Lung-digit syndrome is a well-described occurrence in cats whereby lung cancer (usually bronchogenic adenocarcinoma) selectively spreads or metastasizes to the nailbeds and toes. It is significant because the spread to the digits often occurs before the cat experiences respiratory distress or other signs of pulmonary disease. As such, the observation of lameness or progressive, lytic lesions of the digits should prompt veterinarians to work up for a primary lung mass, especially in high-risk cats. Up to 88% of carcinoma in the feline digit may be metastatic from a pulmonary lesion. While the traditional appearance of a lung mass metastatic to the digits is widely known, there have been numerous reports of non-traditional presentations including spread of lung masses to the eyes, vertebrae, or skeletal muscle. This paper explores 7 non-traditional cases of feline “lung-digit syndrome” to emphasize the importance of recognition of these cases.
The first case described a cat presenting for acute thromboembolic disease to the hind legs. There were no cardiac lesions detected on radiographs or echocardiogram, and later CT scanning detected mass effects within the lungs and limb musculature. This was followed by the detection of firm intramuscular masses within the muscles of the left hind limbs histologically consistent with carcinoma. The remaining cases all presented with lung lesions detected by radiographs, CT or MRI, however the intitial presetation as due to metastatic disease as described:
Case two, nodules within the muscles of the hind limb and within the oral cavity.
Case three, classic digital lesions but also masses in the eyelid, cheek, and popliteal lymph node.
Case four, acute thromboembolic disease and digital lesions.
Case five, subcutaneous masses as well as within the hind limb muscles, and neck muscles.
Case six, elevated creatine kinase indicative of muscle damage and hind end paresis with ultrasound findings suspicious of aortic thrombosis.
Case 7, nodules within the muscles of the neck, head, and flank.
The described mass lesions were cytological or histologically consistent with carcinoma, suggesting that fine needle aspiration or biopsy of these lesions would have raised strong suspicion for the origin of the mass. While not all lesions were identifiable on plain radiographs, the presence of multifocal metastatic adenocarcinoma within muscle and subcutaneous tissue should raise suspicion of a pulmonary lesion. In cats with aortic thromboembolism and no identifiable cardiac disease, screening for masses within the SQ space and musculature is justified.
The authors propose that the metastasis of bronchogenic pulmonary carcinoma in cats occurs by entry of cancerous cells into a pulmonary vein, through the heart and into systemic circulation. Larger masses lodge at the aortic trifurcation resulting in the traditional “Saddle Thrombus” presentation, while smaller emboli may lodge in the arteries supplying the muscles of the thigh, and the smallest make it to the digits.
The authors further hypothesize that metastasis to skeletal muscle and other atypical sites may occur more commonly than the literature suggests. They further suggest that cats presenting with digital lesions or non-cardiogenic aortic thromboembolism should receive thorough palpation of all major muscle groups as well as three view thoracic radiographs to screen for primary lung nodules. Despite advances in cancer management, survival times remain short for cats with pulmonary carcinoma, and so while early detection may not dramatically improve survival in these cats, identification of the cause of disease may prevent unnecessary surgical or diagnostic intervention and allow owners to make an informed decision on the choice of surgical, chemotherapeutic, or palliative options.