Acute weakness

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8.4 Acute weakness

Richard Lennon

Essentials

1 The fact that a child has acute weakness may be obscured by an inability to communicate and a smaller repertoire of activities. The possibility of a neuromuscular problem must be consciously considered by the clinician.
2 Conversely a small child may not move a limb because of pain rather than weakness. A history and examination for evidence of trauma should be performed and appropriate diagnostic imaging considered.
3 Respiratory insufficiency due to weakness, may occur with surprising rapidity and may be compounded by aspiration. Careful assessment and close monitoring for this complication must be made.
4 Abnormal reflexes are important in localising the cause of acute weakness. If some are absent especially distally it suggests a lower motor neurone dysfunction. If all are present it suggests muscle dysfunction and if they are increased it suggests an upper motor neurone weakness.
5 The presence of a sensory level indicates a spinal cord lesion. One should consider investigating this with an urgent MRI to determine whether or not urgent surgery is needed.

Introduction

The acutely weak child is challenging to the emergency physician because she/he can present in many different ways with many different diagnoses. In particular, the preverbal child may present with only regression in motor milestones or bruising on the face from increased frequency of falls. The diagnosis in such children can range from a tick bite to Duchenne pseudohypertrophic muscular dystrophy.

Presentation

In infants of less than three months, parents are usually quite sensitive to abnormalities of the infant’s behaviour. They may bring a baby in complaining of poor feeding, decreased activity or being ‘floppy’. The older infant may be noted to lose milestones such as ceasing to crawl or walk. ‘Ataxia’ may be the predominant feature in preschool and school-age children. Sometimes children may present with an associated feature of illness, such as the rash of dermatomyositis or an obvious engorged tick. In general, the older the child is, the more typical the presentation. However, even in this group, as in adults, there is a wide range of presentations, e.g. cranial nerve palsies in the Miller–Fisher variant of Guillain–Barré syndrome.

Trauma masquerading as weakness

Trauma is dealt with in detail in other chapters of the book. However, it should noted that non-accidental injury (NAI) can present as ‘acute weakness’ in the infant and young child. Shaken baby syndrome may present as a lethargic irritable child with little or no bruising. The apparent focal weakness may be due to an underlying fracture. Likewise, emotional and nutritional neglect may also lead to weakness. Hence, NAI must be considered in the differential diagnosis of the acutely weak child (see Chapter 18.2).

Primary survey approach

General inspection

This is done by looking for both cause and effect of the weakness. An abnormality of the primary survey severe enough to cause generalised weakness (e.g. respiratory distress) is usually the obvious presenting problem. Hypoglycaemia in a child can, in rare circumstances, present with focal weakness. It is important in the ‘weak’, floppy infant to consider the possibility of intussusception or other intra-abdominal pathology. In an infant with intussusception, prostration mimicking severe ‘weakness’ and lethargy may be the only presenting sign. When one comes to exposure in the primary survey of the weak child a thorough search has to take place for ticks (don’t forget to look in deep skin folds), possible venomous bites and stings, and rashes, such as those caused by enteroviruses. Enteroviral rashes are usually scattered maculopapular but may be petechial. A close inspection should be made for bruises and other suggestive signs of NAI. A quick assessment of the child’s nutritional state and the appropriateness of weight for age should be estimated. The size of the child’s head and its proportion to the body should be noted and centiles should be plotted later in the detailed examination phase.

ABC

The effects of weakness may include airway compromise from bulbar palsy (e.g. hoarse voice, stridor, and aspiration of secretions). Severely impaired ventilation will be clinically apparent. However, mild or moderate impairment can be subtle and can rapidly progress to cause respiratory embarrassment. Therefore, if possible, respiratory function tests should be serially performed on all children presenting with neuromuscular weakness. Arterial or venous blood gases should also be considered in this assessment. (Table 8.4.1 for predictors of needing intubation and mechanical ventilation.) Circulatory defects may arise from disturbance of the autonomic nervous system. This is characterised by labile blood pressure, heart rate and postural hypotension.

Table 8.4.1 Predictors of the necessity for ICU/HDU admission

Bulbar palsy Vital capacity <20 mL kg–1 >30%reduction in vital capacity from baseline Flaccid quadriparesis Rapidly progressive weakness Autonomic cardiovascular instability

Disability

The neurological part of the primary survey can give early hints as to the site of the lesion(s) involved. The classic description of peripheral neuromuscular weakness is an alert, but anxious looking, child that has a paucity of limb movements. If respiratory failure or facial weakness have intervened this appearance will not be present. Conversely, intracranial causes of weakness are usually associated with some degree of obtundation.

The weak infant classically lies in a frog leg posture with all limbs lying on the bed, abducted at the hips and flexed at the knees. Asymmetry may be apparent in those with a focal weakness. With central causes of weakness (e.g. acute hydrocephalus) hypertonic posturing may be seen, with scissoring adduction of lower limbs. Autonomic dysfunction may lead to associated fever or hypothermia. In any child with weakness one needs to consider whether the findings could be caused by a toxidrome (e.g. organophosphate poisoning).

Once the primary survey has been completed, and any abnormalities corrected, then a detailed history and examination should be performed.

History

If, on questioning, the weakness has been chronic then the list of possible diagnoses is extensive and is beyond the scope of this text. Such children should be stabilised and referred to the appropriate paediatric service for diagnosis. However, it is possible that a child with chronic weakness could undergo an acute deterioration, such as influenza in a child with a Duchenne muscular dystrophy. In infants the distinction between acute and chronic weakness is less relevant. In those children with conditions covered by this chapter, the history needs to focus initially on finding treatable causes. Detailed enquiry should be made about any possible tick bites or other venomous bites or stings. Enquiry should also be made about the availability of various medications and poisons around the house.

