Leptomeningeal Cyst

Excerpt

The leptomeningeal cyst (LMC) is an encapsulated mass occurring commonly in infants associated with skull fractures. The first description of the entity was made by Howship in 1816, as he described it as parietal bone absorption. The term ‘Growing skull fracture’ was coined by Pia and Tonnis, whereas ‘Leptomeningeal cyst’ was coined by Dyke. Synonyms of the LMC described in the literature include:

1. Traumatic ventricular cysts.

2. Cranio-cerebral erosions.

3. Cranial malacia.

4. Diploic cyst.

5. Growing skull fracture.

6. Cerebrocranial erosion.

However, the term leptomeningeal cyst itself appears as a misnomer as the pathology is an encephalocele consisting of brain parenchyma with CSF and leptomeningeal layers and not an actual cyst. In leptomeningeal cysts, there is a diastatic fracture of the skull involving both the outer and inner table and a laceration of the underlying dura meter. Diastatic fractures occur when there is a separation of the cranial sutures. It is an uncommon complication occurring in young infants and children under three years of age with skull fractures. Post-traumatic intradiploic leptomeningeal cysts are a variant in which the fracture involves the inner table with the dural tear. This leads to CSF accumulation in a sac lined by the arachnoid membrane and located within the diploic space. It is not synonymous with an arachnoid cyst. Since most cases occur in infants, it is postulated that the formation of the mass requires the presence of a rapidly growing brain. The inciting insult seems to be a dural tear accompanying a linear skull fracture creating an orifice for the intracranial contents to herniate. As the intracranial contents begin to escape, they create pressure on the surrounding skull, causing erosion of the bone margins and subsequently expanding the skull fracture. This tissue interposition inhibits osteoblast migration, hence impeding fracture healing. In addition, continued CSF pulsation enlarges the cyst causing the lesions to escape to the subgaleal space. The severity of the trauma and degree of diastasis (>4 mm) also appear to play a role.