Depressed, suicidal, and brittle in her bones

Fracture risk is associated with dura­tion of use rather than dosage. Population-based studies show a higher fracture risk for new users of TCAs compared with continuous users, and the risk of fracture with SSRIs seems to increase slightly over time.⁹ No association has been identified between fracture risk and antidepressant dosage. According to the literature, drugs with low affinity for the serotonin trans­porter, such as maprotiline and mirtazap­ine, likely are the safest antidepressants for patients at increased risk of fracture. Options also include other TCAs and any antidepressant with low affinity for the serotonin receptor.^7,8 

Lithium
Studies on lithium and bone mineral den­sity have shown mixed results. Older stud­ies found that lithium had a negative or no effect on bone mineral density or the para­thyroid hormone level.¹⁰ More recent inves­tigations, however, suggest that the drug has a protective effect on bone mineral den­sity, although this has not been replicated in all studies.

In a mouse model, lithium has been shown to enhance bone formation and improve bone mass, at least in part by activation of the Wnt signaling pathway through an inhibitory effect on glycogen synthase kinase-3β.¹¹ In humans, lithium-treated adults had lower serum alkaline phosphate, osteocalcin, and C-telopeptide levels compared with controls, suggesting a state of decreased bone remodeling and increased turnover.¹² There is a paucity of clinical data on the effect of lithium on frac­ture risk. Additional studies are necessary to elucidate lithium’s mechanism on bone mineral density and determine the magni­tude of the clinical effect.

Anticonvulsants
The association among anticonvulsants, decreased bone mineral density, and increased risk of fracture is well-established in the literature.¹³ However, causality is dif­ficult to determine, because many studies were of patients with a seizure disorder, who often have additional risk factors for fracture, including seizure-related trauma, drowsiness, and slowed reflexes.

Mechanisms through which anticon­vulsants increase fracture risk include increased bone resorption, secondary hypo­parathyroidism, and pseudohypoparathy­roidism. Markers of bone resorption were elevated in patients receiving an antiepi­leptic.¹⁴ This effect might be enhanced by co-administration of cytochrome P450 (CYP450) enzyme-inducing anticon­vulsants and CYP450 enzyme-inhibiting medications, such as valproate. Long-term treatment with valproate may produce reduction of bone mass and increased risk of fractures; however, other studies dis­agree with this finding.¹⁵

In addition to CYP450-inducing effects, phenytoin, carbamezapine, and phenobar­bital can increase catabolism of vitamin D, which is associated with osteomalacia.¹⁴ This results in decreased intestinal absorp­tion of calcium, hypocalcemia, and sec­ondary hyperparathyroidism, which also increases fracture risk. Anticonvulsants also might increase resistance to pseudo­hypoparathyroidism and inhibit calcitonin secretion.

Lamotrigine has not been shown to interfere with bone accrual¹⁶ and may be a safer mood stabilizer for patients at high risk of fracture. For patients at increased risk of fracture, it is important to select an anticonvulsant wisely to minimize frac­ture risk.

How would you treat Ms. E during her hospitalization for bipolar disorder?
   a) carbamazepine
   b) lithium
   c) risperidone
   d) mirtazapine

TREATMENT Minimizing polypharmacy
Because many pharmacotherapeutic options for managing bipolar disorder can increase the risk of fracture, clinicians must be aware of the relative risk of each class of medication and each individual drug. We initiated lithium, 300 mg, 3 times a day, to stabilize Ms. E’s mood. Although clinical data are inconclusive regarding lithium’s effect on fracture risk, we felt that the benefit of acute mood stabiliza­tion outweighed the risk of decreased bone mineral index.

We selected aripiprazole, 10 mg/d, as an adjunctive treatment because of its minimal effect on serum prolactin levels.⁴ We con­sidered prescribing an antidepressant but decided against it because we were concerned about manic switching.

Polypharmacy is another important con­sideration for Ms. E. Several studies have identified polypharmacy, particularly with antipsychotics, as an independent risk factor for fracture.³ Therefore, we sought to minimize the number of medications Ms. E receives. Although lithium monotherapy is an option, we thought that her mood symptoms were severe enough that the risk of inadequately treating her bipolar symptoms outweighed the additional risk of fracture from dual ther­apy with lithium and aripiprazole. Untreated or inadequately treated depression is associ­ated with a higher fracture risk. Therefore, we avoided prescribing >2 medications to mitigate any excessive risk of fracture from polypharmacy.

Bottom Line
Different classes of medications—antipsychotics, anticonvulsants, antidepressants, and lithium—used for treating bipolar disorder have been shown to increase risk of bone fracture through a variety of mechanisms. Anticonvulsants and prolactin-elevating antipsychotics are associated with increased fracture risk; evidence on lithium is mixed. Fracture risk with antidepressants is associated with duration of use, rather than dosage.

Disclosures
The authors report no financial relationship with any company whose products are mentioned in this article or with manufacturers of competing products.