Symmetric lupoid onychodystrophy (SLO)

Introduction

Although claw disease is an uncommon presenting complaint at veterinary hospitals, it is essential that clinicians are aware of diseases affecting the claws and claw folds, as these can greatly affect an animal’s quality of life. “Onychodystrophy” refers to abnormal claw formation, including changes to shape, texture, and growth. However, the term “onychitis” may be considered more appropriate for this disease process, as symmetric lupoid onychodystrophy (SLO) is not a disease of primary claw malformation. Rather, changes to the claws are secondary to inflammation of the claw bed, leading to abnormal claw growth (1). This poorly understood disease is likely a reaction pattern that has a multifactorial pathogenesis including immune-mediated and hereditary components (1–3).

Clinical presentation

Symmetric lupoid onychodystrophy can affect dogs of any age but is most common in those between 2 and 6 y of age (2). The disease mainly affects, but is not limited to, large-breed dogs such as the German shepherd dog, Gordon setter, rottweiler, Labrador and golden retrievers, Akita, bearded collie, and boxer, among others (3–10). Affected dogs are often presented for evaluation of licking and chewing their paws, for acute lameness, or for suspected trauma causing claw avulsion. Physical exam can reveal a variety of findings, including onychomadesis (sloughing of the claws), onychorrhexis (splitting of the claws) (Figure 1), onycholysis (claw separation from the claw bed), and trachyonychia (roughened texture of the claws) (Figure 2). The presence of purulent discharge around the claw folds is suggestive of a secondary bacterial infection. Initially, only a single claw may be affected, but the disease will eventually progress to include multiple claws on multiple paws within weeks to months after initial presentation. Typically, upon regrowth, claws will have a misshapen or brittle appearance.

Figure 1.

Onychorrhexis: Splitting of the claw sheath in a golden doodle dog with symmetric lupoid onychodystrophy (SLO).

Photograph supplied by L. L. Quilling.

Figure 2.

Trachyonychia: Roughened surface of the claw in a dachshund dog with symmetric lupoid onychodystrophy (SLO).

Photograph supplied by L. L. Quilling.

Animals with SLO generally do not display any signs of systemic disease, although follicular dysplasia and systemic lupus erythematosus (SLE) have been reported to occur in conjunction with SLO (7). Allergic dermatitis may also be present in some individuals; however, in the authors’ opinion, this is unlikely to be a predisposing factor for SLO.

Pathogenesis

As there is an increased incidence of SLO in certain dog breeds, a hereditary component is suspected. Genes encoding dog leukocyte antigens (DLA) are responsible for regulation of numerous immune-mediated functions in dogs. In Gordon setters, giant schnauzers, and bearded collies, an increased risk of developing SLO was demonstrated in individuals with homozygosity for DLA Class II (3). Furthermore, the association with DLA Class II strongly supports an immune-mediated component to SLO (3). More recently, bearded collies were reported to have alterations to canine chromosomes (CFA) 12 and 17 (11). Conversely, 1 study failed to demonstrate SLO protein-encoding variants on CFA17 (12). Although the specific chromosomal alterations leading to SLO have not yet been established, CFA12 is a highly probable candidate gene.

Evaluation of thyroid function has been proposed for dogs affected with SLO. In a prior investigation, hypothyroidism was confirmed in 17% of dogs, which was more than the expected prevalence within the general canine population (13). However, an association between hypothyroidism and SLO is still unclear. It has been proposed that the affected cohort may have had a genetic predisposition to developing immune-mediated disease, or that antithyroid antibodies may become bound to claw matrix proteins (13).

In 1 review paper, 4 of 24 dogs with clinical signs of SLO achieved remission after completing an elimination diet trial (13). Two of these dogs were definitively proven to have a food allergy following a dietary provocation challenge. However, none of these dogs had additional clinical signs aside from claw disease (13).

Diagnosis

When investigating the cause of claw disease, cytology should always be done, as secondary bacterial and Malassezia paronychia, and onychomycosis are common. Other differential diagnoses include trauma, parasitosis, leishmaniosis, and immune-mediated diseases such as vasculitis, pemphigus foliaceus, pemphigus vulgaris, bullous pemphigoid, cutaneous drug reaction, and other diseases in the lupus category (2,13). It is important to emphasize that most of these diseases would not be limited to the claws alone, and concurrent dermatologic or systemic signs would also be observed.

