The Approach to Weakness


5	The Approach to Weakness

Weakness is one of the most common presenting neurologic complaints. Many patients may tolerate some degree of numbness, tingling, or even pain, but often it is when weakness sets in that medical attention is finally sought. Similarly, friends or family members will not notice a patient’s sensory problems, but significant weakness will be obvious to all.

At the same time, weakness can be one of the most difficult neurologic problems to sort out, because the pathways that control motor function span the entire axis of the nervous system. Left leg weakness can arise from a peripheral nerve lesion, a lumbosacral plexus problem, or a stroke in the right cerebral hemisphere. Each of these has a different workup, prognosis, and treatment, and it is the job of the physician to use the history and examination to distinguish among them.

PRINCIPLES

Figure 5-1 presents a flowchart to aid in the diagnosis of weakness. The key steps in the clinical approach are outlined below.

1.Make sure that true weakness is the complaint. Sometimes patients will use the term weak to mean a general sense of fatigue; others will say a limb is “weak” when it is clumsy or numb. Having the patient confirm that decreased strength is the symptom may be useful. Likewise, a limb that is painful to move may seem “weak” because of effort; whether there is true underlying weakness may be difficult to discern. Weakness should be characterized as objectively as possible, representing the patient’s understanding of the reported symptom in conjunction with the findings on a detailed physical exam. It is therefore important that patients undergo a systematic motor exam that quantifies the severity of the weakness and discerns whenever possible if the weakness is effort dependent or pain limited.

The history of the onset and pattern of progression of the symptoms is also very important. If the weakness was preceded by a seizure, it may be due to a Todd paralysis (a transient unilateral weakness that resolves typically within several hours after a seizure). If the weakness occurred abruptly and is unilateral, this may be more suggestive of a vascular cause, whereas if the weakness occurred insidiously and is progressive, other etiologies should be considered.

2.Identify which muscles are weak. This seems like an obvious point but must be emphasized. It is not sufficient to know that a patient has left leg weakness. Testing must be done in enough detail to know which muscles in the left leg are weak or, if they are all weak, which are weaker than others.

3.Determine the pattern of weakness. This is frequently the crux of the entire diagnosis. It is the pattern of weakness that will indicate the underlying lesion—whether left leg weakness is due to a peroneal nerve problem or a right hemispheric stroke. One must be familiar with the different patterns of weakness and their implications.

4.Look for associated signs and symptoms. If a leg is weak, determine whether it is also numb, tingling, or painful. Check the reflexes carefully. Examine the muscles themselves looking for atrophy and fasciculations. Tone is also important in helping assess if the problem is peripheral or central. Often the motor deficit overshadows other problems, whose presence may be helpful in supporting or excluding certain diagnoses. Patients may also have an acute cause of weakness superimposed upon a more chronic underlying condition.

FIGURE 5-1. The approach to weakness. EMG/NCS, electromyography/nerve conduction studies; NMJ, neuromuscular junction.

5.Use laboratory and electrophysiologic tests wisely. Blood tests or neuroimaging studies can be useful in the appropriate settings, and electromyography/nerve conduction studies (EMG/NCS) can act as an extension of the clinical exam in localizing the problem to a particular segment of the peripheral nervous system. Lumbar puncture (LP) may also be necessary in patients with suspected Guillain–Barré syndrome. Tests are most useful, however, in the setting of a complete clinical evaluation and formed diagnostic hypothesis.

KEY POINTS

●Weakness can be caused by lesions along the entire neuraxis, from brain to muscle.

●The diagnosis rests on determining what the pattern of weakness is, searching for associated signs and symptoms, and using laboratory tests and EMG/NCS to confirm clinical hypotheses.

DIFFERENTIAL DIAGNOSIS

It is useful to consider the disorders that cause weakness in an anatomic order, from most distal in the nervous system (primary muscle disorders) to most proximal (disorders of the cerebral hemispheres). Below, each anatomic category is presented with the clues that might lead a clinician to suspect a disorder in that location. Individual diseases in each category are discussed in the later chapters covering specific neurologic disorders.

PRIMARY MUSCLE DISORDERS

Pattern of Weakness

Primary muscle problems tend to cause weakness predominantly in proximal muscles, in a symmetric fashion. Distal muscles are affected later or not as severely. In addition, neck flexors and extensors, which are not affected in most nerve or brain lesions, may be weak in a muscle disorder.

Associated Signs and Symptoms

Associated signs and symptoms may occasionally include muscle pain if the muscle disorder is inflammatory, such as polymyositis. By their nature, primary disorders of muscle should not cause sensory signs or other symptoms. Reflexes are characteristically preserved unless the process is so severe that the muscles are nearly paralyzed.

Laboratory Studies

Some disorders of muscle are characterized by an elevated serum creatine kinase (CK) level. The demonstration of characteristic “myopathic” changes on an EMG can help confirm a primary muscle disorder.

Differential Diagnosis

Primary muscle disorders, discussed in Chapter 24, include both acquired problems (myopathies), which can result from inflammatory or toxic etiologies among other causes, and congenital problems (muscular dystrophies).

KEY POINTS

●Primary muscle disorders typically cause symmetric proximal weakness and can affect neck muscles.

●Sensory signs and symptoms are typically not present in primary muscle disorders.

