Hole-within-a-hole sign | Radiology Signs

Answer:

The hole-within-a-hole sign is the appearance of a lytic calvarial lesion in which the inner and outer tables are destroyed to different extents, so that the smaller defect in one table is projected inside the larger defect in the other, giving a double contour. It is characteristic of Langerhans cell histiocytosis of the skull, and it is accompanied in that disease by a sharply defined, punched-out, non-sclerotic margin, a bevelled edge where the inner table is more widely destroyed, and sometimes a central sequestrum described as a button sequestrum. Lesions are usually in the frontal and parietal bones in a child or young adult, may be solitary or multiple, and may be painful with a palpable soft tissue swelling. The differential for a lytic skull lesion depends on age and number: metastasis and myeloma in an adult, in which multiple lesions have less sharply defined margins and myeloma classically produces uniform punched-out lesions without sclerosis; epidermoid or dermoid cyst, which has a sclerotic rim; venous lakes and arachnoid granulations, which are normal and follow known anatomical sites; osteomyelitis with a ragged margin; and haemangioma with a radiating trabecular pattern. Skeletal survey or whole-body MRI defines the extent of disease, and the presence of vertebra plana, mandibular floating teeth or diabetes insipidus supports the diagnosis.

Why is it called so?

The smaller defect in one cortical table is seen inside the larger defect of the other, so the lesion appears as a hole within a hole.

Pathophysiology

Langerhans cell histiocytosis is a clonal neoplastic proliferation of myeloid dendritic cells, most carrying BRAF V600E or other MAP kinase pathway mutations, which infiltrate bone and provoke an eosinophil-rich inflammatory response with local osteoclast activation. The proliferating tissue lies in the diploic space and destroys bone outward and inward, but the rate of destruction of the inner and outer tables differs, usually with the inner table eroded more extensively. Because the two tables are separated in space and are seen superimposed on a radiograph or on a single CT section, the mismatch generates the double outline. The same asymmetric destruction gives the lesion its bevelled edge in profile.

Alternative names: Double contour lytic lesion; bevelled edge lesion

Other associated named signs: Beveled edge sign, the button sequestrum, vertebra plana and floating teeth in the same disease

References

1. Azouz EM, Saigal G, Rodriguez MM, Podda A. Langerhans’ cell histiocytosis: pathology, imaging and treatment of skeletal involvement. Pediatr Radiol. 2005;35(2):103-15. PMID: 15289942.