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Typical trigeminal neuralgia is characterised by recurring, paroxysmal, lancinating, shock-like pain within the distribution of one or more branches of the trigeminal nerve. Light tactile stimulation may trigger such an attack. Although the contribution of central and peripheral mechanisms to the aetiopathogenesis of trigeminal neuralgia still remains unclear, the concept of vascular compression of the trigeminal root as the main causal factor in idiophatic “tic douloureux” has achieved widespread acceptance. Trigeminal neuralgia may also afflict patients with multiple sclerosis. In these cases, the demyelination of central trigeminal pathways is the accepted aetiology, and the presence of a T2 hyperintensity along the intrapontine course of trigeminal fibres is generally considered a contraindication to microvascular decompression. We recently saw a case of successful microvascular decompression in a patient without multiple sclerosis, despite an intrapontine trigeminal lesion.
This 66 year old previously healthy man presented with a 6 year history of intense, paroxysmal, electric shock-like pain in the territory of the second branch of the right trigeminal nerve. The pain was triggered by washing his face and shaving and it lasted for a few seconds. Painful attacks, initially rare, gradually increased in frequency and intensity and spread to the first trigeminal branch. When admitted to our hospital 1200 mg carbamazepine were ineffective in relieving the pain. Before admission phenitoin, baclofen, and lamotrigine had been tried without success. Neurological examination was negative and in particular there were no gross sensory deficits in his right trigeminal territory. Magnetic resonance imaging showed a T2 hyperintensity of intrapontine trigeminal fibres and nucleus (fig 1), without evidence of vascular conflicts with the trigeminal root entry zone. A controlateral, smaller, symmetric lesion was also evident. Multiple sclerosis and Lyme disease were ruled out by clinical history and appropriate investigations. The patient had no vascular risk factors. Holter monitoring, neck vessels colour echo Doppler, transthoracic echocardiographic examination, and brain MR angiography did not disclose alterations suggesting a possible ischaemic origin.
At operation, performed through a keyhole retromastoid craniectomy, the root entry zone of the nerve was found crossed by an “intratrigeminal” vein, which was electrocoagulated and divided. No other vascular contacts could be detected by careful exploration of the intracysternal tract of the nerve. The postoperative course was uneventful. Paroxysmal pain slowly faded away during subsequent weeks. Carbamazepine could be completely withdrawn after 2 months.
This case is intriguing for two reasons: firstly, because we obtained an apparently paradoxical therapeutic answer in a case where microvascular decompression should have been generally contraindicated; secondly, because of the rarity of the T2 hyperintensity of intrapontine trigeminal fibres and nucleus in patients without multiple sclerosis.
Although the intimate aetiopathogenetic mechanisms of trigeminal neuralgia still remain unknown, peripheral lesions affecting the trigeminal nerve entry zone (tortuous vessels, meningiomas, schwannomas, aneurysms, artereovenous malfunctions, lipomas, epidermoid cysts, osteomas, etc) and multiple sclerosis are certainly involved in the mechanisms causing paroxysmal pain. Demyelination of trigeminal fibres at the level of trigeminal root entry zone in case of vascular cross compression and demyelination of intrapontine trigeminal fibres in case of multiple sclerosis (personal observation of MRI in more than 80% of cases) may result in ephaptic, abnormal transmission of impulses. A neurovascular conflict with focal demyelination at the root entry zone was surgically and pathologically confirmed in a patient with multiple sclerosis and trigeminal neuralgia by Lazar and Kirkpatrick.1 The anatomopathological evidence of demyelination of intrapontine trigeminal fibres in a patient with multiple sclerosis and trigeminal neuralgia was provided by Crooks and Miles.2
Recent studies3 hypothesised that vascular compression (and possible consequent demyelination) of the trigeminal root and demyelination of intrapontine trigeminal fibres due to multiple sclerosis can coexist and perhaps cooperate in the genesis of painful attacks.
These findings led us to propose microvascular decompression to the patient rather than percutaneous lesive methods. At 2 year follow up the patient is still pain free, confirming the hypothesis that vascular compression and central demyelination can coexist and cooperate in provoking pain paroxysms. Hence, the classic distinction between the supposed “all central” mechanism for trigeminal neuralgia associated with multiple sclerosis and the “all peripheral” mechanism for the trigeminal neuralgia related to vascular compression should be overcome in favour of a unique (patients with trigeminal neuralgia and multiple sclerosis are included), mixed central-peripheral mechanism in which abnormal impulses coming from demyelinated axons (multiple sclerosis, vascular compression, and any other possible cause of demyelination along the central and the peripheral course of gasserian ganglion fibres) modulate the nuclear activity.
An alternative view of this case might be the one advocating the hypothesis that pain relief might be due to surgical damage to the trigeminal root.4 This view has been strongly challenged by the results of recent studies on sensory effects of microvascular decompression.5 In our patient, too, no evidence of sensory deficits could be found postoperatively.
Generally a minimum myelin damage, without any gross nerve hypofunction, is involved in the aetiopathogenesis of trigeminal neuralgia. In rare cases demyelination is so widespread along trigeminal fibres to be visualised by MRI and only patients with multiple sclerosis show the classic T2 hyperintensity along the intra-axial trigeminal pathways.
To the best of our knowledge this case is the third reported in which such a lesion was found in a patient without multiple sclerosis. In the two previous cases6,7 a pontine ischaemia was supposed: in one, multiple cerebral ischaemic lesions with widespread cortical atrophy were found; in the other the pontine lesion was isolated, but an MR angiogram showed a focal stenosis of the vertebral artery. A few patients with small pontine infarcts were reported with trigeminal symptoms, but without paroxysmal pain.
In our case the MRI evident T2 hyperintensity of intrapontine trigeminal pathways was considered a demyelinating or an ischaemic lesion of unknown aetiology.
This case suggests that the presence of a pontine trigeminal lesion is not an absolute contraindication for microvascular decompression in cases of drug resistant typical trigeminal neuralgia.
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