In many patients there are no symptoms or signs that betray the pontine lesion, presumably because it is so small, extending only 2 to 3 mm on either side of the median raphe and involving only a small portion of the corticopontine or pontocerebellar fibers. In others, its presence is obscured by coma from a metabolic or other associated disease. Prior to the inception of MRI only a minority of cases, exemplified by the first patient observed by Adams et al, were recognized during life. In this patient, a serious alcoholic with delirium tremens and pneumonia, there evolved, over a period of several days, a flaccid paralysis of all 4 limbs and an inability to chew, swallow, or speak (thus simulating occlusion of the basilar artery). Pupillary reflexes, movements of the eyes and lids, corneal reflexes, and facial sensation were spared. In some instances, however, conjugate eye movements are limited, and there may be nystagmus. With survival for several days, the tendon reflexes become more active, followed by spasticity and extensor posturing of the limbs on painful stimulation. Some patients are left in a state of mutism and paralysis with relative intactness of sensation and comprehension (pseudocoma, or locked-in syndrome).
The capacity of CT and especially MRI to visualize the pontine lesion has greatly increased the frequency of premortem diagnoses. The MRI discloses a characteristic lesion of the pons in typical cases (Fig. 39-6), although this change may become evident only several days after the onset of symptoms. Brainstem auditory evoked responses also disclose the lesions that encroach upon the pontine tegmentum.
Variants of this syndrome are being encountered with increasing frequency. Two of our elderly patients, with confusion and stupor but without signs of corticospinal or pseudobulbar palsy, recovered; however, they were left with a severe dysarthria and cerebellar ataxia lasting many months. After 6 months, these patients’ nervous system function was essentially restored to normal. In reference to the pathogenesis of this lesion, originally both patients had serum Na levels of 99 mEq/L, but information about the rate of correction of serum Na was not available. Another of our patients developed a typical locked-in syndrome after the rapid correction of a serum sodium of 104 mEq/L. He had large symmetrical lesions of the frontal cortex and underlying white matter but no pontine lesion.
Brainstem infarction caused by basilar artery occlusion may be simulated by pontine myelinolysis. Sudden onset or step-like progression of the clinical state, asymmetry of long tract signs, and more extensive involvement of tegmental structures of the pons as well as the midbrain and thalamus are the distinguishing characteristics of vertebrobasilar thrombosis or embolism. On MRI studies, an evolving infarction shows signal changes on diffusion-weighted imaging, while the primary finding in osmotic demyelination is abnormal hyperintensity on T2-weighted images without associated restricted diffusion. Massive pontine demyelination in acute or chronic relapsing multiple sclerosis rarely produces a pure pontine syndrome. The clinical features and context provide the clues to correct diagnosis.