Before starting to apply this clinical mind map, a clinician needs to make sure that the patient’s chief complaint is indeed true muscular weakness, and not fatigue or asthenia. True weakness is characterized as the inability to initiate, continue, or complete a task which requires the use of muscles. Fatigue is the inability to continue performing a task after multiple repetitions, whereas true weakness is the inability to perform a first repetition of the task. Asthenia is a sense of weariness or exhaustion without weakness.
This clinical mind map is organized based upon the site of the weakness, as generalized and focal. Differential diagnoses under these two groups are further divided based upon the pathophysiologic feature of upper motor (UMN) versus lower motor neuron (LMN) lesions. Upper motor neuron lesions are further divided into central nervous system and spinal cord lesions, and lower motor neuron lesions are further divided into mono/poly neuropathies, neuromuscular junction pathologies, and muscle diseases. Patterns or distributions of weakness is stated under each differential. To navigate this clinical mind map, features related to pathophysiologic characterization (focal, general, upper motor neurons, and lower motor neurons) are noted in blue and bold font. Generally, upper motor neuron lesions cause area or task specific deficits, and lower motor neuron lesions cause nerve distribution specific deficit (if mono neuropathy) or bilateral deficit (if peripheral poly neuropathy). Pathologies in the neuromuscular junction cause fluctuating weakness. Lastly, muscle-related pathologies, or myopathies, cause mostly proximal muscle groups weakness, although a few cause weakness in the distal groups of muscles. Myopathies do not cause numbness and tingling, such that the absence of this feature can help narrow the differentials.
Applying this clinical mind map, a clinician should think of all the probable diagnoses when evaluating a patient with weakness, then address urgent/emergent situations by reviewing vital signs, the patient’s level of alertness, general appearance, pattern of progression of weakness, and toxin exposure. Life-threatening conditions which present as urgent/emergent situations are cerebrovascular accidents, GBS, or any condition which is likely to progress fast, such as heavy toxin exposure, and infection. The life-threatening conditions mentioned above present as urgent/emergent situations only after their characteristic symptoms and signs meet a certain “threshold,” such as a decline in mental alertness, alteration in vital signs, distressed appearance, and/or rapid progression of neuro/muscular deficit. A neurological deficit can usually be observed or elicited within the first few moments of a patient evaluation. A widely used mnemonic for this is FAST for Cerebrovascular Accident (facial asymmetry, arm weakness, speech difficulty, and timing). This is one patient presentation in which a brief physical exam may need to be done before finishing the history, in order to address urgent/emergent situations. In order to weigh and remove anchor bias, a clinician must ask questions about the duration, location and pattern of weakness to determine whether it is focal/general/proximal/distal/fluctuating with or without sensory loss. Once a narrow list of differentials has been formulated with these questions, the clinician can ask condition-specific questions, and gather or review additional information, such as risk factors for certain diseases. Physical exam features are described briefly as motor exam features which differentiate between upper motor neuron and lower motor neuron lesions, and which are found at the bottom of the clinical mind map.
Additional information in the form of physical exam and lab tests helps arrive at the final diagnosis.
A helpful mnemonic to remember differential diagnoses on this mind map is to think 3/3/3/3 meaning 3 conditions related to central nervous system, peripheral nerves, neuromuscular junction/anterior horn and muscles each.