Recurrent painful ophthalmoplegic neuropathy (RPON), previously known as ophthalmoplegic migraine (OM), is a rare neurological disorder that is characterized by repeated headache attacks and reversible ipsilateral paresis of one or more ocular cranial nerves (CN). Oculomotor nerve (CNIII) is by far the most common cranial nerve involves in RPON, while abducens nerve (CNVI) and trochlear nerve (CNIV) involvements are also reported. Globally, RPON was estimated to have an annual incidence rate of 0.7 per million as of 1990, no further epidemiological studies have been conducted. It occurs more often in children and females. Although the etiology of RPON is unknown owing to the rarity of this disease, various potential theories including migrainous and neuropathic mechanisms have been developed. Diagnosis is based on physical examination, magnetic resonance imaging (MRI), and ruling out other possible causes. Despite the fact that there are currently no evidence-based treatment guidelines for RPON, the most commonly used medications are corticosteroids that have been proven effective in alleviating the symptoms. As for people who are unresponsive to steroids, other treatments, such as anti-migraine drugs, botulinum toxin injection and strabismus surgery, may be utilized. For prevention, several medications including calcium channel blockers may be used. However, none have been proved consistently effective. RPON has a good overall prognosis as a result of the self-limiting nature of the condition, but permanent neurological damage may accumulate over repeated episodes.
Signs and symptoms
The typical symptoms of RPON are recurrent headaches and ipsilateral paralysis of the extraocular muscles (ophthalmoplegia) that are responsible for controlling eye movements. People with RPON experience different severity of pain, duration of symptoms, and frequency of attacks, which are also dependent on the treatment they received. The attacks of RPON usually occur on the same side of the head in subsequent episodes.
Headache The most common type of headache is migraine-like, usually accompanied by symptoms such as nausea, vomiting, photophobia, or phonophobia. Less commonly, RPON can occur without migrainous headache nor accompanied by the classical migrainous symptoms. The frequency of headaches can range from several times per month to once in several years, while the duration varies from several days to a week, which lasts longer than that of a typical migraine.
Ophthalmoplegia Ophthalmoplegia usually occurs either concurrently or within 1 week after the onset of headache, and less commonly up to 2 weeks. The ocular manifestation of RPON depend on which ocular cranial nerve is being affected. CNIII is involved in the vast majority of cases, but the involvement of CNVI and CNIV is also reported in studies. Typical ocular signs involving CNIII include outwards and downwards drifting of eyes (strabismus), difficulty in controlling inward and upward eye movements, drooping of eyelid (ptosis) and double vision. Pupil dilation (mydriasis) and reduction in pupillary light reflexes can also be observed as pupillomotor fibers are also affected by CNIII paralysis. Rare cases involve abducens nerve paralysis affects lateral eye movement while trochlear nerve paralysis affects vertical eye movement. Similar to headache, ophthalmoplegia usually resolve gradually and completely over time, but recovery may be incomplete after several episodes.
Pathophysiology The etiology of RPON is still unknown and remains controversial - some scientists believe that RPON is a subtype of migraine, while others believe that it is a neuropathic disorder. As such, different potential etiologic mechanisms have been proposed based on case reports, including migrainous and neuropathic mechanisms. Yet, the renaming of RPON from OM indicates that the neuropathic cause may play a more important role than the migrainous cause in the pathophysiology of RPON.
Migrainous mechanism RPON was previously thought to be having a migrainous origin since it shares some common characteristics with migraine, and their only differences are the intensity and duration of headaches. Therefore, it was postulated that migraine caused ophthalmoplegia in RPON. However, the major argument against a migrainous origin is the long latency period between the onset of headache and ophthalmoplegia (up to 14 days), which is not consistent with a typical migraine. Nevertheless, several migrainous mechanisms have been proposed.
Nerve compression Compression mechanism is the first theory postulated regarding the pathophysiology of RPON. It is proposed that headache is caused by direct pressure on CNIII (compression neuropathy) within the cavernous sinus due to edema of the walls of the internal carotid artery (ICA) or posterior cerebral artery (PCA) during migraine episodes.
Ischemia Cavernous section of the ICA and a perforating vessel from the proximal PCA are responsible for supplying CNIII, CNIV, and CNVI. During migraine attacks, vasoconstriction of the ICA and PCA may lead to a reversible and ischemic breakdown of the blood-brain barrier. The blood-brain barrier breakdown results in vasogenic edema, which promotes the development of RPON.
Neuropeptides-induced Alternatively, it is proposed that migraine-related release of neuropeptides from trigeminal nerve fibers terminating on the circle of Willis might be the cause. Some of the neuropeptides released are potentially toxic, and may induce blood-brain barrier breakdown that accounts for ophthalmoplegia.
Neuropathic mechanism Additional models have been proposed to explain the cause of RPON. The thickened and enhanced symptomatic nerve, as shown in recent MRI findings, might be indicative of the existence of structural nerve damage in RPON. Therefore, neuropathy is suggested as the primary cause of RPON, either induced by recurrent viral infections or immune-mediated inflammation. However, cerebrospinal fluid (CSF) analyses are normal in the vast majority of RPON cases, which is inconsistent with the findings of autoimmune or inflammatory-mediated pathomechanism. Nevertheless, several neuropathic mechanisms have been proposed.
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