The dorsal cyst in holoprosencephaly and the role of the thalamus in its formation

The dorsal cyst is poorly understood, although it is commonly encountered i n holoprosencephaly. We endeavor to establish the role of diencephalic malf ormations in the formation of the dorsal cyst and speculate on the developm ental factors responsible. We reviewed the imaging, of 70 patients with hol oprosencephaly (MRI of 50 and high-quality CT of 20). The presence or absen ce of a dorsal cyst, thalamic noncleavage and abnormal thalamic orientation were assessed for statistical association, using Fisher's Exact Test and l ogistical regression. The presence of a dorsal cyst correlated strongly wit h the presence of noncleavage of the thalamus (P = 0.0007) and with its deg ree (P < 0.00005). There was a trend toward an association between abnormal ities in the orientation of the thalamus and the dorsal cyst, but this was not statistically significant (P = 0.07). We speculate that the unseparated thalamus physically blocks egress of cerebrospinal fluid from the third ve ntricle, resulting in expansion of the posterodorsal portion of the ventric le to form the cyst.