Alternating hemiplegia

Lore & Background

Alternating hemiplegia of childhood is a genetic disorder caused by mutations in the ATP1A3 gene, which encodes a sodium-potassium pump crucial for neuronal function. Episodes of paralysis may be brief or last several days, often resolving after sleep. Unlike brainstem stroke syndromes (Weber's, Foville's, medial medullary), AHC does not involve fixed ipsilateral cranial nerve palsies or contralateral limb weakness. Instead, it presents with transient, alternating hemiplegia, often accompanied by dystonic posturing, ataxia, or nystagmus. The condition is not due to a structural brainstem lesion but rather a functional channelopathy.

Reader's Guide

The significance of alternating hemiplegia lies in its distinct pattern of crossed neurological deficits, which aids in localizing brainstem lesions. The condition's diagnosis relies on criteria including onset before 18 months, frequent hemiplegic episodes, paroxysmal disorders like dystonia or nystagmus, symptom resolution after sleep, and developmental delays. Treatment focuses on avoiding triggers (e.g., cold, stress, fatigue) and using flunarizine to reduce attack severity and duration, though it does not stop attacks. Sleep is also used as a management technique. The discovery of ATP1A3 gene mutations in alternating hemiplegia of childhood has advanced understanding of its genetic basis, though similar syndromes may follow brainstem infarction.

Did You Know?

• Episodes of paralysis may resolve after sleep and often return upon waking during an attack.
• Flunarizine, a calcium channel blocker, is the most common drug prescribed and can shorten attack duration and reduce severity.
• Alternating hemiplegia of childhood is caused by mutations in the ATP1A3 gene, not by brainstem strokes.
• A port-wine stain on the face is associated with Sturge-Weber syndrome, not Weber's syndrome or alternating hemiplegia.

The Brainstem Crossroads: How One Lesion Creates Two Sides of Paralysis

Alternating hemiplegia derives its name from a striking neurological paradox: a single unilateral lesion in the brainstem simultaneously produces paralysis on opposite sides of the body. The mechanism hinges on the anatomy of motor pathways. Upper motor neurons travel through the pyramidal tract within the brainstem before crossing to the opposite side, so damage at this level leaves the majority of limb and trunk muscles on the contralateral side with spastic paralysis. Meanwhile, cranial nerve nuclei sit in the same brainstem region, and because most cranial nerves do not cross before reaching their target muscles (the trochlear nerve being the notable exception), the ipsilateral face and head structures are affected. Cranial nerves III, VI, and XII are most vulnerable because of their close anatomical relationship with the pyramidal tract. The result is a signature pattern: one side of the body loses voluntary movement while the same side of the face or head shows cranial nerve palsy, creating the crossed presentation that defines this syndrome.

Three Levels, Three Syndromes: Mapping the Brainstem Lesion

Because the brainstem is a layered structure, the precise vertical level of a lesion determines which cranial nerve is knocked out alongside the corticospinal fibers, giving rise to three distinct clinical pictures. At the superior level, Weber syndrome presents with contralateral weakness of the limbs and facial muscles paired with impaired eye movement on the same side, reflecting damage to oculomotor nerve fibers. In the middle level, Foville syndrome produces extremity weakness together with paralysis of the lateral rectus muscle on the opposite side, pointing to a caudal and medial pontine lesion that catches both the abducens nerve root and the descending motor pathways. At the inferior level, the medial medullary syndrome manifests as limb weakness combined with ipsilateral tongue paralysis, visible as deviation of the tongue when the patient protrudes it, indicating involvement of the corticospinal fibers within the pyramid and the exiting hypoglossal nerve roots. Each syndrome is essentially a fingerprint of where the brainstem was struck.

A Single Gene, A Newborn's Burden: The Childhood Form

Unlike the acquired brainstem lesions that produce the classic crossed syndromes, alternating hemiplegia of childhood is rooted in a specific genetic defect: a mutation in the ATP1A3 gene. In a cohort of twenty-four affected children—fifteen girls and nine boys—this mutation was consistently identified. The spectrum of mutations varied: three patients carried a heterozygous de novo missense change, six had a de novo missense mutation, and one child harbored a de novo splice-site mutation. The term de novo is critical here; it means the mutation arose spontaneously in the germ cell of one parent. Neither mother nor father carried the altered gene in their own tissues, yet the defective variant was transmitted through the sperm or egg to the child. This genetic origin distinguishes the childhood form sharply from the stroke-related or traumatic brainstem injuries that produce the same crossed-paralysis pattern in adults, and it underscores that the condition is present from the earliest stages of development rather than acquired later in life.

Diagnosing a Condition That Sleeps Away: Clinical Criteria

Diagnosing alternating hemiplegia of childhood relies on a constellation of criteria rather than a single test. Symptoms must appear before the child reaches eighteen months of age, and the hemiplegic episodes must recur frequently, striking either side of the body. Clinicians also look for accompanying paroxysmal events such as tonic attacks, dystonia, nystagmus, strabismus, and dyspnoea, though eye-related signs may fade in older children and go unremembered. A hallmark that strongly supports the diagnosis is the pattern of symptoms resolving during sleep and reappearing upon waking. Developmental delays, learning difficulties, or other neurological irregularities typically emerge as the child grows, even if they are subtle in infancy. Crucially, all of these findings must be unexplained by any other condition before a definitive diagnosis can be made. For the rarer Weber variant, a port-wine stain involving the ophthalmic division of the trigeminal nerve around the eye raises suspicion, and MRI becomes the preferred imaging tool to assess intracranial leptomeningeal angiomatosis.