 2008 issue of Rheumatology, Healy and co-workers [32] report their MRI study on psoriatic dactyltis. Scans were performed before and after treatment and pre- and post-gadolinium contrast. Joint synovitis and soft tissue oedema were the most frequent abnormalities. The oedema extended all around the circumference and was not ever associated with flexor tenosynovitis which was, however, a common finding. Bone oedema ranges from a small area adjacent to the joint capsule insertions to a lesion involving the whole phalanx. This last finding seems to confirm the hypothesis by McGonagle on a primary involvement of the entheses in dactylitis. However, the authors wonder why the dactylitic digit develops such widespread inflammation across different tissues and why the same process does not occur in severe synovitis of RA.

Another important aspect of Healy and his colleagues’ study is the MRI score that they propose. This could be useful in clinical trials even if poorly correlated with the clinical scores. Among these, only the validated Leeds Dactylitis Index (LDI) [33, 34] was significantly associated with the MRI score.

In conclusion, the last MRI studies on dactylitis have shown the involvement of digit entheses different from the insertion of the flexor tendons. It remains to understand if this involvement can explain the development of the vigorous inflammation across the different soft tissue of the digit.

Disclosure statement: The authors have declared no conflicts of interest.

References

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Accepted 11 January 2008


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