Exercise 18 discussion

Clinical history and questions

Close-up view of a popliteal lymph node, exposed and sliced.  From an old dog from the local pound (no history was available).  The animal had been killed for a student necropsy class.

The skin of the rump and hind legs was diffusely wrinkled, hyperpigmented and symmetrically alopecic (a typical example is visible at lower right).

  1. Describe the abnormalities
  2. What basic disease processes might be present?
  3. Pathological diagnosis (give other possibilities in order of preference)?
  4. What is the most likely cause of this change?

An interpretation of Diagnostic Exercise No. 18 – Roger Kelly

  1. Description:  The sliced node appears enlarged and the cut surface is discoloured brown, with randomly distributed fine black stippling.  The bit of skin at lower right is consistent with the clinical description.
  2. Basic process/es:  Pigmentation is considered to be a degenerative process, unless it involves melanin produced by melanocytic neoplasia.  Lymph node enlargement is also consistent with neoplasia or inflammation, and inflammation (chronic) is also indicated by the history.  The focal black pigmentation raises the question of metastatic melanocytic malignancy, but the diffuse brown discoloration is most likely a degenerative change that has produced accumulation of either lipofuscin or haemosiderin or both.  To sum up, then; a combination of degeneration and chronic inflammation seems most likely, with neoplasia a possibility.
  3. Pathological diagnoses:  Chronic lymphadenitis with diffuse haemosiderosis and focal melanosis; possible metastatic malignancy.
  4. The most likely cause of these changes is accumulation of haemosiderin and melanin in the node after transport in macrophages from chronically inflamed and hyperpigmented skin.  The most common cause of symmetrical hyperpigmentation and alopecia hindquarter skin of old dogs would be chronic flea allergy.  Chronic erythrocyte diapedesis due to self-trauma leads to haemoglobin being taken up and transformed to haemosiderin in local macrophages, which eventually find their way to the local nodes, along with some of the melanin which is produced to excess and accumulates beneath the epidermis.  Endocrine disorders may produce somewhat similar changes in the skin, but would not produce these changes in the local nodes.