What's wrong with that foot? (Proceedings)

Plasma Cell Pododermatitis

The etiology of this unusual condition in cats is not known. It probably falls into the category of other lymphoproliferative disorders of cats.

Clinical signs include soft, puffy swelling of one or more of the footpads. The metacarpal and metatarsal pads are most frequently affected. The condition usually affects more than one foot but occasionally unipedal involvement is observed. Surprisingly, affected feet do not seem to be painful unless ulceration of the pads occurs. Secondary infection may occur in ulcerated pads.

The clinical appearance is fairly classical if several feet are involved. Confirmation can be made by fine needle aspiration (FNA) or biopsy. The aspirate will usually include numerous RBCs mixed with plasma cells and some neutrophils. Neutrophils may be more numerous if there is secondary infection in an ulcerated pad.

Occasionally this condition will regress spontaneously, however, treatment is recommended. Doxycycline (10 mg/kg PO q24h) may be effective, most likely because of its anti-TNFα effects. Glucocorticoids (oral or repositol) will usually produce regression. Other immunosuppressive drugs such as cyclosporine may be helpful but have not been used as often. The prognosis for recovery from this condition is good.

Cutaneous Horns

Cutaneous horns are composed of keratin overgrowth. They may affect one or multiple footpads. Often thin and horn-like (hence the name), they may appear like second "nails" close to the nails on the digital pads. If not on a weight bearing surface, these lesions usually do not cause lameness. If on the plantar surfaces of the pads, they may cause discomfort in walking.

Cutaneous horns may be spontaneous, and this is often the case on the footpads. Horns are also associated with papillomavirus infection, FeLV, and squamous cell carcinoma. Diagnosis of the condition is usually based on the appearance of the lesions. Affected cats should be tested for FeLV because this is an easy rule-out diagnosis. Single horns associated with scaly skin lesions should be biopsied to rule out SCC.

If the horns are not causing lameness the lesions are often ignored. The horny growth can removed by trimming, however, the horns will often recur. Horns causing discomfort should be removed and, if it is possible to do so without causing a large pad defect, the base of the lesion should be excised to prevent re-growth.

Lung-Digit Syndrome

Lung carcinomas in cats may metastasize to the digit, usually the third phalanx, to cause bony lysis, painful, firm swelling of the toe, and paronychia. If a single digit is involved, the swelling is often attributed to infection and the underlying lesion may not be recognized until routine therapy fails to alleviate the problem. Lesions may occur in multiple digits and that should alert the astute clinician to look at the lung early in the course.

Radiographs of affected digits usually demonstrate bony lysis of the third phalanx with significant soft tissue swelling. As is typical of neoplasms, the bony lysis usually does not cross the joint space to the second phalanx but periosteal reaction may be seen in this area. Lung radiographs may reveal a single pulmonary lesion or diffuse lung carcinoma.

Excision of the affected digit(s) may help alleviate pain, however, the primary lesion is in the lung and microscopic metastasis to other digits or other body areas may already be present. Treatment of the primary lung tumor will not improve the condition of the feet. Therapy is therefore usually palliative and consists of analgesia and supportive care. NSAIDs may be beneficial because of their analgesic/anti-inflammatory effects as well as COX-2 inhibitory activity. COX-2 is involved in tumor angiogenesis and these drugs may slow tumor growth. Drugs that have been used in cats include Piroxicam (0.3 mg/kg PO q24-48h) and Meloxicam (0.1 mg/kg X 1-2 days, then 0.1 mg [total dose] PO q24-72h). As with the use of any NSAID, renal function should be closely monitored to avoid renal toxicity with these drugs.

Tumor Infarction

In addition to causing lung-digit syndrome, tumor emboli from lung carcinomas may block arterial flow to the limb(s) resulting in infarction and tissue necrosis. These emboli often cause a very well-demarcated pattern of involvement. Initially, there is acute, painful swelling of the affected limb(s). The tissue then rapidly turns dark red and progresses to areas of black. Skin and tissue sloughing often follows over time.

Diagnosis consists of recognition of this pattern of necrosis and identification of the pulmonary neoplastic lesions by radiography. Differential diagnoses for this type of lesion would include embolism from cardiac disease although this usually occurs higher in a major limb vessel, and other types of embolic phenomena.

It is unusual for affected limbs to recover circulation after neoplastic infarction and there are or will be emboli in other body parts. Because of the pain and necrosis of the limb (gangrene), amputation is the only recourse if the owners want to palliate and to give the cat some additional time. However, because there is primary pulmonary neoplasia and because other metastatic lesions are often already present, the prognosis for prolonged survival is very poor even with surgery. Follow-up palliative treatment with NSAIDs as previous described may be used if amputation is performed.

