Biomedical subjects
Publications and source records attributed to W D Robertson.
A patient with a history of previous head injury presented with an isodense subdural hematoma with extension into a preexisting middle fossa arachnoid cyst. The latter, suspected on the basis of findings pointing to chronic expansion of the middle fossa, was confirmed in a repeat CT study carried out after evacuation of the hematoma.
Review of phosphate behavior in four mature septic system plumes on similar textured sand has revealed a strong correlation between carbonate mineral content and phosphate concentrations. A plume on calcareous sand (Cambridge site, 27 wt % CaCO3 equiv.) has proximal zone PO4 concentrations (4.8 mg/L P average) that are about 75% of the septic tank effluent value, whereas three plumes on noncalcareous sand (Muskoka, L. Joseph, and Nobel sites, <1 wt % CaCO3 equiv.) have proximal zone phosphate concentrations (<0.1 mg/L P) that are consistently less than 2% of the effluent values. Phosphate attenuation at the noncalcareous sites appears to be an indirect result of the development of acidic conditions (site average pH 3.5 to 5.9) and elevated Al concentrations (up to 24 mg/L), which subsequently causes the precipitation of Al-P minerals such as variscite (AlPO4 x 2H2O). This is supported by scanning electron microscope analyses, which show the widespread occurrence of (Al+P)--rich secondary mineral coatings on sand grains below the infiltration beds. All of these septic systems are more than 10 years old, indicating that these attenuation reactions have substantial longevity. A field lysimeter experiment demonstrated that this reaction sequence can be readily incorporated into engineered waste water treatment systems. We feel this important P removal mechanism has not been adequately recognized, particularly for its potential significance in reducing P loading from septic systems in lakeshore environments.
A nitrate-reactive porous media layer comprising wood particles with very high hydraulic conductivity (K approximately 1 cm/s) was used to successfully treat nitrate in a shallow sand-and-gravel aquifer in southern Ontario. Nitrate concentrations of 1.3 to 14 mg/L as N in the aquifer were attenuated to <0.5 mg/L as N in the reactive layer. Borehole dilution testing indicated that ground water velocities in the reactive layer, although variable, averaged five times higher than in the surrounding aquifer, suggesting that the layer was capturing ground water flow from deeper in the aquifer. The use of high-K reactive media opens up the possibility of installing permeable reactive barriers as horizontal layers in the shallow water table zone that do not necessarily have to penetrate the full depth of a contaminant plume to be effective. Model simulations show that the depth of capture of a high-K layer increases as the layer width in the direction of flow increases. Shallower emplacement could decrease barrier costs at some sites.
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Globe tenting is a change in the posterior globe configuration that results in a tented or conical appearance and is objectively defined as a posterior globe angle of less than 130 degrees. Globe tenting is caused by an acute/subacute intraorbital mass effect producing significant proptosis with tethering of the globe by the stretched optic nerve. In this series of 10 patients, globe tenting was produced by subperiosteal abscess (five cases), hemorrhage into a lymphangioma (two cases), a varix (one case), traumatic carotid hemorrhage into a lymphangioma (two cases), a varix (one case), traumatic carotid cavernous fistula (one case), and multiple epithelial implantation cysts (one case). Progressive narrowing of the posterior globe angle correlated with an increase in proptosis and in optic nerve length, as well as with more severe visual impairment. Tenting with a posterior globe angle of 120-130 degrees correlated with mild visual symptoms and a good recovery. A posterior globe angle of less than 120 degrees with acute proptosis constitutes a surgical emergency; a delay in surgical decompression in these patients may prevent complete recovery of visual function. CT is useful in providing good characterization of globe tenting and, thus, in helping to determine the clinical significance of this abnormality.
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Seven patients with orbital plexiform neurofibromas (PNFs) were evaluated by computed tomography (CT). These lesions are pathognomonic of von Recklinghausen disease and manifest a spectrum of radiologic abnormalities ranging from thickening of the eyelid and periorbital soft tissues to enlargement of the bony orbit and extensive infiltration of the orbital soft tissues. In analyzing the CT features of orbital PNF, three findings that have not been previously reported were identified. Increased density of the intraconal fat present in four of seven patients was believed to represent PNF involvement of small intraconal nerves. Three patients demonstrated enhancing, irregularly nodular thickening of the optic nerve sheath complex thought to be caused by PNF of the posterior ciliary nerves surrounding the optic nerve. Three patients with buphthalmos and congenital glaucoma demonstrated thickening and enhancement of the uveal/scleral layer believed to represent PNF of these structures. Involvement of the choroid and sclera, posterior ciliary nerves, and small intraconal nerves has been recognized pathologically but has not been previously identified by CT.
Seventy-one patients with acute subdural hematomas were examined by CT within 72 hr of a documented head injury. Lesions often did not have the classical appearance of a homogeneous, high-density extracerebral collection of blood in a crescentic configuration. Specifically, 28 patients (39%) had mixed-density subdural hematomas (MDSDH) with various degrees of low-density blood within the subdural space. In 10 of these 28 patients, the hematoma had a relatively localized mass effect with a convex inner margin, occasionally mimicking the appearance of an epidural hematoma. The MDSDH group differed from the typical homogeneous high-density subdural hematomas in that they were larger (average maximal thickness was 18.1 mm versus 8.0 mm), had more midline shift, and had a higher mortality rate (50% versus 26%). Four patients with MDSDH demonstrated an unusual pattern of ventricular compression with trapping of cerebrospinal fluid in the body of the ipsilateral ventricle and compression of the body of the contralateral ventricle. This pattern has to our knowledge not been previously described. Possible causes of the low-density regions within the hematomas include unclotted blood in an early stage of hematoma development, serum extruded during the early phase of clot retraction, or cerebrospinal fluid within the subdural space due to an arachnoid tear.
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