but a similar phenomenon has been observed in a quadriplegic patient with a posttraumatic syringomyelic cyst.440 The pathophysiology of this phenomenon is unclear, but it may be due to either episodic oculosympathetic spasm producing alternating Claude Bernard–Horner’s syndrome or episodic oculosympathetic interruption producing Horner’s syndrome. Some other pupillary disorders have been intermittent rather than rhythmic and include intermittent mydriasis, cyclic sympathetic spasm with concentric dilatation lasting 40–60 s, and “tadpole pupils,” in which the sympathetic spasm is sectoral.746

Conditions with cycle duration of a few minutes sometimes become apparent when the clinician notices a reversal of a previously noted finding – for example, observation of conjugately turned eyes to the right in a patient whose eyes were previously noted to be conjugately turned to the left brings forth the diagnosis of PAGD. Conditions with a cycle duration of 24 h or longer cannot be definitively diagnosed from data collected during a single office visit. For instance, a patient with cyclic esotropia may be initially evaluated on a “crossed” day and be labeled as congenital or acquired esotrope. If the subsequent visit occurs on a “crossed” day, it may only consolidate the earlier false diagnosis. If it occurs on a “straight” day, the clinician may suspect spontaneous resolution or an accommodative element with variable angle and either question the observation of esotropia in earlier visits or correctly suspect cyclic esotropia. Conversely, initial evaluation of a patient with cyclic esotropia on a “straight” day may lead to missing the diagnosis, dubbing the condition pseudostrabismus.

Cyclic phenomena in children are not limited to the neuroophthalmologic disorders discussed here, but involve other organ systems as well. Other biologic phenomena, such as sweating, salivation, and pulse rate, also have intrinsic periodicity, as do many manifestations of psychiatric dysfunction. Periodicity appears to be the norm in many biologic phenomena, and a normal person displays a complex array of biorhythms involving the various body systems.640 Numerous periodic or rhythmic disorders have been described – for example, periodic recurrence of fever, swelling of joints, fluctuations of circulating blood cells (periodic hematopoiesis), and edema. Accumulating evidence points to the existence of a biologic clock mechanism that keeps time with extraordinary accuracy and is independent of internal and external stimuli.

discharge in a peripheral nerve. Neuromyotonia describes neurotonia accompanied by fibrillations, fasciculations, myokymia, or sustained contraction of a muscle group. Ocular neuromyotonia describes sustained contraction of one or more extraocular muscles due to involuntary firing of the supplying ocular motor nerve.

Ocular neuromyotonia is a relatively rare ocular motility disorder that manifests with brief paroxysmal monocular deviations with associated diplopia.701 Episodes generally last 10–60 s (range, 5 s to 3 min) and may recur 20 or more times per day. Some episodes occur spontaneously, while others are triggered by gaze in the direction of the involved muscle. Between attacks, affected patients usually show normal ocular motility, although some may show subtle evidence of aberrant innervation manifesting as minimal lid retraction in downgaze. The paroxysms result from tonic involuntary contraction of extraocular muscles innervated by the third (all muscles supplied by the nerve or any combination thereof), fourth, or sixth cranial nerves.57,486 Spontaneous discharges from axons with unstable cell membranes are presumed to underlie this condition. In some cases, ocular neuromyotonia may coexist with primary aberrant regeneration.160

Histopathologic studies of peripheral nerves in nonocular cases of neuromyotonia have shown segmental demyelination as well as axonal degeneration, sprouting, and remyelination.794 Most patients have a history of brain irradiation, typically for the treatment of tumors of the skull base, such as pituitary tumors or craniopharyngiomas.486,798 Idiopathic cases with no specific cause have been reported.798 The interval between radiotherapy and onset of neuromyotonia may be months to years. Two cases have been reported in teenagers following radiation therapy for parasellar lesions.

A favorable response to membrane-stabilizing medications, such as carbamazepine is reported in many patients, supporting pathoetiology consisting of spontaneous or impulse-induced repetitive discharge of hyperexcitable trigger zones in ocular motor nerves. In some instances, the neuromyotonia has not recurred on cessation of the medication. Some cases may remit spontaneously.

Several conditions not associated with radiation therapy may come to be classified as ocular neuromyotonia. Adie’s pupil and superior oblique myokymia are two common forms of ocular neuromyotonia seen in neuro-ophthalmologic practice.

Myotonia denotes delayed muscle relaxation after sustained contraction as a result of muscle membrane dysfunction. In contrast, neurotonia or pseudomyotonia represents delayed muscle relaxation as a result of impulse-induced repetitive

Cerebral hemispheric abnormalities are often associated with ocular motor abnormalities that may be subtle or profound depending upon the size, location, and other characteristics of the lesion. These are reviewed in detail elsewhere.