Corticospinal Tract and Pontocerebellar Fiber of Central Pontine Myelinolysis
The patient in our case was a 53-year-old man who was diagnosed with liver cirrhosis due to hepatitis B. After liver transplantation on July 15, 2007, central pontine myelinolysis was diagnosed on the basis of clinical presentation and magnetic resonance imaging. He visited our clinic to evaluate a gait disturbance in April, 2010. When central pontine myelinolysis occurred in 2005, he went into a coma and was quadriplegia. After recovering from these symptoms, he developed a gait disturbance and displayed a tremor in both hands.\nUpon physical examination in April 2010, the upper and lower extremities strength was determined to be 4/5. The results of the nine-hole peg test to assess hand coordination was 108/58 sec (27/25 sec, normative data mean) for the left-/right- sided hand. He had a tendency to fall with or without his eye open in the Rhomberg test. In the cerebellar function tests, such as finger to finger, finger to nose and heel to shin test, he showed dysmetria and went past the target or walked at a non-regular speed, bilaterally. His speech was slurred and dysarthric. His gait was remarkably irregular in terms of his stance and he displayed poor balance, which was indicative of cerebellar ataxia.\nA transcranial magnetic stimulation was performed using a Magstim 200 stimulator (Magstm, Wales, UK) equipped with a 65 mm figure-eight coil, Medtronic Keypoint® electromyography unit (Medtronic Inc., Skovlunde, Denmark), at a digitization rate of 10 kHz, high-pass filter at 100 Hz, and low-pass filter at 5 kHz. The motor evoked potentials of the first dorsal interosseous muscles of both hands and the tibialis anterior muscles of both legs were recorded simultaneously. TMS mapping over the hand area of each cortex was obtained along a 2 by 2-cm grid at rest. The coil handle pointed backwards and 45° away from the midline.\nFor each detected MEP, the following parameters were determined; the optimal point defined as the scalp position at which a reproducible muscle response was elicited with the lowest stimulation intensity and the motor threshold at rest (defined as the minimum stimulation intensity that produced at least five MEPs exceeding 50 µV in 10 trials). The stimulation intensity was set at the 110% of the motor threshold. Each site was stimulated 4 times, from which the shortest latency and the average peak to peak amplitudes were recorded.\nA 60 mm circular stimulating coil was centered over the C7 spinous process level for cervical motor root stimulation and the L5-spinous process level for lumbar motor root stimulation, which was used to obtain the shortest latency and the average peak to peak amplitudes of MEPs. The central motor conduction time is defined as the latency difference between the motor evoked potentials from the first dorsal interosseous and tibialis anterior muscles induced by stimulation of the motor cortex and those evoked by spinal (motor root) stimulation ().\nThe onset latency and CMCT on the first dorsal interosseous muscles of both hands after transcranial magnetic stimulation of the cerebral cortex were in the normal range. However, the onset latency on tibialis anterior after transcranial magnetic stimulation of the cerebral cortex was 40.8/42.1 ms and the CMCT on the tibialis anterior was 27.3/29 ms for the left-/right- sided peaks. The TMS showed delayed latency in the patient when compared to normal subjects ().\nAt the time of admission to our hospital when the patient was in a coma and was quadriplegia in August 2005, T2-weighted MR images of the brain showed a bilateral symmetrical hyperintense lesion with a spared corticospinal tract at the midbrain and pons. These findings suggested that the central pontine myelinolysis occurred in the midbrain and pons. After 5 years, T2-weighted MR images of the brain showed an oval cavitary lesion at the center of the basis pontis and bilateral symmetrical linear lesions that indicated pontocerebellar fibers ().\nDiffusion tensor images were obtained with a head coil on a 3.0T Siemens Verio scanner (Siemens, Erlangen, Germany) using a single-shot echo planar imaging sequence with two diffusion sensitizing gradients. To reduce the scan time, a Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) technique, which is a component of the parallel imaging technique, was used. In addition, this technique can produce a higher image quality because it reduces image distortion caused by the echo-planar imaging sequence. To correct for potential image distortion, an automated image registration program was employed. The imaging parameters used were: echo time=105 ms, repetition time=7,000 ms, field of view=200 mm2, matrix size=128×128, number of excitation=2, and b-value=1,000 s/mm2. We acquired 46 contiguous slices parallel to the anterior commissure-posterior commissure line at a slice thickness of 3.0 mm with no gap in 30 different diffusion directions. To evaluate the pontocerebellar fibers, the fractional anisotropy and apparent diffusion coefficient of the bilateral middle cerebellar peduncle (MCP) were measured, and regions of interest were identified on the basis of a previously published fiber tract-based atlas of human white matter anatomy. The section level for MCP delineation was chosen at one section posterior to the corticospinal tract in the coronal section, and the MCP was tracked through 2 regions of interest at the left and right of the lateral pontine tegmentum, which was clearly identified on the color map.\nThe corticospinal tract was preserved based on the blue color in the DTI color maps at a low pons level (). However, the pontocerebellar fiber was found to be abnormal in the DTI based on the decreased fractional anisotropy and increased apparent diffusion coefficient values in the middle cerebellar peduncle when compared to normal subjects, which was indicative of reflected degenerative axonal and/or myelin loss of the pontocerebellar fiber (, ).