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Overshunting-associated myelopathy, also known as Miyazaki syndrome, is a very rare form of compressive myelopathy that is a complication of intracranial hypotension due to excessive cerebrospinal fluid (CSF) diversion.
Patients present with progressive cervical myelopathy 1-5:
weakness in the arms and/or legs, classically with upper motor neuron signs in the lower limbs and lower motor neuron signs in the upper limbs
sensory loss, classically with a cervical sensory level
neck pain, which may radiate to the head or arms
bowel and bladder dysfunction may be present
The temporal relationship between time of CSF diversion and development of symptoms is highly variable, with time frames in the literature ranging from months to decades 2, with a mean of approximately 12 years 1. Interestingly, other general clinical features of intracranial hypotension, such as a postural headache, are not usually present in this syndrome 1.
Overshunting-associated myelopathy is a complication of excessive CSF diversion, most commonly in the setting of a ventriculoperitoneal shunt 1,2. The prominent etiological theory suggests that with excessive CSF diversion, intracranial hypotension ensues, and there is venous engorgement as per the Monro-Kellie doctrine 1-3.
This results in engorgement and congestion of the valveless epidural venous plexus, presumably directly from the intracranial venous system 1-3. The dilated plexus then has compressive effects on the cervical cord and/or nerve roots, resulting in the clinical syndrome 1-3.
MRI/MRV is the imaging modality of choice, showing:
dilated/engorged epidural veins 1-5
cervical spinal canal compression/stenosis with increased intramedullary T2 signal 1-4
there may also be compression of cervical nerve roots 1
other general features of intracranial hypotension (see main article) 1,2
dilated ventricles, instead of slit ventricles, may instead be present 1
Treatment and prognosis
Management is with shunt revision or valve adjustment 1. Many case reports report excellent clinical outcomes with prompt management 2.
History and etymology
The condition is named after T Miyazaki, Japanese neurologist, who first described the condition with his colleagues in 1998 5.