Which condition has clonus (involuntary muscle contractions) as a symptom more frequently, Serotonin Syndrome or Neuroleptic Malignant Syndrome?

Clonus in Serotonin Syndrome vs. Neuroleptic Malignant Syndrome

Clonus is more frequently a symptom of Serotonin Syndrome than Neuroleptic Malignant Syndrome. 1

Serotonin Syndrome and Clonus

Serotonin Syndrome is characterized by a clinical triad of:

1. Mental status changes
2. Autonomic hyperactivity
3. Neuromuscular abnormalities

Clonus is a central feature in the diagnosis of Serotonin Syndrome, specifically highlighted in the Hunter Criteria, which are the recommended diagnostic criteria with high sensitivity (84%) and specificity (97%) 1. The Hunter Criteria specifically include:

• Spontaneous clonus
• Inducible clonus (with agitation or diaphoresis)
• Ocular clonus (with agitation or diaphoresis)
• Tremor and hyperreflexia
• Hypertonia with temperature >38°C and ocular or inducible clonus

The prominence of clonus in these diagnostic criteria underscores its importance as a characteristic feature of Serotonin Syndrome 2.

Differentiating Features

When comparing Serotonin Syndrome to Neuroleptic Malignant Syndrome:

• Serotonin Syndrome: Characterized by neuromuscular excitation including clonus, hyperreflexia, and myoclonus 3
• Neuroleptic Malignant Syndrome: More commonly presents with "lead pipe" rigidity rather than clonus

The presence of clonus has been found to be more specific to Serotonin Toxicity, making it a key differentiating feature between these two conditions 3, 4.

Clinical Implications

For clinicians evaluating patients with suspected drug-induced syndromes:

• Look specifically for clonus (spontaneous, inducible, or ocular) when Serotonin Syndrome is suspected 5
• The presence of clonus, especially when combined with hyperreflexia, strongly suggests Serotonin Syndrome rather than Neuroleptic Malignant Syndrome
• Clonus may present at various stages of Serotonin Syndrome, from moderate to severe cases 6

Common Pitfalls to Avoid

• Failing to test specifically for clonus when evaluating a patient with suspected drug toxicity
• Misattributing clonus to other neurological conditions when serotonergic medications are involved
• Overlooking mild forms of clonus that may be present only upon specific examination techniques
• Confusing rigidity (more common in Neuroleptic Malignant Syndrome) with hypertonicity accompanied by clonus (more typical of Serotonin Syndrome)

In summary, while both conditions can present with overlapping symptoms, clonus is a hallmark feature of Serotonin Syndrome and is significantly more frequent in this condition compared to Neuroleptic Malignant Syndrome.