A precise time course for the illness should be obtained, along with the pattern of evolution of the weakness. Whether it is ascending from lower limbs up, whether it is lateralised and whether it has progressed or not. A sudden onset may suggest a vascular or epileptic event. Rapid onset weakness may follow an intoxication or envenomation. Other causes are more subacute and may have progressed over weeks. The family history should be reviewed, and whether there is consanguinity. A history of recent infectious illnesses may be relevant. Immunisation history regarding polio vaccination is important and diphtheria is still a common cause of weakness in third world countries. Any overseas travel should be noted.

Examination

A detailed but focused examination is then performed, that attempts to establish the level and the nature of the lesion. The following lists the important distinguishing features of the different levels of the neuromuscular system which may present with weakness.

Muscular:

• usually proximal more than distal weakness;
• reflexes preserved until very late;
• may have tender muscles;
• these children often have a positive Gower’s sign (see below).

Neuromuscular junction:

• fatigability;
• reflexes are preserved if the muscle is not fatigued or the disease is not severe.

Lower motor neuron weakness:

• more peripheral than proximal weakness;
• reflexes are lost early in the illness.

Upper motor neuron weakness:

• apart from an initial flaccid phase, tone and reflexes are usually increased;
• spinal cord lesions are usually associated with a sensory level;
• intracranial problems are associated with features of encephalopathy, decreased level of consciousness, speech dysfunction, ataxia, and bulbar dysfunction.

It may be difficult to perform a formal neurological examination in an unco-operative infant or small child. Often in these situations, one can get a lot of information just by watching the child play. Another useful test is known as Gower’s sign. The child is laid on their back on a firm surface and encouraged to stand. A child with proximal muscle weakness will not be able to sit up but will have to roll on to its abdomen get up on all fours and then ‘climb up its legs’ using its hands. Although reflexes are a good guide they are by no means foolproof. Upper motor lesions will usually have increased reflexes with increased tone. However, immediately after a spinal cord insult there may be a flaccid paralysis below the level of the lesion with absence of reflexes. Also, in transverse myelitis, there may be patchy combinations of upper and lower motor neuron signs. Early in Guillain–Barré syndrome the reflexes may be preserved and, likewise, very late in myopathic weakness, distal reflexes may be lost. Acute myositis is usually associated with tender muscles.

Investigations

Laboratory

Tests that may be useful in the acutely weak child include:

• a full blood count, which may reveal anaemia, nutritional disorders, leukaemia, or signs of infection;
• abnormalities of electrolytes especially potassium, calcium, phosphate, sodium may cause weakness;
• urea and creatinine – renal failure may present as generalised weakness, but more likely cause an abnormality of electrolytes;
• creatine kinase is often raised in muscular causes of acute weakness;
• endocrine function tests – especially thyroid and adrenal (see below);
• lumbar puncture (see below)
• electrophysiological studies (electromyogram (EMG), electroneurogram (ENG)).

Imaging

• Urgent magnetic resonance imaging (MRI) of spine (where progressive spinal cord lesion is suspected);
• Computerised tomography (CT) or MRI of brain (where central cause is suspected);
• Chest X-ray (e.g. for suspected aspiration).

Specific conditions causing acute weakness

Though not exhaustive, the following sections give some detail on the more common causes of acute weakness seen in children and also the rarer ones that must be diagnosed and treated in emergency.

Guillain–Barré syndrome

Introduction

Guillain–Barré syndrome (GBS) is an acute polyradiculopathy. It is now considered a group of diseases with two main pathological mechanisms; demyelination and axonal degeneration. In the paediatric population it is more common in late childhood (4–9 years of age) and in males (1.5 times higher risk). It is uncommon, estimated at less than 1/100 000 person years in people less than 17 years of age. It is primarily a lower motor neuron disease affecting the myelin sheath with variable damage to axons. It is an immune-mediated reaction, usually to a recent infection. Antibodies to various gangliosides are found in the serum of patients with GBS and matching antigens are often found in the preceding infectious agent. Infections known to be associated with GBS include Campylobacter, Mycoplasma, Epstein–Barr virus, Coxsackie viruses, influenza viruses, echoviruses and cytomegalovirus. The greater the axonal damage, the longer and the less complete is the recovery.

History and examination

Children usually present with weakness, falls, regression of motor milestones or ataxia. They also complain of muscle pain in the early part of the illness. Cranial nerves are involved in 40–50% of cases, with the facial nerve most commonly involved. The Miller–Fisher variant presents with oculomotor palsies, ataxia and areflexia.

On questioning, the parents often give a history of a generalised viral, respiratory or gastroenteritis illness in the preceding 2 weeks. In the early part of the illness the child may have paraesthesiae. Classically, the paralysis is ascending and symmetrical. The majority present with mostly distal weakness; however, about 15% have extensive proximal muscular involvement.

Papilloedema is rare, but may occur in GBS and is associated with raised intracranial pressure. Paralysis of the respiratory muscles is common and must be monitored carefully. Sympathetic nervous system involvement can produce profuse sweating, hypertension, postural hypotension and disturbances of sphincter function. Fatal cardiac arrhythmias have been reported in association with these signs. Although primarily a motor problem, sensory disturbance does occur, especially impairment of position sense. As mentioned above, reflexes are usually absent though increased reflexes and extensor plantar responses are occasionally found in the early phase of the illness.

The weakness may evolve rapidly within hours. However, it usually takes 1–2 weeks to reach the maximal weakness. Then, in the 2nd to 4th week of the illness, recovery is apparent and most children have recovered by 2 months, although some take as long as 18 months. Rarely, GBS will present in the newborn and is known as congenital GBS. They present as floppy babies that are areflexic and have elevated cerebrospinal fluid (CSF) protein.

Laboratory findings

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