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ball at the time of generation. The difference in the size of the final lesion may be due to the extent of edema, which is dependent upon the size of the lesion and the type of brain tissue, more edema in grey matter than white.
In addition to imaging modalities the ice ball can be monitored with thermocouples. These probes can be placed based upon CT or MRI data prior to lesioning. The thermocouple's temperature would be proportional to the distance away from the cryoprobe and influenced by the characteristics of the surrounding tissue. Up to 0.75 cm away from the cryoprobe this axiom holds true and the temperature is proportional only to the distance from the probe. After this distance the temperature is not linear and therefore, the most accurate reading of the important lesion/brain interface would be determined by placing thermocouples at the edge of the planned ice ball. A thermocouple is a 21 gauge wire, which would be placed stereotactically and carries only minimal additional risks of hemorrhage and infection (12).
USES OF CRYOSURGERY
The application of cryosurgery has been extensive and is almost unlimited. It has proven useful in the treatment of glioblastoma multiforme, movement disorders, vascular malformations and pain control. Cooper (6) popularized the procedure of Hassler and Riechert from 1954 by performing 800 cryothalamectomies between 1961 and 1963. He treated Parkinson's disease by placing lesions in the ventrolateral region of the thalamus. He demonstrated a 90 percent relief of contralateral tremor and rigidity without manifestation of major morbidity or mortality. His technique of thalamotomy however, did not improve one of the three disabling signs of Parkinson's disease, bradykinesia, but in fact made it worse. This same procedure was carried out in patients with intentional tremor of multiple sclerosis, cerebellar movement disorders and various dystonias and revealed similar results. The lesion of choice is a comma shaped double lesion involving the posterior half of the ventrolateral nucleus, the anterior portion of the ventroposterolateral and ventroposteromedial nuclei and the outer 2/3 of the centrum medianum. The lesion is 8 to 12 mm in diameter. The procedure is first performed by cooling the ventroposterolateral nucleus in the conscious patient. The target is found nowadays by performing CT stereotactic planning. The target can be identified on the slice 5 mm superior to the calcified pineal gland, 12.5 mm anterior and 13.5 mm lateral. If correctly positioned the patient will experience paresthesia or numbness in the contralateral hand that will disappear upon rewarming. If paresthesia appear at the corner of the mouth the lesion is to medial, if contralateral hand paresthesia appear too early when the temperature is -30 C then the lesion is too posterior, if too late when the temperature is-100 C then too anterior, if there is a positive Babinski or drift of the extended hand then the lesion is too lateral (Continued on page 85)
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