Immune-mediated Disease

Pemphigus foliaceous is the most common of the feline cutaneous IM diseases and often involves the footpads and nailbeds. The footpads are often involved early and may appear scaly or hyperkeratotic. The nailbeds often contain caseous exudate. Pain is usually absent but pruritus may be present. Lesions occur on other parts of the body, particularly on the ear pinnae. These may consist only of subtle pustules or crusts early in the course of the disease. More generalized skin involvement with crusting and hyperkeratosis occurs as the disease progresses.

The diagnosis of PF may be suggested on impression cytology from skin underlying the crusts. Acantholytic epithelial cells are characteristic of PF. If intact pustules are found, aspiration cytology from these usually demonstrates neutrophilic exudate with occasional eosinophils. Biopsies may be necessary to confirm the diagnosis and multiple samples should be taken from several areas on the cat.

Treatment options include: prednisolone 2.2-4.4 mg/kg PO q24h, then tapering after 2 weeks if effective, dexamethasone 0.1-0.2 mg/kg q 24 h, then tapered. In a recent study, triamcinolone at 0.5-1.0 mg/kg was also effective. If glucocortioids alone are not effective, chlorambucil 2 mg PO q48h can be added to the regimen. Cyclosporine has recently been shown to be a very effective drug in the treatment of a variety of feline immune-mediated disease and may be an effective option for treatment. The usual dose of CyA used for other skin diseases is 25 mg PO q24h.

Fungal Infections

Fungal infections, particularly the opportunistic soil-borne organisms such as: phaeohyphomycosis (pigmented hyphal forms); hyalohyphomycosis (unpigmented hyphal forms); eumycotic mycetoma (fibrosing granuloma with tissue grains containing pigmented or unpigmented fungal elements); and pythiosis/lagenidiosis/zygomycosis (pyogranulomatous and eosinophilic inflammation associated with wide, infrequently septate, nonpigmented hyphae). Systemic mycotic organisms (Histoplasma, Blastomyces, Coccidioides, Cryptococcus), may also affect the skin but foot involvement is unusual Most often these organisms are introduced by a traumatic injury such as a puncture wound or foreign body. Fungi cause granulomatous inflammation in soft tissue and bone. Ulceration of the overlying skin with secondary bacterial infection may occur. Regional lymphadenopathy may or may not be present. Systemic signs of illness may or may not be present.

Radiographs of affected areas may demonstrate only soft tissue swelling or may reveal underlying bone reaction and necrosis. The diagnosis is made by identification of the organism in fine needle aspiration cytology or on biopsy. If fungi are suspected, special stains should be requested at the time of sample submission. Tissue samples for fungal culture should be collected at the time of biopsy and submitted for further specific identification of the species of infecting organism.

Treatment with systemic antifungal drugs may or may not be effective. There is wide variation in the susceptibility of these unusual fungi to azole antifungals and AMB. Itraconazole (10 mg/kg PO q24h) and fluconazole (5-10 mg/kg PO q12-24h) are the drugs of choice. If treatment with azoles is ineffective, it may be helpful to try to get an antifungal sensitivity profile on the infecting organism. If the infection is localized and drug therapy is not effective, amputation may be the only option for cure.

L-form Bacterial Infections

One of the more common causes of chronic bacterial abscessation that is resistant to routine treatment is L-form bacterial infection. L-forms are cell wall-free bacteria, usually a common species that has undergone alteration due to some environmental pressure in the wound site. Lesions occur most often on the limbs and systemic signs of illness such as fever and anorexia accompany the abscesses. Joints may be involved and can collapse due to cartilage destruction.

The discharge associated with L-form infection appears purulent and cytologically is granulomatous with many PMNs and macrophages. Organisms cannot be identified on cytologic examination and culture on routine bacteriologic media is unsuccessful. The diagnosis is usually suspected because of resistance to other antibiotic therapy and failure to respond to routine abscess management. Confirmation is by rapid response to doxycycline therapy.

L-form infections do not respond to commonly used antibacterial agents used to treat abscesses but do respond promptly to tetracycline drugs with significant improvement occurring within 48-72 hours. Doxycycline 10 mg/kg PO q24h given for 3 weeks is effective in eliminating L-form infection. The clinical appearance of Mycoplasma infections is similar to that described for L-forms and doxycycline is also effective against these organisms.

Other Miscellaneous Neoplasms

Other occasional neoplasms involving the feet include primary squamous cell carcinoma of the nail bed and mast cell tumor.

Primary SCC looks clinically and radiographically like lung-digit disease but no lung lesions are found. Digital SCC is often highly malignant and the regional node(s) should be examined for metastasis in addition to surgical amputation of the digit. Follow up therapy with NSAIDs may be palliative in patients with disseminated disease.

MCT of the foot is unusual and may cause significant edema of the affected distal limb. Diagnosis is usually straightforward from fine needle aspiration cytology or biopsy. These lesions may not be responsive to corticosteroids and amputation may need to be considered. Regional nodes should be assessed for metastasis before radical procedures are recommended. Additional therapy with H2 blockers and other adjunctive agents are recommended.