123I‐β‐CIT and 123I‐IBZM‐SPECT scanning in levodopa‐naive Parkinson's disease

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Abstract
Striatal dopamine transporter function and dopamine D2 receptor status were evaluated in 15 patients with early untreated Parkinson's disease using single photon emission tomography (SPECT) with 123I‐Iodo‐2β‐carboxymethoxy‐3β‐(4‐idiophenyl)tropane (β‐CIT) and 123I‐Iodobenzamide (IBZM) as pre‐ and postsynaptic ligands. Symptoms were unilateral in five patients and bilateral but asymmetric in 10 patients. Patients with bilateral symptoms had significantly lower 18‐hour striatal/cerebellar β‐CIT binding ratios (3.59 ± 0.79) than hemiparkinsonian patients (5.76 ± 1.48, p < 0.05) reflecting more advanced disease in this subgroup. Patients with bilateral parkinsonism were also found to have a significant side‐to‐side difference in striatal β‐CIT binding with more marked reduction contralateral to the presenting limb (18‐hour striatal/cerebellar ratio: 4.13 ± 0.78 [ipsilateral] versus 3.59 ± 0.79 [contralateral], p < 0.05). Dopamine D2 receptor binding as measured by IBZM was significantly elevated contralateral to the affected side in hemiparkinsonian patients (striatal/cerebellar ratio: 2.42 ± 0.90 [contralateral] versus 2.19 ± 0.80 [ipsilateral], p < 0.05). This asymmetric upregulation was absent in the patients with bilateral parkinsonism (striatal/cerebellar ratio: 1.85 ± 0.43 [contralateral to more severely affected side] versus 1.83 ± 0.34 [ipsilateral], p > 0.05). Our data suggest that postsynaptic dopamine receptor upregulation contralateral to the presenting side occurs in untreated unilateral PD and disappears in untreated bilateral (asymmetric) PD despite a greater loss of dopamine transporter function. Combined β‐CIT and IBZM SPECT studies may be helpful to monitor the progression of nigrostriatal dysfunction in early PD.

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