High-Frequency Oscillatory Ventilation: “Please, Keep Your Eyes on Me,” Said the Patient

To the Editor.—We read the case report of subdural hemorrhage in an infant with glutaric aciduria type 1 (GA1) by Hartley et al1 with intense interest and wish to make a few pertinent observations about the biomechanics of the infant head.The gross pathology of GA1 is pleomorphic: infant patients are commonly macrocephalic. In some instances the macrocephaly is attributable to megalencephaly.2 In other instances prominence of the subarachnoid spaces (especially of the sylvian fissures) is a factor,3 with or without an additional contribution from chronic or acute subdural hematomas.4 Subcortical white matter changes, lesions of the basal ganglia, and ventricular dilatation are also described. Very likely there is a comprehensible evolutionary relationship among these states related to age, treatment status, and the occurrence or avoidance of the acute encephalopathic crises that typify this condition.Subdural hemorrhage in GA1 is a predictable, mechanical complication of the prominence of the subarachnoid spaces. The prototypical condition, characterized by prominence of the subarachnoid spaces, is benign external hydrocephalus (BEH), and the predilection of patients with BEH to “spontaneous” subdural hemorrhage or to hemorrhage after minor trauma has been reported repeatedly.5–7 Recently a biomechanical model has been developed to illuminate this phenomenon.8 The exaggerated depth of the subarachnoid space in BEH is associated with elongation of the bridging veins that drain the cortical surface into the dural venous sinuses. (These veins can be seen on T2-weighted magnetic resonance images, and their identification is a key to distinguishing between cerebrospinal fluid in the subarachnoid space and chronic hematoma or hygroma in the subdural space.9) Because the brain is not packed snugly into the cranial cavity, the brain lags behind the skull when the skull is accelerated suddenly by an impact. The elongated bridging veins are stretched and can rupture into the subdural space. A similar process presumably accounts for the susceptibility to subdural hemorrhage of patients with intracranial arachnoid cysts.10Of particular interest in the case reported by Hartley et al is the observation of a single hemorrhage in the retina of one eye. Because retinal hemorrhages are seen so seldom in other settings, they have been considered virtually pathognomonic of abuse. Recently one of us has suggested that BEH may predispose patients to retinal hemorrhage after minor trauma.11 We wish to raise the possibility that the altered craniocephalic biomechanics related to prominence of the subarachnoid spaces may create a susceptibility to retinal hemorrhage in GA1 as well.We thank the authors for drawing attention to the challenge of the differential diagnosis of acute subdural hemorrhage in infancy.Addendum. Dr Verity does not think that we “should reach any definite conclusion” about retinal hemorrhages in GA1. We wrote “to raise the possibility” rather than to “reach any definite conclusion.”