Abstract

Drs Pirakitikulr and Tse describe their experience managing a patient with progressive enophthalmos following cerebrospinal fluid (CSF) shunting. The case is interesting and nicely illustrates several valuable features of this uncommon syndrome. The syndrome has gone by a number of names. To our knowledge, the first use of the term “silent brain syndrome” was by Thomas Hwang during a North American Neuoro-ophthlmogy Walsh Society meeting in 2008. 1 We later referred to this as intracranial-hypotension-related skull remodeling (IHSR) with enophthalmos, reflective of the dominant theory that the mechanism lies in alteration of the pressure gradient between the orbit and intracranial spaces. In this case, imaging findings suggestive of intracranial hypotension were lacking, and intracranial pressure (ICP) was measured to be low normal, 110 mmH2O. Based upon this, Drs Pirakitikulr and Tse entertain alternate mechanisms. Our understanding of IHSR has evolved since the first report in 1996, where Meyer et al. described three patients with congenital hydrocephalus who developed bilateral enophthalmos following ventriculoperitoneal shunting (VPS). In our manuscripts published in 2011, we reported the results of an orbit volumetric analysis on the largest cohort of patients to date. We demonstrated that skull remodeling occurred, with marked upward bowing of the orbital roof into the intracranial space. These bony changes are seen in the later images of the patient managed by Drs. Pirakitikulr and Tse. The altered pressure gradient between the orbital and intracranial spaces theoretically causes not only a net resorption of bone because of decreased stress but an effective intracranially directed force across the roof causing orbital expansion. Drs Pirakitikulr and Tse state that their patient had orbit fat atrophy. This appears to be presumed, based upon the patients’ overall gaunt appearance and low body mass index (BMI). Actual measures of orbit fat volume are not provided. As part of our volumetric analysis, the orbit fat volume was measured to be increased in all study patients, compared to an age matched control group. This was due to soft tissue collapsing into the expanded orbit. It is possible that in this particular patient, orbit fat volume had decreased along with their overall depletion of body fat. If measures confirmed loss of orbit fat, this is more likely related to the overall fat loss and is not generalizable to the majority of patients with IHSR-related enophthalmos. The clinical consequences of IHSR are significant. With advanced disease, the enophthalmos becomes so severe that the eyes may actually lose contact with the eyelids with resultant severe ocular surface disease. Many patients develop significant ocular motility problems, in particular a marked reduction in supraduction. Abnormal vertical ductions are explained by the mechanical effect of the superior bowing of the orbital roof and displacement of the superior recti muscles. A number of patients also develop optic neuropathies, likely related to malformation of the bony optic canals. Most of these findings are due to bony changes, and are independent of fat volume. Although IHSR appears to be the major contributor to enophthalmos, we agree that other factors contribute. In some cases, we have observed a small but rapid improvement in enophthalmos within days of correcting the intracranial hypotension. Forward movement of the globe after shunt revision may result from increased hydraulic force generated by the increased CSF pressure in the subarachnoid space of the optic nerve just as raised pressures can flatten the globe in cases of intracranial hypertension. Second, a rise in ICP causes an elevation in venous pressure within the cavernous sinus and thus the orbital venous pressure, which would cause increased orbital soft tissue blood volume. Finally, the cuff of CSF around the optic nerve would also expand with normalization of the ICP. Other bony structures are often, if not usually, involved. Enlargement of the sphenoid sinus is easily appreciated in most cases of IHSR. So long ago as 1992, Schayck and Niedeggen described two patients who developed enlargement of the sphenoid, ethmoid and frontal sinuses after cerebrospinal fluid (CSF) shunting. The sinuses do not contain fat, precluding fat atrophy as a mechanism, further supporting the dominant theory of the altered pressure gradient causing bone remodeling. We respectfully disagree with the authors’ assertion that the ICP was established as normal in their patient. Their determination is based primarily on the findings ORBIT 2021, VOL. 40, NO. 5, 444–446 https://doi.org/10.1080/01676830.2021.1939729