A single study analyzing abnormalities in the laboratory findings of affected individuals indicated mild, variable changes on hematology and biochemistry that were not considered clinically relevant (13). Because changes are inconsistent, blood work is unlikely to offer additional clinical insight and would better serve as a general baseline health assessment prior to initiating treatment for SLO. Antinuclear antibody titers were negative in all dogs and are not warranted as part of the diagnostic workup (13). However, it may be worthwhile to consider an elimination diet trial to further investigate the role of food allergy.

Definitive diagnosis requires a biopsy of affected claws, which involves amputation of the third phalanx (P3) (2,4). This allows for examination of the entire structure as well as the claw matrix, which is not possible when avulsed or sloughed claws alone are submitted for histopathological evaluation (2). Amputation of an affected dew claw is always recommended, if available, to avoid surgical complications on weight-bearing digits. An alternative approach for biopsy of the lateral claw matrix has been described to avoid amputation (14); however, this method frequently yields nondiagnostic results and is now less commonly recommended. Histopathologic findings include interface inflammation composed of lymphocytes and macrophages, with fewer neutrophils and plasma cells at the junction of the claw bed epithelium and the dermis, with or without a lichenoid band (1). Because there are very few diseases that cause claw abnormalities without concurrent dermatologic or systemic signs, many dermatologists make a presumptive diagnosis of SLO based on clinical findings and history alone, foregoing digit amputation altogether.

Management

One of the most common treatment options for SLO is doxycycline (or tetracycline) and niacinamide. This well-tolerated combination of an antibiotic and a B vitamin has immunomodulatory properties, although the exact mechanism is poorly understood. In 1 study, 22 of 30 dogs were treated with this combination therapy; there was an excellent response in 7 of those dogs and a partial response in another 7 dogs (6). It is worth mentioning that, despite its efficacy and safety, this combination therapy is starting to fall out of favor among dermatologists given the recent evidence highlighting the importance of practicing responsible antimicrobial stewardship.

Corticosteroids have potent anti-inflammatory to immunosuppressive activity. In 1 review in which prednisolone was used as a sole or multidrug therapy, a good to excellent response was reported in 4 of 5 dogs. However, treatment was suspended in 3 dogs due to adverse effects, and 1 dog died of acute pancreatitis (6). Although corticosteroid therapy appears to be highly efficacious, the authors recommended that it be reserved for refractory cases or short-term use only.

Oral essential fatty acids, particularly omega-3 and omega-6 fatty acids including γ-linoleic acid, may be helpful as adjunct or maintenance therapies (2,6,15). Several studies have shown variable responses, ranging from complete to partial to total lack of resolution, when used in combination with other treatments such as pentoxifylline and cyclosporine (5,6,16–18). One study comparing the efficacy of cyclosporine and essential fatty acid supplementation in dogs being fed a diet high in omega-3 fatty acids reported improvement of clinical signs in 13 of 14 dogs, and no statistically significant difference between treatment groups; however, neither treatment resulted in long-term cure (17). Monotherapy with essential fatty acids and pentoxifylline has also been shown to produce complete or partial resolution (5,6,10,19). Although no studies have evaluated the use of cyclosporine as monotherapy, the authors consider this to also be a reasonable option for managing SLO, as no single therapy (or combination of therapies) appears to be superior.

Prognosis

Response to treatment is usually slow. In healthy laboratory colony dogs, claws grew at a rate of 0.7 to 2.1 mm/wk (20); therefore, assessment of clinical response should not be rushed, and client expectations must be adjusted accordingly. As new claws may not be normal in appearance, the presence of well-anchored claws with no signs of inflammation at the level of the claw bed and no pain on palpation around the claw folds should be considered a satisfactory response to therapy. In the authors’ experience, response to therapy is dependent on the individual and may require trying > 1 treatment option before finding the best fit. Treatment failure is uncommon, and most dogs can achieve a good quality of life with long-term management.

Conclusion

When discussing claw disease with clients, it is important to emphasize that SLO typically requires lifelong maintenance therapy. Additionally, diagnostic tests, such as cytology to rule out infection or an elimination diet trial to investigate food allergies, are important for formulating an initial treatment plan. Although diagnostic tests are important, in most cases an underlying etiology is seldom uncovered. Treatment is generally safe and well-tolerated. Given the good response to mild immunomodulation, immunosuppressive therapy is rarely warranted. Many dogs are expected to achieve and maintain a good quality of life.