●Serum CK level is elevated in some muscle disorders, and EMG may show a characteristic “myopathic” pattern.

NEUROMUSCULAR JUNCTION DISORDERS

Pattern of Weakness

Neuromuscular junction (NMJ) problems can vary in the pattern of weakness they cause, though most affect proximal limb muscles. Some NMJ disorders can lead to ptosis as well as weakness of extraocular, bulbar, and neck muscles. The characteristic feature of NMJ disorders is not the location of weakness, however, but the fluctuation. The degree of weakness may change from hour to hour. Depending on the specific disease, strength may be worse after using the muscles or toward the end of the day; it may improve after resting or in the morning (fatigability). Alternatively, strength may improve paradoxically after exercise in other conditions.

Associated Signs and Symptoms

By their nature, NMJ problems, which affect only the junction between the motor axon terminal and the muscle, should not lead to sensory signs or symptoms. Some NMJ disorders may have associated autonomic features and can also be associated with malignancies. A detailed review of systems can help identify these features.

Laboratory Studies

EMG/NCS can demonstrate nearly pathognomonic findings for certain NMJ disorders on specialized testing. Some of the diseases in this category have specific serum markers, such as anti-acetylcholine receptor antibodies and muscle specific kinase in myasthenia gravis. These are discussed in further detail in Chapter 24.

Differential Diagnosis

NMJ disorders are discussed in Chapter 24; they include myasthenia gravis and Lambert–Eaton myasthenic syndrome, among others.

KEY POINTS

●NMJ disorders can cause weakness of proximal muscles; some characteristically affect extraocular and bulbar muscles.

●The key to diagnosing NMJ disorders is fluctuation in the degree of weakness.

●Sensory signs and symptoms are not generally present in NMJ disorders.

●EMG/NCS can be nearly pathognomonic in some cases of NMJ disorders.

PERIPHERAL NERVE DISORDERS

Pattern of Weakness

Each muscle in the upper or lower limbs is innervated by an individual peripheral nerve (Table 5-1). A lesion involving a particular peripheral nerve will lead to weakness in the muscles innervated by that nerve while sparing other, often neighboring muscles.

Disorders affecting a single peripheral nerve are known as mononeuropathies. Certain systemic conditions can lead to dysfunction of multiple peripheral nerves in succession, a disorder known as mononeuropathy multiplex. Finally, when peripheral nerves are all affected diffusely, in a polyneuropathy, dysfunction typically occurs in the longest nerves first. Thus, weakness from a polyneuropathy usually appears first in the distal muscles, symmetrically.

Associated Signs and Symptoms

Mononeuropathies may cause sensory symptoms—such as numbness, tingling, or pain—in the distribution of the relevant peripheral nerve. Mononeuropathy multiplex is characteristically associated with pain. Polyneuropathies, depending on etiology, usually have associated sensory loss and depressed or absent reflexes, particularly in the distal extremities.

Laboratory Studies

EMG/NCS can confirm the clinical suspicion of a problem localized to the peripheral nerves. NCS can identify whether the pathologic process affects primarily the axons or the myelin of the nerve, an essential step in formulating a differential diagnosis. EMG may yield insight into the relative acuity or chronicity of a nerve disorder.

Differential Diagnosis

Mononeuropathies most commonly occur as a result of entrapment (as in carpal tunnel syndrome). Mononeuropathy multiplex is associated with systemic vasculitis and other metabolic or rheumatologic diseases. Demyelinating polyneuropathies can be hereditary (such as Charcot–Marie–Tooth disease) or acquired (as in Guillain–Barré syndrome), whereas axonal polyneuropathies have many potential underlying causes from systemic conditions or ingestions (e.g., alcohol or toxins). Peripheral nerve disorders are discussed in Chapter 23.

TABLE 5-1. Commonly Tested Movements
Movement	Muscle	Nerve	Root
Shoulder abduction	Deltoid	Axillary	C5
Elbow flexion	Biceps	Musculocutaneous	C5/C6
Elbow extension	Triceps	Radial	C7
Wrist extension	Wrist extensors	Radial	C7
Finger flexion	Finger flexors	Median, ulnar	C8/T1
Finger extension	Finger extensors	Radial	C7
Finger abduction	Interossei	Ulnar	C8/T1
Hip flexion	Iliopsoas	Nerve to iliopsoas	L1/L2/L3
Hip abduction	Gluteus medius, minimus	Superior gluteal	L5
Hip adduction	Hip adductors	Obturator	L3
Hip extension	Gluteus maximus	Sciatic	S1
Knee flexion	Hamstrings	Sciatic	L5/S1
Knee extension	Quadriceps	Femoral	L3/L4
Plantar flexion	Gastrocnemius, soleus	Tibial	S1
Dorsiflexion	Tibialis anterior	Peroneal	L5
Foot eversion	Peroneus muscles	Peroneal	S1
Foot inversion	Tibialis posterior	Tibial	L5
Great toe extension	Extensor hallucis longus	Peroneal	L5

KEY POINTS

●Mononeuropathies lead to weakness in muscles innervated by a single peripheral nerve.

●Polyneuropathies first affect the muscles of the distal extremities symmetrically.

●EMG/NCS can confirm peripheral nerve involvement, identify axonal or demyelinating features, and evaluate the relative chronicity of a nerve disorder.

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